erlang

erlang

The Erlang BIFs

By convention, most built-in functions (BIFs) are seen as being in the module erlang. A number of the BIFs are viewed more or less as part of the Erlang programming language and are auto-imported. Thus, it is not necessary to specify the module name and both the calls atom_to_list(Erlang) and erlang:atom_to_list(Erlang) are identical.

In the text, auto-imported BIFs are listed without module prefix. BIFs listed with module prefix are not auto-imported.

BIFs may fail for a variety of reasons. All BIFs fail with reason badarg if they are called with arguments of an incorrect type. The other reasons that may make BIFs fail are described in connection with the description of each individual BIF.

Some BIFs may be used in guard tests, these are marked with "Allowed in guard tests".

ext_binary()
  a binary data object,
  structured according to the Erlang external term format

iodata() = iolist() | binary()

iolist() = [char() | binary() | iolist()]
  a binary is allowed as the tail of the list
    

Types:

Returns an integer or float which is the arithmetical absolute value of Number.

> abs(-3.33).
3.33000
> abs(-3).
3
        

Allowed in guard tests.

Types:

Returns a new tuple which has one element more than Tuple1, and contains the elements in Tuple1 followed by Term as the last element. Semantically equivalent to list_to_tuple(tuple_to_list(Tuple ++ [Term]), but much faster.

> erlang:append_element({one, two}, three).
{one,two,three}
        

Types:

Call a fun, passing the elements in Args as arguments.

Note: If the number of elements in the arguments are known at compile-time, the call is better written as Fun(Arg1, Arg2, ... ArgN).

Earlier, Fun could also be given as {Module, Function}, equivalent to apply(Module, Function, Args). This usage is deprecated and will stop working in a future release of Erlang/OTP.

Types:

Returns the result of applying Function in Module to Args. The applied function must be exported from Module. The arity of the function is the length of Args.

> apply(lists, reverse, [[a, b, c]]).
[c,b,a]
        

apply can be used to evaluate BIFs by using the module name erlang.

> apply(erlang, atom_to_list, ['Erlang']).
"Erlang"
        

Note: If the number of arguments are known at compile-time, the call is better written as Module:Function(Arg1, Arg2, ..., ArgN).

Failure: error_handler:undefined_function/3 is called if the applied function is not exported. The error handler can be redefined (see process_flag/2). If the error_handler is undefined, or if the user has redefined the default error_handler so the replacement module is undefined, an error with the reason undef is generated.

Types:

Returns a string which corresponds to the text representation of Atom.

> atom_to_list('Erlang').
"Erlang"
        

Types:

Returns a list of integers which correspond to the bytes of Binary.

Types:

As binary_to_list/1, but returns a list of integers corresponding to the bytes from position Start to position Stop in Binary. Positions in the binary are numbered starting from 1.

Types:

Returns an Erlang term which is the result of decoding the binary object Binary, which must be encoded according to the Erlang external term format. See also term_to_binary/1.

Types:

This implementation-dependent function increments the reduction counter for the calling process. In the Beam emulator, the reduction counter is normally incremented by one for each function and BIF call, and a context switch is forced when the counter reaches 1000.

This BIF might be removed in a future version of the Beam machine without prior warning. It is unlikely to be implemented in other Erlang implementations.

Types:

Cancels a timer, where TimerRef was returned by either erlang:send_after/3 or erlang:start_timer/3. If the timer is there to be removed, the function returns the time in milliseconds left until the timer would have expired, otherwise false (which means that TimerRef was never a timer, that it has already been cancelled, or that it has already delivered its message).

See also erlang:send_after/3, erlang:start_timer/3, and erlang:read_timer/1.

Note: Cancelling a timer does not guarantee that the message has not already been delivered to the message queue.

Types:

Returns true if the process Pid is executing old code for Module. That is, if the current call of the process executes old code for this module, or if the process has references to old code for this module, or if the process contains funs that references old code for this module. Otherwise, it returns false.

> check_process_code(Pid, lists).
false
        

See also code(3).

Do not use; use list_to_binary/1 instead.

Types:

Returns the current date as {Year, Month, Day}.

The time zone and daylight saving time correction depend on the underlying OS.

> date().
{1995, 2, 19}
        

Types:

Makes the current code for Module become old code, and deletes all references for this module from the export table. Returns undefined if the module does not exist, otherwise true.

This BIF is intended for the code server (see code(3)) and should not be used elsewhere.

Failure: badarg if there is already an old version of Module.

Types:

If MonitorRef is a reference which the calling process obtained by calling erlang:monitor/2, this monitoring is turned off. If the monitoring is already turned off, nothing happens.

Once erlang:demonitor(MonitorRef) has returned it is guaranteed that no {'DOWN', MonitorRef, _, _, _} message due to the monitor will be placed in the callers message queue in the future. A {'DOWN', MonitorRef, _, _, _} message might have been placed in the callers message queue prior to the call, though. Therefore, in most cases, it is advisable to remove such a 'DOWN' message from the message queue after monitoring has been stopped. erlang:demonitor(MonitorRef, [flush]) can be used instead of erlang:demonitor(MonitorRef) if this cleanup is wanted.

Prior to OTP release R11B (erts version 5.5) erlang:demonitor/1 behaved completely asynchronous, i.e., the monitor was active until the "demonitor signal" reached the monitored entity. This had one undesirable effect, though. You could never know when you were guaranteed not to receive a DOWN message due to the monitor. Current behavior can be viewed as two combined operations: asynchronously send a "demonitor signal" to the monitored entity and ignore any future results of the monitor.

Failure: It is an error if MonitorRef refers to a monitoring started by another process. Not all such cases are cheap to check; if checking is cheap, the call fails with badarg (for example if MonitorRef is a remote reference).

Types:

erlang:demonitor(MonitorRef, []) is equivalent to erlang:demonitor(MonitorRef).

Currently the following Options are valid:

flush

Remove (one) {'DOWN', MonitorRef, _, _, _} message, if there is one, from the callers message queue after monitoring has been stopped.
Calling erlang:demonitor(MonitorRef, [flush]) is equivalent to:

    erlang:demonitor(MonitorRef),
    receive
        {'DOWN', MonitorRef, _, _, _} ->
            true
    after 0 ->
            true
    end

            

More options may be added in the future.

Failure: badarg if OptionList is not a list, or if Option is not a valid option, or the same failure as for erlang:demonitor/1

Types:

Forces the disconnection of a node. This will appear to the node Node as if the local node has crashed. This BIF is mainly used in the Erlang network authentication protocols. Returns true if disconnection succeeds, otherwise false. If the local node is not alive, the function returns ignored.

Types:

Prints a text representation of Term on the standard output.

This BIF is intended for debugging only.

Types:

Returns the Nth element (numbering from 1) of Tuple.

> element(2, {a, b, c}).
b
        

Allowed in guard tests.

Types:

Returns the process dictionary and deletes it.

> put(key1, {1, 2, 3}),
  put(key2, [a, b, c]),
  erase().
[{key1,{1,2,3}},{key2,[a,b,c]}]
        

Types:

Returns the value Val associated with Key and deletes it from the process dictionary. Returns undefined if no value is associated with Key.

> put(key1, {merry, lambs, are, playing}),
  X = erase(key1),
  {X, erase(key1)}.
{{merry,lambs,are,playing},undefined}
        

Types:

Stops the execution of the calling process with the reason Reason, where Reason is any term. The actual exit reason will be {Reason, Where}, where Where is a list of the functions most recently called (the current function first). Since evaluating this function causes the process to terminate, it has no return value.

> catch erlang:error(foobar).
{'EXIT',{foobar,[{erl_eval,do_apply,5},
                 {erl_eval,expr,5},
                 {shell,exprs,6},
                 {shell,eval_loop,3}]}}
        

Types:

Stops the execution of the calling process with the reason Reason, where Reason is any term. The actual exit reason will be {Reason, Where}, where Where is a list of the functions most recently called (the current function first). Args is expected to be the list of arguments for the current function; in Beam it will be used to provide the actual arguments for the current function in the Where term. Since evaluating this function causes the process to terminate, it has no return value.

Types:

Stops the execution of the calling process with the exit reason Reason, where Reason is any term. Since evaluating this function causes the process to terminate, it has no return value.

> exit(foobar).
** exited: foobar **
> catch exit(foobar).
{'EXIT', foobar}
        

Types:

Sends an exit signal with exit reason Reason to the process Pid.

The following behavior apply if Reason is any term except normal or kill:

If Pid is not trapping exits, Pid itself will exit with exit reason Reason. If Pid is trapping exits, the exit signal is transformed into a message {'EXIT', From, Reason} and delivered to the message queue of Pid. From is the pid of the process which sent the exit signal. See also process_flag/2.

If Reason is the atom normal, Pid will not exit. If it is trapping exits, the exit signal is transformed into a message {'EXIT', From, normal} and delivered to its message queue.

If Reason is the atom kill, that is if exit(Pid, kill) is called, an untrappable exit signal is sent to Pid which will unconditionally exit with exit reason killed.

Types:

Stops the execution of the calling process with the reason Reason. This is an old equivalent to erlang:error(Reason).

Types:

Stops the execution of the calling process with the reason Reason. This is an old equivalent to erlang:error(Reason, Args) .

Types:

Returns a float by converting Number to a float.

> float(55).
55.0000
        

Allowed in guard tests.

Note that if used on the top-level in a guard, it will test whether the argument is a floating point number; for clarity, use is_float/1 instead. When float/1 is used in an expression in a guard, such as 'float(A) == 4.0', it converts a number as described above.

Types:

Returns a string which corresponds to the text representation of Float.

> float_to_list(7.0).
"7.00000000000000000000e+00"
        

Types:

Returns a list containing information about the fun Fun. Each element of the list is a tuple. The order of the tuples is not defined, and more tuples may be added in a future release.

This BIF is mainly intended for debugging, but it can occasionally be useful in library functions that might need to verify, for instance, the arity of a fun.

There are two types of funs with slightly different semantics:

A fun created by fun M:F/A is called an external fun. Calling it will always call the function F with arity A in the latest code for module M. Note that module M does not even need to be loaded when the fun fun M:F/A is created.

All other funs are called local. When a local fun is called, the same version of the code that created the fun will be called (even if newer version of the module has been loaded).

The following elements will always be present in the list for both local and external funs:

{type, Type}

Type is either local or external.

{module, Module}

Module (an atom) is the module name.
If Fun is a local fun, Module is the module in which the fun is defined.
If Fun is an external fun, Module is the module that the fun refers to.

{name, Name}

Name (an atom) is a function name.
If Fun is a local fun, Name is the name of the local function that implements the fun. (This name was generated by the compiler, and is generally only of informational use. As it is a local function, it is not possible to call it directly.) If no code is currently loaded for the fun, [] will be returned instead of an atom.
If Fun is an external fun, Name is the name of the exported function that the fun refers to.

{arity, Arity}

Arity is the number of arguments that the fun should be called with.

{env, Env}

Env (a list) is the environment or free variables for the fun. (For external funs, the returned list is always empty.)

The following elements will only be present in the list if Fun is local:

{pid, Pid}

Pid is the pid of the process that originally created the fun.

{index, Index}

Index (an integer) is an index into the module's fun table.

{new_index, Index}

Index (an integer) is an index into the module's fun table.

{new_uniq, Uniq}

Uniq (a binary) is a unique value for this fun.

{uniq, Uniq}

Uniq (an integer) is a unique value for this fun.

Types:

Returns information about Fun as specified by Item, in the form {Item,Info}.

For any fun, Item can be any of the atoms module, name, arity, or env.

For a local fun, Item can also be any of the atoms index, new_index, new_uniq, uniq, and pid. For an external fun, the value of any of these items is always the atom undefined.

See erlang:fun_info/1.

Types:

Returns a string which corresponds to the text representation of Fun.

Types:

Returns true if the module Module is loaded and contains an exported function Function/Arity; otherwise false.

Returns false for any BIF (functions implemented in C rather than in Erlang).

This function is retained mainly for backwards compatibility.

Forces an immediate garbage collection of the currently executing process. The function should not be used, unless it has been noticed -- or there are good reasons to suspect -- that the spontaneous garbage collection will occur too late or not at all. Improper use may seriously degrade system performance.

Compatibility note: In versions of OTP prior to R7, the garbage collection took place at the next context switch, not immediately. To force a context switch after a call to erlang:garbage_collect(), it was sufficient to make any function call.

Types:

Works like erlang:garbage_collect() but on any process. The same caveats apply. Returns false if Pid refers to a dead process; true otherwise.

Types:

Returns the process dictionary as a list of {Key, Val} tuples.

> put(key1, merry),
  put(key2, lambs),
  put(key3, {are, playing}),
  get().
[{key1,merry},{key2,lambs},{key3,{are,playing}}]
        

Types:

Returns the value Valassociated with Key in the process dictionary, or undefined if Key does not exist.

> put(key1, merry),
  put(key2, lambs),
  put({any, [valid, term]}, {are, playing}),
  get({any, [valid, term]}).
{are,playing}
        

Types:

Returns the magic cookie of the local node, if the node is alive; otherwise the atom nocookie.

Types:

Returns a list of keys which are associated with the value Val in the process dictionary.

> put(mary, {1, 2}),
  put(had, {1, 2}),
  put(a, {1, 2}),
  put(little, {1, 2}),
  put(dog, {1, 3}),
  put(lamb, {1, 2}),
  get_keys({1, 2}).
[mary,had,a,little,lamb]
        

Types:

Get the call stack backtrace (stacktrace) of the last exception in the calling process as a list of {Module,Function,Arity} tuples. The Arity field in the first tuple may be the argument list of that function call instead of an arity integer, depending on the exception.

If there has not been any exceptions in a process, the stacktrace is []. After a code change for the process, the stacktrace may also be reset to [].

The stacktrace is the same data as the catch operator returns, for example:

{'EXIT',{badarg,Stacktrace}} = catch abs(x)

See also erlang:error/1 and erlang:error/2.

Types:

Returns the pid of the group leader for the process which evaluates the function.

Every process is a member of some process group and all groups have a group leader. All IO from the group is channeled to the group leader. When a new process is spawned, it gets the same group leader as the spawning process. Initially, at system start-up, init is both its own group leader and the group leader of all processes.

Types:

Sets the group leader of Pid to GroupLeader. Typically, this is used when a processes started from a certain shell should have another group leader than init.

See also group_leader/0.

Halts the Erlang runtime system and indicates normal exit to the calling environment. Has no return value.

> halt().
os_prompt%
        

Types:

Status must be a non-negative integer, or a string. Halts the Erlang runtime system. Has no return value. If Status is an integer, it is returned as an exit status of Erlang to the calling environment. If Status is a string, produces an Erlang crash dump with String as slogan, and then exits with a non-zero status code.

Note that on many platforms, only the status codes 0-255 are supported by the operating system.

Returns a hash value for Term within the range 1..Range. The allowed range is 1..2^27-1.

This BIF is deprecated as the hash value may differ on different architectures. Also the hash values for integer terms larger than 2^27 as well as large binaries are very poor. The BIF is retained for backward compatibility reasons (it may have been used to hash records into a file), but all new code should use one of the BIFs erlang:phash/2 or erlang:phash2/1,2 instead.

Types:

Returns the head of List, that is, the first element.

> hd([1,2,3,4,5]).
1
        

Allowed in guard tests.

Failure: badarg if List is the empty list [].

Types:

Puts the calling process into a wait state where its memory allocation has been reduced as much as possible, which is useful if the process does not expect to receive any messages in the near future.

The process will be awaken when a message is sent to it, and control will resume in Module:Function with the arguments given by Args with the call stack emptied, meaning that the process will terminate when that function returns. Thus erlang:hibernate/3 will never return to its caller.

If the process has any message in its message queue, the process will be awaken immediately in the same way as described above.

In more technical terms, what erlang:hibernate/3 does is the following. It discards the call stack for the process. Then it garbage collects the process. After the garbage collection, all live data is in one continuous heap. The heap is then shrunken to the exact same size as the live data which it holds (even if that size is less than the minimum heap size for the process).

If the size of the live data in the process is less than the minimum heap size, the first garbage collection occurring after the process has been awaken will ensure that the heap size is changed to a size not smaller than the minimum heap size.

Note that emptying the call stack means that any surrounding catch is removed and has to be re-inserted after hibernation. One effect of this is that processes started using proc_lib (also indirectly, such as gen_server processes), should use proc_lib:hibernate/3 instead to ensure that the exception handler continues to work when the process wakes up.

This BIF is now equivalent to erlang:system_info/1.

Types:

Returns a string which corresponds to the text representation of Integer.

> integer_to_list(77).
"77"
        

Types:

Returns a string which corresponds to the text representation of Integer in base Base.

> erlang:integer_to_list(1023, 16).
"3FF"
        

Types:

Returns a binary which is made from the integers and binaries in IoListOrBinary.

> Bin1 = <<1,2,3>>.
<<1,2,3>>
> Bin2 = <<4,5>>.
<<4,5>>
> Bin3 = <<6>>.
<<6>>
> iolist_to_binary([Bin1,1,[2,3,Bin2],4|Bin3]).
<<1,2,3,1,2,3,4,5,4,6>>
        

Types:

Returns an integer which is the size in bytes of the binary that would be the result of iolist_to_binary(Item).

> iolist_size([1,2|<<3,4>>]).
4
        

Returns true if the local node is alive; that is, if the node can be part of a distributed system. Otherwise, it returns false.

Types:

Returns true if Term is an atom; otherwise returns false.

Allowed in guard tests.

Types:

Returns true if Term is a binary; otherwise returns false.

Allowed in guard tests.

Types:

Returns true if Term is either the atom true or the atom false (i.e. a boolean); otherwise returns false.

Allowed in guard tests.

Types:

Returns true if Module:Function/Arity is a BIF implemented in C; otherwise returns false. This BIF is useful for builders of cross reference tools.

Types:

Returns true if Term is a floating point number; otherwise returns false.

Allowed in guard tests.

Types:

Returns true if Term is a fun; otherwise returns false.

Allowed in guard tests.

Types:

Returns true if Term is a fun that can be applied with Arity number of arguments; otherwise returns false.

Allowed in guard tests.

Currently, is_function/2 will also return true if the first argument is a tuple fun (a tuple containing two atoms). In a future release, tuple funs will no longer be supported and is_function/2 will return false if given a tuple fun.

Types:

Returns true if Term is an integer; otherwise returns false.

Allowed in guard tests.

Types:

Returns true if Term is a list with zero or more elements; otherwise returns false.

Allowed in guard tests.

Types:

Returns true if Term is either an integer or a floating point number; otherwise returns false.

Allowed in guard tests.

Types:

Returns true if Term is a pid (process identifier); otherwise returns false.

Allowed in guard tests.

Types:

Returns true if Term is a port identifier; otherwise returns false.

Allowed in guard tests.

Types:

Pid must refer to a process at the local node. Returns true if the process exists and is alive, that is, has not exited. Otherwise, returns false.

Types:

Returns true if Term is a tuple and its first element is RecordTag. Otherwise, returns false.

Normally the compiler treats calls to is_record/2 specially. It emits code to verify that Term is a tuple, that its first element is RecordTag, and that the size is correct. However, if the RecordTag is not a literal atom, the is_record/2 BIF will be called instead and the size of the tuple will not be verified.

Allowed in guard tests, if RecordTag is a literal atom.

Types:

RecordTag must be an atom. Returns true if Term is a tuple, its first element is RecordTag, and its size is Size. Otherwise, returns false.

Allowed in guard tests, provided that RecordTag is a literal atom and Size is a literal integer.

This BIF is documented for completeness. In most cases is_record/2 should be used.

Types:

Returns true if Term is a reference; otherwise returns false.

Allowed in guard tests.

Types:

Returns true if Term is a tuple; otherwise returns false.

Allowed in guard tests.

Types:

Returns the length of List.

> length([1,2,3,4,5,6,7,8,9]).
9
        

Allowed in guard tests.

Types:

Creates a link between the calling process and another process (or port) Pid, if there is not such a link already. If a process attempts to create a link to itself, nothing is done. Returns true.

If Pid does not exist, the behavior of the BIF depends on if the calling process is trapping exits or not (see process_flag/2):

• If the calling process is not trapping exits, and checking Pid is cheap -- that is, if Pid is local -- link/1 fails with reason noproc.
• Otherwise, if the calling process is trapping exits, and/or Pid is remote, link/1 returns true, but an exit signal with reason noproc is sent to the calling process.

Types:

Returns the atom whose text representation is String.

> list_to_atom("Erlang").
'Erlang'
        

Types:

Returns a binary which is made from the integers and binaries in IoList.

> Bin1 = <<1,2,3>>.
<<1,2,3>>
> Bin2 = <<4,5>>.
<<4,5>>
> Bin3 = <<6>>.
<<6>>
> list_to_binary([Bin1,1,[2,3,Bin2],4|Bin3]).
<<1,2,3,1,2,3,4,5,4,6>>
        

Types:

Returns the atom whose text representation is String, but only if there already exists such atom.

Failure: badarg if there does not already exist an atom whose text representation is String.

Types:

Returns the float whose text representation is String.

> list_to_float("2.2017764e+0").
2.20178
        

Failure: badarg if String contains a bad representation of a float.

Types:

Returns an integer whose text representation is String.

> list_to_integer("123").
123
        

Failure: badarg if String contains a bad representation of an integer.

Types:

Returns an integer whose text representation in base Base is String.

> erlang:list_to_integer("3FF", 16).
1023
        

Failure: badarg if String contains a bad representation of an integer.

Types:

Returns a pid whose text representation is String.

This BIF is intended for debugging and for use in the Erlang operating system. It should not be used in application programs.

> list_to_pid("<0.4.1>").
<0.4.1>
        

Failure: badarg if String contains a bad representation of a pid.

Types:

Returns a tuple which corresponds to List. List can contain any Erlang terms.

> list_to_tuple([share, ['Ericsson_B', 163]]).
{share, ['Ericsson_B', 163]}
        

Types:

If Binary contains the object code for the module Module, this BIF loads that object code. Also, if the code for the module Module already exists, all export references are replaced so they point to the newly loaded code. The previously loaded code is kept in the system as old code, as there may still be processes which are executing that code. It returns either {module, Module}, or {error, Reason} if loading fails. Reason is one of the following:

badfile

The object code in Binary has an incorrect format.

not_purged

Binary contains a module which cannot be loaded because old code for this module already exists.

badfile

The object code contains code for another module than Module

This BIF is intended for the code server (see code(3)) and should not be used elsewhere.

Types:

Returns a list of all loaded Erlang modules (current and/or old code), including preloaded modules.

See also code(3).

Types:

Returns the current local date and time {{Year, Month, Day}, {Hour, Minute, Second}}.

The time zone and daylight saving time correction depend on the underlying OS.

> erlang:localtime().
{{1996,11,6},{14,45,17}}
        

Types:

Converts local date and time to Universal Time Coordinated (UTC), if this is supported by the underlying OS. Otherwise, no conversion is done and {Date1, Time1} is returned.

> erlang:localtime_to_universaltime({{1996,11,6},{14,45,17}}).
{{1996,11,6},{13,45,17}}
        

Failure: badarg if Date1 or Time1 do not denote a valid date or time.

Types:

Converts local date and time to Universal Time Coordinated (UTC) just like erlang:localtime_to_universaltime/1, but the caller decides if daylight saving time is active or not.

If IsDst == true the {Date1, Time1} is during daylight saving time, if IsDst == false it is not, and if IsDst == undefined the underlying OS may guess, which is the same as calling erlang:localtime_to_universaltime({Date1, Time1}).

> erlang:localtime_to_universaltime({{1996,11,6},{14,45,17}}, true).
{{1996,11,6},{12,45,17}}
> erlang:localtime_to_universaltime({{1996,11,6},{14,45,17}}, false).
{{1996,11,6},{13,45,17}}
> erlang:localtime_to_universaltime({{1996,11,6},{14,45,17}}, undefined).
{{1996,11,6},{13,45,17}}
        

Failure: badarg if Date1 or Time1 do not denote a valid date or time.

Returns an almost unique reference.

The returned reference will reoccur after approximately 2^82 calls; therefore it is unique enough for practical purposes.

> make_ref().
#Ref<0.0.0.135>
        

Types:

Returns a new tuple of the given Arity, where all elements are InitialValue.

> erlang:make_tuple(4, []).
{[],[],[],[]}
        

Types:

Computes an MD5 message digest from Data, where the length of the digest is 128 bits (16 bytes). Data is a binary or a list of small integers and binaries.

See The MD5 Message Digest Algorithm (RFC 1321) for more information about MD5.

Types:

Finishes the update of an MD5 Context and returns the computed MD5 message digest.

Types:

Creates an MD5 context, to be used in subsequent calls to md5_update/2.

Types:

Updates an MD5 Context with Data, and returns a NewContext.

Types:

Returns a list containing information about memory dynamically allocated by the Erlang emulator. Each element of the list is a tuple {Type, Size}. The first element Typeis an atom describing memory type. The second element Sizeis memory size in bytes. A description of each memory type follows:

total

The total amount of memory currently allocated, which is the same as the sum of memory size for processes and system.

processes

The total amount of memory currently allocated by the Erlang processes.

processes_used

The total amount of memory currently used by the Erlang processes.
This memory is part of the memory presented as processes memory.

system

The total amount of memory currently allocated by the emulator that is not directly related to any Erlang process.
Memory presented as processes is not included in this memory.

atom

The total amount of memory currently allocated for atoms.
This memory is part of the memory presented as system memory.

atom_used

The total amount of memory currently used for atoms.
This memory is part of the memory presented as atom memory.

binary

The total amount of memory currently allocated for binaries.
This memory is part of the memory presented as system memory.

code

The total amount of memory currently allocated for Erlang code.
This memory is part of the memory presented as system memory.

ets

The total amount of memory currently allocated for ets tables.
This memory is part of the memory presented as system memory.

maximum

The maximum total amount of memory allocated since the emulator was started.
This tuple is only present when the emulator is run with instrumentation.
For information on how to run the emulator with instrumentation see instrument(3) and/or erl(1).

The system value is not complete. Some allocated memory that should be part of the system value are not. For example, memory allocated by drivers is missing. When the emulator is run with instrumentation, the system value is more accurate, but memory directly allocated by malloc (and friends) are still not part of the system value. Direct calls to malloc are only done from OS specific runtime libraries and perhaps from user implemented Erlang drivers that do not use the memory allocation functions in the driver interface. Since the total value is the sum of processes and system the error in system will propagate to the total value.

The different values has the following relation to each other. Values beginning with an uppercase letter is not part of the result.

        total = processes + system
        processes = processes_used + ProcessesNotUsed
        system = atom + binary + code + ets + OtherSystem
        atom = atom_used + AtomNotUsed

        RealTotal = processes + RealSystem
        RealSystem = system + MissedSystem
        

The total value is supposed to be the total amount of memory dynamically allocated by the emulator. Shared libraries, the code of the emulator itself, and the emulator stack(s) are not supposed to be included. That is, the total value is not supposed to be equal to the total size of all pages mapped to the emulator. Furthermore, due to fragmentation and pre-reservation of memory areas, the size of the memory segments which contain the dynamically allocated memory blocks can be substantially larger than the total size of the dynamically allocated memory blocks.

More tuples in the returned list may be added in the future.

Types:

Returns the memory size in bytes allocated for memory of type Type. The argument can also be given as a list of Type atoms, in which case a corresponding list of {Type, Size} tuples is returned.

See erlang:memory/0.

Failure: badarg if the emulator is not run with instrumentation when Type == maximum.

Types:

Returns true if the module Module is loaded, otherwise returns false. It does not attempt to load the module.

This BIF is intended for the code server (see code(3)) and should not be used elsewhere.

Types:

The calling process starts monitoring Item which is an object of type Type.

Currently only processes can be monitored, i.e. the only allowed Type is process, but other types may be allowed in the future.

Item can be:

pid()

The pid of the process to monitor.

{RegName, Node}

A tuple consisting of a registered name of a process and a node name. The process residing on the node Node with the registered name RegName will be monitored.

RegName

The process locally registered as RegName will be monitored.

When a process is monitored by registered name, the process that has the registered name at the time when erlang:monitor/2 is called will be monitored. The monitor will not be effected, if the registered name is unregistered.

A 'DOWN' message will be sent to the monitoring process if Item dies, if Item does not exist, or if the connection is lost to the node which Item resides on. A 'DOWN' message has the following pattern:

{'DOWN', MonitorRef, Type, Object, Info}
        

where MonitorRef and Type are the same as described above, and:

Object

A reference to the monitored object:

• the pid of the monitored process, if Item was specified as a pid.
• {RegName, Node}, if Item was specifed as {RegName, Node}.
• {RegName, Node}, if Item was specified as RegName. Node will in this case be the name of the local node (node()).

Info

Either the exit reason of the process, noproc (non-existing process), or noconnection (no connection to Node).

If/when erlang:monitor/2 is extended (e.g. to handle other item types than process), other possible values for Object, and Info in the 'DOWN' message will be introduced.

The monitoring is turned off either when the 'DOWN' message is sent, or when erlang:demonitor/1 is called.

If an attempt is made to monitor a process on an older node (where remote process monitoring is not implemented or one where remote process monitoring by registered name is not implemented), the call fails with badarg.

Making several calls to erlang:monitor/2 for the same Item is not an error; it results in as many, completely independent, monitorings.

The format of the 'DOWN' message changed in the 5.2 version of the emulator (OTP release R9B) for monitor by registered name. The Object element of the 'DOWN' message could in earlier versions sometimes be the pid of the monitored process and sometimes be the registered name. Now the Object element is always a tuple consisting of the registered name and the node name. Processes on new nodes (emulator version 5.2 or greater) will always get 'DOWN' messages on the new format even if they are monitoring processes on old nodes. Processes on old nodes will always get 'DOWN' messages on the old format.

Types:

Monitors the status of the node Node. If Flag is true, monitoring is turned on; if Flag is false, monitoring is turned off.

Making several calls to monitor_node(Node, true) for the same Node is not an error; it results in as many, completely independent, monitorings.

If Node fails or does not exist, the message {nodedown, Node} is delivered to the process. If a process has made two calls to monitor_node(Node, true) and Node terminates, two nodedown messages are delivered to the process. If there is no connection to Node, there will be an attempt to create one. If this fails, a nodedown message is delivered.

Nodes connected through hidden connections can be monitored as any other node.

Failure: badargif the local node is not alive.

Types:

Returns the name of the local node. If the node is not alive, nonode@nohost is returned instead.

Allowed in guard tests.

Types:

Returns the node where Arg is located. Arg can be a pid, a reference, or a port. If the local node is not alive, nonode@nohost is returned.

Allowed in guard tests.

Types:

Returns a list of all visible nodes in the system, excluding the local node. Same as nodes(visible).

Types:

Returns a list of nodes according to argument given. The result returned when the argument is a list, is the list of nodes satisfying the disjunction(s) of the list elements.

Arg can be any of the following:

visible

Nodes connected to this node through normal connections.

hidden

Nodes connected to this node through hidden connections.

connected

All nodes connected to this node.

this

This node.

known

Nodes which are known to this node, i.e., connected, previously connected, etc.

Some equalities: [node()] = nodes(this), nodes(connected) = nodes([visible, hidden]), and nodes() = nodes(visible).

If the local node is not alive, nodes(this) == nodes(known) == [nonode@nohost], for any other Arg the empty list [] is returned.

Types:

Returns the tuple {MegaSecs, Secs, MicroSecs} which is the elapsed time since 00:00 GMT, January 1, 1970 (zero hour) on the assumption that the underlying OS supports this. Otherwise, some other point in time is chosen. It is also guaranteed that subsequent calls to this BIF returns continuously increasing values. Hence, the return value from now() can be used to generate unique time-stamps. It can only be used to check the local time of day if the time-zone info of the underlying operating system is properly configured.

Types:

Returns a port identifier as the result of opening a new Erlang port. A port can be seen as an external Erlang process. PortName is one of the following:

{spawn, Command}

Starts an external program. Command is the name of the external program which will be run. Command runs outside the Erlang work space unless an Erlang driver with the name Command is found. If found, that driver will be started. A driver runs in the Erlang workspace, which means that it is linked with the Erlang runtime system.
When starting external programs on Solaris, the system call vfork is used in preference to fork for performance reasons, although it has a history of being less robust. If there are problems with using vfork, setting the environment variable ERL_NO_VFORK to any value will cause fork to be used instead.

{fd, In, Out}

Allows an Erlang process to access any currently opened file descriptors used by Erlang. The file descriptor In can be used for standard input, and the file descriptor Out for standard output. It is only used for various servers in the Erlang operating system (shell and user). Hence, its use is very limited.

PortSettings is a list of settings for the port. Valid settings are:

{packet, N}

Messages are preceded by their length, sent in N bytes, with the most significant byte first. Valid values for N are 1, 2, or 4.

stream

Output messages are sent without packet lengths. A user-defined protocol must be used between the Erlang process and the external object.

{line, L}

Messages are delivered on a per line basis. Each line (delimited by the OS-dependent newline sequence) is delivered in one single message. The message data format is {Flag, Line}, where Flag is either eol or noeol and Line is the actual data delivered (without the newline sequence).
L specifies the maximum line length in bytes. Lines longer than this will be delivered in more than one message, with the Flag set to noeol for all but the last message. If end of file is encountered anywhere else than immediately following a newline sequence, the last line will also be delivered with the Flag set to noeol. In all other cases, lines are delivered with Flag set to eol.
The {packet, N} and {line, L} settings are mutually exclusive.

{cd, Dir}

This is only valid for {spawn, Command}. The external program starts using Dir as its working directory. Dir must be a string. Not available on VxWorks.

{env, Env}

This is only valid for {spawn, Command}. The environment of the started process is extended using the environment specifications in Env.
Env should be a list of tuples {Name, Val}, where Name is the name of an environment variable, and Val is the value it is to have in the spawned port process. Both Name and Val must be strings. The one exception is Val being the atom false (in analogy with os:getenv/1), which removes the environment variable. Not available on VxWorks.

exit_status

This is only valid for {spawn, Command} where Command refers to an external program.
When the external process connected to the port exits, a message of the form {Port,{exit_status,Status}} is sent to the connected process, where Status is the exit status of the external process. If the program aborts, on Unix the same convention is used as the shells do (i.e., 128+signal).
If the eof option has been given as well, the eof message and the exit_status message appear in an unspecified order.
If the port program closes its stdout without exiting, the exit_status option will not work.

use_stdio

This is only valid for {spawn, Command}. It allows the standard input and output (file descriptors 0 and 1) of the spawned (UNIX) process for communication with Erlang.

nouse_stdio

The opposite of use_stdio. Uses file descriptors 3 and 4 for communication with Erlang.

stderr_to_stdout

Affects ports to external programs. The executed program gets its standard error file redirected to its standard output file. stderr_to_stdout and nouse_stdio are mutually exclusive.

in

The port can only be used for input.

out

The port can only be used for output.

binary

All IO from the port are binary data objects as opposed to lists of bytes.

eof

The port will not be closed at the end of the file and produce an exit signal. Instead, it will remain open and a {Port, eof} message will be sent to the process holding the port.

The default is stream for all types of port and use_stdio for spawned ports.

Failure: If the port cannot be opened, the exit reason is the Posix error code which most closely describes the error, or einval if no Posix code is appropriate. The following Posix error codes may appear:

enomem

There was not enough memory to create the port.

eagain

There are no more available operating system processes.

enametoolong

The external command given was too long.

emfile

There are no more available file descriptors.

enfile

A file or port table is full.

During use of a port opened using {spawn, Name}, errors arising when sending messages to it are reported to the owning process using signals of the form {'EXIT', Port, PosixCode}. See file(3) for possible values of PosixCode.

The maximum number of ports that can be open at the same time is 1024 by default, but can be configured by the environment variable ERL_MAX_PORTS.

Types:

Portable hash function that will give the same hash for the same Erlang term regardless of machine architecture and ERTS version (the BIF was introduced in ERTS 4.9.1.1). Range can be between 1 and 2^32, the function returns a hash value for Term within the range 1..Range.

This BIF could be used instead of the old deprecated erlang:hash/2 BIF, as it calculates better hashes for all datatypes, but consider using phash2/1,2 instead.

Types:

Portable hash function that will give the same hash for the same Erlang term regardless of machine architecture and ERTS version (the BIF was introduced in ERTS 5.2). Range can be between 1 and 2^32, the function returns a hash value for Term within the range 0..Range-1. When called without the Range argument, a value in the range 0..2^27-1 is returned.

This BIF should always be used for hashing terms. It distributes small integers better than phash/2, and it is faster for bignums and binaries.

Note that the range 0..Range-1 is different from the range of phash/2 (1..Range).

Types:

Returns a string which corresponds to the text representation of Pid.

This BIF is intended for debugging and for use in the Erlang operating system. It should not be used in application programs.

Types:

Closes an open port. Roughly the same as Port ! {self(), close} except for the error behaviour (see below), and that the port does not reply with {Port, closed}. Any process may close a port with port_close/1, not only the port owner (the connected process).

For comparison: Port ! {self(), close} fails with badarg if Port cannot be sent to (i.e., Port refers neither to a port nor to a process). If Port is a closed port nothing happens. If Port is an open port and the calling process is the port owner, the port replies with {Port, closed} when all buffers have been flushed and the port really closes, but if the calling process is not the port owner the port owner fails with badsig.

Note that any process can close a port using Port ! {PortOwner, close} just as if it itself was the port owner, but the reply always goes to the port owner.

In short: port_close(Port) has a cleaner and more logical behaviour than Port ! {self(), close}.

Failure: badarg if Port is not an open port or the registered name of an open port.

Types:

Sends data to a port. Same as Port ! {self(), {command, Data}} except for the error behaviour (see below). Any process may send data to a port with port_command/2, not only the port owner (the connected process).

For comparison: Port ! {self(), {command, Data}} fails with badarg if Port cannot be sent to (i.e., Port refers neither to a port nor to a process). If Port is a closed port the data message disappears without a sound. If Port is open and the calling process is not the port owner, the port owner fails with badsig. The port owner fails with badsig also if Data is not a valid IO list.

Note that any process can send to a port using Port ! {PortOwner, {command, Data}} just as if it itself was the port owner.

In short: port_command(Port, Data) has a cleaner and more logical behaviour than Port ! {self(), {command, Data}}.

Failure: badarg if Port is not an open port or the registered name of an open port.

Types:

Sets the port owner (the connected port) to Pid. Roughly the same as Port ! {self(), {connect, Pid}} except for the following:

• The error behavior differs, see below.
  
• The port does not reply with {Port,connected}.
  
• The new port owner gets linked to the port.
  

The old port owner stays linked to the port and have to call unlink(Port) if this is not desired. Any process may set the port owner to be any process with port_connect/2.

For comparison: Port ! {self(), {connect, Pid}} fails with badarg if Port cannot be sent to (i.e., Port refers neither to a port nor to a process). If Port is a closed port nothing happens. If Port is an open port and the calling process is the port owner, the port replies with {Port, connected} to the old port owner. Note that the old port owner is still linked to the port, and that the new is not. If Port is an open port and the calling process is not the port owner, the port owner fails with badsig. The port owner fails with badsig also if Pid is not an existing local pid.

Note that any process can set the port owner using Port ! {PortOwner, {connect, Pid}} just as if it itself was the port owner, but the reply always goes to the port owner.

In short: port_connect(Port, Pid) has a cleaner and more logical behaviour than Port ! {self(),{connect,Pid}}.

Failure: badarg if Port is not an open port or the registered name of an open port, or if Pid is not an existing local pid.

Types:

Performs a synchronous control operation on a port. The meaning of Operation and Data depends on the port, i.e., on the port driver. Not all port drivers support this control feature.

Returns: a list of integers in the range 0 through 255, or a binary, depending on the port driver. The meaning of the returned data also depends on the port driver.

Failure: badarg if Port is not an open port or the registered name of an open port, if Operation cannot fit in a 32-bit integer, if the port driver does not support synchronous control operations, or if the port driver so decides for any reason (probably something wrong with Operation or Data).

Types:

Performs a synchronous call to a port. The meaning of Operation and Data depends on the port, i.e., on the port driver. Not all port drivers support this feature.

Port is a port identifier, referring to a driver.

Operation is an integer, which is passed on to the driver.

Data is any Erlang term. This data is converted to binary term format and sent to the port.

Returns: a term from the driver. The meaning of the returned data also depends on the port driver.

Failure: badarg if Port is not an open port or the registered name of an open port, if Operation cannot fit in a 32-bit integer, if the port driver does not support synchronous control operations, or if the port driver so decides for any reason (probably something wrong with Operation or Data).

Types:

Returns a list containing tuples with information about the Port, or undefined if the port is not open. The order of the tuples is not defined, nor are all the tuples mandatory.

{registered_name, RegName}

RegName (an atom) is the registered name of the port. If the port has no registered name, this tuple is not present in the list.

{id, Index}

Index (an integer) is the internal index of the port. This index may be used to separate ports.

{connected, Pid}

Pid is the process connected to the port.

{links, Pids}

Pids is a list of pids to which processes the port is linked.

{name, String}

String is the command name set by open_port.

{input, Bytes}

Bytes is the total number of bytes read from the port.

{output, Bytes}

Bytes is the total number of bytes written to the port.

Failure: badarg if Port is not a local port.

Types:

Returns information about Port as specified by Item, or undefined if the port is not open. Also, if Item == registered_name and the port has no registered name, [] is returned.

For valid values of Item, and corresponding values of Info, see erlang:port_info/1.

Failure: badarg if Port is not a local port.

Types:

Returns a string which corresponds to the text representation of the port identifier Port.

This BIF is intended for debugging and for use in the Erlang operating system. It should not be used in application programs.

Returns a list of all ports on the local node.

Types:

Returns a list of Erlang modules which are pre-loaded in the system. As all loading of code is done through the file system, the file system must have been loaded previously. Hence, at least the module init must be pre-loaded.

Types:

Writes information about the local process Pid on standard error. The currently allowed value for the atom Type is backtrace, which shows the contents of the call stack, including information about the call chain, with the most recent data printed last. The format of the output is not further defined.

Types:

Sets certain flags for the process which calls this function. Returns the old value of the flag.

process_flag(trap_exit, Boolean)

When trap_exit is set to true, exit signals arriving to a process are converted to {'EXIT', From, Reason} messages, which can be received as ordinary messages. If trap_exit is set to false, the process exits if it receives an exit signal other than normal and the exit signal is propagated to its linked processes. Application processes should normally not trap exits.
See also exit/2.

process_flag(error_handler, Module)

This is used by a process to redefine the error handler for undefined function calls and undefined registered processes. Inexperienced users should not use this flag since code autoloading is dependent on the correct operation of the error handling module.

process_flag(min_heap_size, MinHeapSize)

This changes the minimum heap size for the calling process.

process_flag(priority, Level)

This sets the process priority. Level is an atom. All implementations support three priority levels, low, normal, and high. The default is normal.

process_flag(save_calls, N)

N must be an integer in the interval 0..10000. If N > 0, call saving is made active for the process, which means that information about the N most recent global function calls, BIF calls, sends and receives made by the process are saved in a list, which can be retrieved with process_info(Pid, last_calls). A global function call is one in which the module of the function is explicitly mentioned. Only a fixed amount of information is saved: a tuple {Module, Function, Arity} for function calls, and the mere atoms send, 'receive' and timeout for sends and receives ('receive' when a message is received and timeout when a receive times out). If N = 0, call saving is disabled for the process, which is the default. Whenever the size of the call saving list is set, its contents are reset.

Types:

Sets certain flags for the process Pid, in the same manner as process_flag/2. Returns the old value of the flag. The allowed values for Flag are only a subset of those allowed in process_flag/2, namely: save_calls.

Failure: badarg if Pid is not a local process.

Types:

Returns a list containing tuples with information about the process Pid, or undefined if the process is not alive. The order of the tuples is not defined, nor are all the tuples mandatory.

This BIF is intended for debugging only.

{current_function, {Module, Function, Args}}

Module, Function, Args is the current function call of the process.

{dictionary, Dictionary}

Dictionary is the dictionary of the process.

{error_handler, Module}

Module is the error handler module used by the process (for undefined function calls, for example).

{group_leader, GroupLeader}

GroupLeader is group leader for the IO of the process.

{heap_size, Size}

Size is the heap size of the process in words.

{initial_call, {Module, Function, Arity}}

Module, Function, Arity is the initial function call with which the process was spawned.

{links, Pids}

Pids is a list of pids, with processes to which the process has a link.

{message_queue_len, MessageQueueLen}

MessageQueueLen is the number of messages currently in the message queue of the process. This is the length of the list MessageQueue returned as the info item messages (see below).

{messages, MessageQueue}

MessageQueue is a list of the messages to the process, which have not yet been processed.

{priority, Level}

Level is the current priority level for the process. Only low and normal are always supported.

{reductions, Number}

Number is the number of reductions executed by the process.

{registered_name, Atom}

Atom is the registered name of the process. If the process has no registered name, this tuple is not present in the list.

{stack_size, Size}

Size is the stack size of the process in words.

{status, Status}

Status is the status of the process. Status is waiting (waiting for a message), running, runnable (ready to run, but another process is running), or suspended (suspended on a "busy" port or by the erlang:suspend_process/1 BIF).

{trap_exit, Boolean}

Boolean is true if the process is trapping exits, otherwise it is false.

Failure: badarg if Pid is not a local process.

Types:

Returns information about the process Pid as specified by Item, or undefined if the process is not alive. Also, if Item == registered_name and the process has no registered name, [] is returned.

The value of Item, and corresponding value of Info, can be any of the values specified for process_info/1.

In addition to the above, also the following items -- with corresponding values -- are allowed:

{backtrace, Bin}

The binary Bin contains the same information as the output from erlang:process_display(Pid, backtrace). Use binary_to_list/1 to obtain the string of characters from the binary.

{last_calls, false|Calls}

The value is false if call saving is not active for the process (see process_flag/3). If call saving is active, a list is returned, in which the last element is the most recent called.

{memory, Size}

Size is the size of the process in bytes. This includes call stack, heap and internal structures.

{monitored_by, Pids}

A list of pids that are monitoring the process (with erlang:monitor/2).

{monitors, Monitors}

A list of monitors (started by erlang:monitor/2) that are active for the process. For a local process monitor or a remote process monitor by pid, the list item is {process, Pid}, and for a remote process monitor by name, the list item is {process, {RegName, Node}}.

Note however, that not all implementations support every one of the above Items.

Failure: badarg if Pid is not a local process.

Returns a list of all processes on the local node.

> processes().
[<0.0.0>,
 <0.2.0>,
 <0.4.0>,
 <0.5.0>,
 <0.7.0>,
 <0.8.0>]
        

Types:

Removes old code for Module. Before this BIF is used, erlang:check_process_code/2 should be called to check that no processes are executing old code in the module.

This BIF is intended for the code server (see code(3)) and should not be used elsewhere.

Failure: badarg if there is no old code for Module.

Types:

Adds a new Key to the process dictionary, associated with the value Val, and returns undefined. If Key already exists, the old value is deleted and replaced by Val and the function returns the old value.

The values stored when put is evaluated within the scope of a catch will not be retracted if a throw is evaluated, or if an error occurs.

> X = put(name, walrus), Y = put(name, carpenter),
  Z = get(name),
  {X, Y, Z}.
{undefined,walrus,carpenter}
        

Types:

Stops the execution of the calling process with an exception of given class, reason and call stack backtrace (stacktrace).

This BIF is intended for debugging and for use in the Erlang operating system. In general, it should be avoided in applications, unless you know very well what you are doing.

Class is one of error, exit or throw, so if it were not for the stacktrace erlang:raise(Class, Reason, Stacktrace) is equivalent to erlang:Class(Reason). Reason is any term and Stacktrace is a list as returned from get_stacktrace(), that is a list of 3-tuples {Module, Function, Arity | Args} where Module and Function are atoms and the third element is an integer arity or an argument list. The stacktrace may also contain {Fun, Args} tuples where Fun is a local fun and Args is an argument list.

The stacktrace is used as the exception stacktrace for the calling process; it will be truncated to the current maximum stacktrace depth.

Because evaluating this function causes the process to terminate, it has no return value - unless the arguments are invalid, in which case the function returns the error reason, that is badarg. If you want to be really sure not to return you can call erlang:error(erlang:raise(Class, Reason, Stacktrace)) and hope to distinguish exceptions later.

Types:

TimerRef is a timer reference returned by erlang:send_after/3 or erlang:start_timer/3. If the timer is active, the function returns the time in milliseconds left until the timer will expire, otherwise false (which means that TimerRef was never a timer, that it has been cancelled, or that it has already delivered its message).

See also erlang:send_after/3, erlang:start_timer/3, and erlang:cancel_timer/1.

Types:

Returns a string which corresponds to the text representation of Ref.

This BIF is intended for debugging and for use in the Erlang operating system. It should not be used in application programs.

Types:

Associates the name RegName with a pid or a port identifier. RegName, which must be an atom, can be used instead of the pid / port identifier in the send operator (RegName ! Message).

> register(db, Pid).
true
        

Failure: badarg if Pid is not an existing, local process or port, if RegName is already in use, if the process or port is already registered (already has a name), or if RegName is the atom undefined.

Types:

Returns a list of names which have been registered using register/2.

> registered().
[code_server, file_server, init, user, my_db]
        

Types:

Resume a suspended process Pid.

This BIF is intended for debugging only.

Failure: badarg if Pid does not exist.

Types:

Returns an integer by rounding Number.

> round(5.5).
6
        

Allowed in guard tests.

Returns the pid (process identifier) of the calling process.

> self().
<0.26.0>
        

Allowed in guard tests.

Types:

Sends a message and returns Msg. This is the same as Dest ! Msg.

Dest may be a remote or local pid, a (local) port, a locally registered name, or a tuple {RegName, Node} for a registered name at another node.

Types:

Sends a message and returns ok, or does not send the message but returns something else (see below). Otherwise the same as erlang:send/2. See also erlang:send_nosuspend/2,3. for more detailed explanation and warnings.

The possible options are:

nosuspend

If the sender would have to be suspended to do the send, nosuspend is returned instead.

noconnect

If the destination node would have to be autoconnected before doing the send, noconnect is returned instead.

As with erlang:send_nosuspend/2,3: Use with extreme care!

Types:

Starts a timer which will send the message Msg to Dest after Time milliseconds.

If Dest is an atom, it is supposed to be the name of a registered process. The process referred to by the name is looked up at the time of delivery. No error is given if the name does not refer to a process.

If Dest is a pid, the timer will be automatically canceled if the process referred to by the pid is not alive, or when the process exits. This feature was introduced in erts version 5.4.11. Note that timers will not be automatically canceled when Dest is an atom.

See also erlang:start_timer/3, erlang:cancel_timer/1, and erlang:read_timer/1.

Failure: badarg if the arguments does not satisfy the requirements specified above.

Types:

The same as erlang:send(Dest, Msg, [nosuspend]), but returns true if the message was sent and false if the message was not sent because the sender would have had to be suspended.

This function is intended for send operations towards an unreliable remote node without ever blocking the sending (Erlang) process. If the connection to the remote node (usually not a real Erlang node, but a node written in C or Java) is overloaded, this function will not send the message but return false instead.

The same happens, if Dest refers to a local port that is busy. For all other destinations (allowed for the ordinary send operator '!') this function sends the message and returns true.

This function is only to be used in very rare circumstances where a process communicates with Erlang nodes that can disappear without any trace causing the TCP buffers and the drivers que to be overfull before the node will actually be shut down (due to tick timeouts) by net_kernel. The normal reaction to take when this happens is some kind of premature shutdown of the other node.

Note that ignoring the return value from this function would result in unreliable message passing, which is contradictory to the Erlang programming model. The message is not sent if this function returns false.

Note also that in many systems, transient states of overloaded queues are normal. The fact that this function returns false does not in any way mean that the other node is guaranteed to be nonrespoinsive, it could be a temporary overload. Also a return value of true does only mean that the message could be sent on the (TCP) channel without blocking, the message is not guaranteed to have arrived at the remote node. Also in the case of a disconnected nonresponsive node, the return value is true (mimics the behaviour of the ! operator). The expected behaviour as well as the actions to take when the function returns false are application and hardware specific.

Use with extreme care!

Types:

The same as erlang:send(Dest, Msg, [nosuspend | Options]), but with boolean return value.

This function behaves like erlang:send_nosuspend/2), but takes a third parameter, a list of options. The only currently implemented option is noconnect. The option noconnect makes the function return false if the remote node is not currently reachable by the local node. The normal behaviour is to try to connect to the node, which may stall the process for a shorter period. The use of the noconnect option makes it possible to be absolutely sure not to get even the slightest delay when sending to a remote process. This is especially useful when communicating with nodes who expect to always be the connecting part (i.e. nodes written in C or Java).

Whenever the function returns false (either when a suspend would occur or when noconnect was specified and the node was not already connected), the message is guaranteed not to have been sent.

Use with extreme care!

Types:

Sets the magic cookie of Node to the atom Cookie. If Node is the local node, the function also sets the cookie of all other unknown nodes to Cookie (see Distributed Erlang in the Erlang Reference Manual).

Failure: function_clause if the local node is not alive.

Types:

Returns a tuple which is a copy of the argument Tuple1 with the element given by the integer argument Index (the first element is the element with index 1) replaced by the argument Value.

> setelement(2, {10, green, bottles}, red).
{10, red, bottles}
        

Types:

Returns an integer which is the size of the argument Item, which must be either a tuple or a binary.

> size({morni, mulle, bwange}).
3
        

Allowed in guard tests.

Types:

Returns the pid of a new process started by the application of Fun to the empty list []. Otherwise works like spawn/3.

Types:

Returns the pid of a new process started by the application of Fun to the empty list [] on Node. If Node does not exist, a useless pid is returned. Otherwise works like spawn/3.

Types:

Returns the pid of a new process started by the application of Module:Function to Args. The new process created will be placed in the system scheduler queue and be run some time later.

error_handler:undefined_function(Module, Function, Args) is evaluated by the new process if Module:Function/Arity does not exist (where Arity is the length of Args). The error handler can be redefined (see process_flag/2). If error_handler is undefined, or the user has redefined the default error_handler its replacement is undefined, a failure with the reason undef will occur.

> spawn(speed, regulator, [high_speed, thin_cut]).
<0.13.1>
        

Types:

Returns the pid of a new process started by the application of Module:Function to Args on Node. If Node does not exists, a useless pid is returned. Otherwise works like spawn/3.

Types:

Returns the pid of a new process started by the application of Fun to the empty list []. A link is created between the calling process and and the new process, atomically. Otherwise works like spawn/3.

Types:

Returns the pid of a new process started by the application of Fun to the empty list [] on Node. A link is created between the calling process and and the new process, atomically. If Node does not exist, a useless pid is returned (and due to the link, an exit signal with exit reason noconnection will be received). Otherwise works like spawn/3.

Types:

Returns the pid of a new process started by the application of Module:Function to Args. A link is created betwen the calling process and the new process, atomically. Otherwise works like spawn/3.

Types:

Returns the pid of a new process started by the application of Module:Function to Args on Node. A link is created betwen the calling process and the new process, atomically. If Node does not exist, a useless pid is returned (and due to the link, an exit signal with exit reason noconnection will be received). Otherwise works like spawn/3.

Types:

Returns the pid of a new process started by the application of Fun to the empty list []. Otherwise works like spawn_opt/4.

Types:

Returns the pid of a new process started by the application of Fun to the empty list [] on Node. If Node does not exist, a useless pid is returned. Otherwise works like spawn_opt/4.

Types:

Works exactly like spawn/3, except that an extra option list is given when creating the process.

This BIF is only useful for performance tuning. Random tweaking of the parameters without measuring execution times and memory consumption may actually make things worse. Furthermore, most of the options are inherently implementation-dependent, and they can be changed or removed in future versions of OTP.

link

Sets a link to the parent process (like spawn_link/3 does).

{priority, Level}

Sets the priority of the new process. Equivalent to executing process_flag(priority, Level) in the start function of the new process, except that the priority will be set before the process is scheduled in the first time.

{fullsweep_after, Number}

The Erlang runtime system uses a generational garbage collection scheme, using an "old heap" for data that has survived at least one garbage collection. When there is no more room on the old heap, a fullsweep garbage collection will be done.
The fullsweep_after option makes it possible to specify the maximum number of generational collections before forcing a fullsweep even if there is still room on the old heap. Setting the number to zero effectively disables the general collection algorithm, meaning that all live data is copied at every garbage collection.
Here are a few cases when it could be useful to change fullsweep_after. Firstly, if binaries that are no longer used should be thrown away as soon as possible. (Set Number to zero.) Secondly, a process that mostly have short-lived data will be fullsweeped seldom or never, meaning that the old heap will contain mostly garbage. To ensure a fullsweep once in a while, set Number to a suitable value such as 10 or 20. Thirdly, in embedded systems with limited amount of RAM and no virtual memory, one might want to preserve memory by setting Number to zero. (The value may be set globally, see erlang:system_flag/2.)

{min_heap_size, Size}

Gives a minimum heap size in words. Setting this value higher than the system default might speed up some processes because less garbage collection is done. Setting too high value, however, might waste memory and slow down the system due to worse data locality. Therefore, it is recommended to use this option only for fine-tuning an application and to measure the execution time with various Size values.

Types:

Returns the pid of a new process started by the application of Module:Function to Args on Node. If Node does not exist, a useless pid is returned. Otherwise works like spawn_opt/4.

Types:

Returns a tuple containing the binaries which are the result of splitting Bin into two parts at position Pos. This is not a destructive operation. After the operation, there will be three binaries altogether.

> B = list_to_binary("0123456789").
<<48,49,50,51,52,53,54,55,56,57>>
> size(B).
10
> {B1, B2} = split_binary(B,3).
{<<48,49,50>>,<<51,52,53,54,55,56,57>>}
> size(B1).
3
> size(B2).
7
        

Types:

Starts a timer which will send the message {timeout, TimerRef, Msg} to Dest after Time milliseconds.

If Dest is an atom, it is supposed to be the name of a registered process. The process referred to by the name is looked up at the time of delivery. No error is given if the name does not refer to a process.

If Dest is a pid, the timer will be automatically canceled if the process referred to by the pid is not alive, or when the process exits. This feature was introduced in erts version 5.4.11. Note that timers will not be automatically canceled when Dest is an atom.

See also erlang:send_after/3, erlang:cancel_timer/1, and erlang:read_timer/1.

Failure: badarg if the arguments does not satisfy the requirements specified above.

Types:

Returns information about the system as specified by Type:

context_switches

Returns {ContextSwitches, 0}, where ContextSwitches is the total number of context switches since the system started.

exact_reductions

Returns {Total_Exact_Reductions, Exact_Reductions_Since_Last_Call}.
NOTE: statistics(exact_reductions) is a more expensive operation than statistics(reductions) especially on an Erlang machine with smp support.

garbage_collection

Returns {Number_of_GCs, Words_Reclaimed, 0}. This information may not be valid for all implementations.

io

Returns {{input, Input}, {output, Output}}, where Input is the total number of bytes received through ports, and Output is the total number of bytes output to ports.

reductions

Returns {Total_Reductions, Reductions_Since_Last_Call}.
NOTE: From erts version 5.5 (OTP release R11B) this value does not include reductions performed in current time slices of currently scheduled processes. If an exact value is wanted, use statistics(exact_reductions).

run_queue

Returns the length of the run queue, that is, the number of processes that are ready to run.

runtime

Returns {Total_Run_Time, Time_Since_Last_Call}.

wall_clock

Returns {Total_Wallclock_Time, Wallclock_Time_Since_Last_Call}. wall_clock can be used in the same manner as runtime, except that real time is measured as opposed to runtime or CPU time.

All times are in milliseconds.

> statistics(runtime).
{1690,1620}
> statistics(reductions).
{2046,11}
> statistics(garbage_collection).
{85,23961,0}
        

Types:

Suspends the process Pid.

This BIF is intended for debugging only.

Failure: badarg if Pid does not exist.

Types:

Sets various system properties of the Erlang node. Returns the old value of the flag.

erlang:system_flag(backtrace_depth, Depth)

Sets the maximum depth of call stack backtraces in the exit reason element of 'EXIT' tuples.

erlang:system_flag(fullsweep_after, Number)

Number is a non-negative integer which indicates how many times generational garbages collections can be done without forcing a fullsweep collection. The value applies to new processes; processes already running are not affected.
In low-memory systems (especially without virtual memory), setting the value to 0 can help to conserve memory.
An alternative way to set this value is through the (operating system) environment variable ERL_FULLSWEEP_AFTER.

erlang:system_flag(min_heap_size, MinHeapSize)

Sets the default minimum heap size for processes. The size is given in words. The new min_heap_size only effects processes spawned after the change of min_heap_size has been made. The min_heap_size can be set for individual processes by use of spawn_opt/N or process_flag/2.

erlang:system_flag(schedulers, NO)

Sets the number of scheduler threads to used when the runtime system has been compiled with smp support. Valid range of NO is 1-1024. Returns {OpRes, NoOfSchedulers, OldNoOfSchedulers} where OpRes = atom() (ok or a posix error), NoOfSchedulers = integer() (current number of scheduler threads), and OldNoOfSchedulers = integer() (previous number of scheduler threads).
NOTE! This feature might be removed without prior notice.
NOTE! This is a quite expensive operation. Do not use it for mutual exclusion.

erlang:system_flag(trace_control_word, TCW)

Sets the value of the node's trace control word to TCW. TCW should be an unsigned integer. For more information see documentation of the set_tcw function in the match specification documentation in the ERTS User's Guide.

Types:

Returns various information about the current system (emulator) as specified by Type:

allocated_areas

Returns a list of tuples with information about miscellaneous allocated memory areas.
Each tuple contains an atom describing type of memory as first element and amount of allocated memory in bytes as second element. In those cases when there is information present about allocated and used memory, a third element is present. This third element contains the amount of used memory in bytes.
erlang:system_info(allocated_areas) is intended for debugging, and the content is highly implementation dependent. The content of the results will therefore change when needed without prior notice.
Note: The sum of these values is not the total amount of memory allocated by the emulator. Some values are part of other values, and some memory areas are not part of the result. If you are interested in the total amount of memory allocated by the emulator see erlang:memory/0,1.

allocator

Returns {Allocator, Version, Features, Settings}.
Types:

• Allocator = undefined | elib_malloc | glibc
• Version = [int()]
• Features = [atom()]
• Settings = [{Subsystem, [{Parameter, Value}]}]
• Subsystem = atom()
• Parameter = atom()
• Value = term()

Explanation:

• Allocator corresponds to the malloc() implementation used. If Allocator equals undefined, the malloc() implementation used could not be identified. Currently elib_malloc and glibc can be identified.
  
• Version is a list of integers (but not a string) representing the version of the malloc() implementation used.
  
• Features is a list of atoms representing allocation features used.
  
• Settings is a list of subsystems, their configurable parameters, and used values. Settings may differ between different combinations of platforms, allocators, and allocation features. Memory sizes are given in bytes.
  

See also "System Flags Effecting erts_alloc" in erts_alloc(3).

{allocator, Alloc}

Returns information about the specified allocator. If Alloc is not a recognized allocator, undefined is returned. If Alloc is disabled, false is returned.
Note: The information returned is highly implementation dependent and may be changed, or removed at any time without prior notice. It was initially intended as a tool when developing new allocators, but since it might be of interest for others it has been briefly documented.
The recognized allocators are listed in erts_alloc(3), and after reading this also the returned information should more or less speak for itself. But it can be worth explaining some things. Call counts are presented by two values. The first value is giga calls, and the second value is calls. mbcs, and sbcs are abbreviations for, respectively, multiblock carriers, and singleblock carriers. Sizes are presented in bytes. When it is not a size that is presented, it is the amount of something. Sizes and amounts are often presented by three values, the first is current value, the second is maximum value since the last call to erlang:system_info({allocator, Alloc}), and the third is maximum value since the emulator was started. If only one value is present, it is the current value. fix_alloc memory block types are presented by two values. The first value is memory pool size and the second value used memory size.

compat_rel

Returns the compatibility mode of the local node as an integer. The integer returned represents the Erlang/OTP release which the current emulator has been set to be backward compatible with. The compatibility mode can be configured at startup by using the command line flag +R, see erl(1).

creation

Returns the creation of the local node as an integer. The creation is changed when a node is restarted. The creation of a node is stored in process identifiers, port identifiers, and references. This makes it (to some extent) possible to distinguish between identifiers from different incarnations of a node. Currently valid creations are integers in the range 1..3, but this may (probably will) change in the future. If the node is not alive, 0 is returned.

dist

Returns a binary containing a string of distribution information formatted as in Erlang crash dumps. For more information see the "How to interpret the Erlang crash dumps" chapter in the ERTS User's Guide.

dist_ctrl

Returns a list of tuples {Node, ControllingEntity}, one entry for each connected remote node. The Node is the name of the node and the ControllingEntity is the port or pid responsible for the communication to that node. More specifically, the ControllingEntity for nodes connected via TCP/IP (the normal case) is the socket actually used in communication with the specific node.

elib_malloc

If the emulator uses the elib_malloc memory allocator, a list of two-element tuples containing status information is returned; otherwise, false is returned. The list currently contains the following two-element tuples (all sizes are presented in bytes):

{heap_size, Size}

Where Size is the current heap size.

{max_alloced_size, Size}

Where Size is the maximum amount of memory allocated on the heap since the emulator started.

{alloced_size, Size}

Where Size is the current amount of memory allocated on the heap.

{free_size, Size}

Where Size is the current amount of free memory on the heap.

{no_alloced_blocks, No}

Where No is the current number of allocated blocks on the heap.

{no_free_blocks, No}

Where No is the current number of free blocks on the heap.

{smallest_alloced_block, Size}

Where Size is the size of the smallest allocated block on the heap.

{largest_free_block, Size}

Where Size is the size of the largest free block on the heap.

fullsweep_after

Returns {fullsweep_after, int()} which is the fullsweep_after garbage collection setting used by default. For more information see garbage_collection described below.

garbage_collection

Returns a list describing the default garbage collection settings. A process spawned on the local node by a spawn or spawn_link will use these garbage collection settings. The default settings can be changed by use of system_flag/2. spawn_opt/4 can spawn a process that does not use the default settings.

global_heaps_size

Returns the current size of the shared (global) heap.

heap_sizes

Returns a list of integers representing valid heap sizes in words. All Erlang heaps are sized from sizes in this list.

heap_type

Returns the heap type used by the current emulator. Currently the following heap types exist:

private

Each process has a heap reserved for its use and no references between heaps of different processes are allowed. Messages passed between processes are copied between heaps.

shared

One heap for use by all processes. Messages passed between processes are passed by reference.

hybrid

A hybrid of the private and shared heap types. A shared heap as well as private heaps are used.

info

Returns a binary containing a string of miscellaneous system information formatted as in Erlang crash dumps. For more information see the "How to interpret the Erlang crash dumps" chapter in the ERTS User's Guide.

loaded

Returns a binary containing a string of loaded module information formatted as in Erlang crash dumps. For more information see the "How to interpret the Erlang crash dumps" chapter in the ERTS User's Guide.

machine

Returns a string containing the Erlang machine name.

process_count

Returns the number of processes currently existing at the local node as an integer. The same value as length(processes()) returns.

process_limit

Returns the maximum number of concurrently existing processes at the local node as an integer. This limit can be configured at startup by using the command line flag +P, see erl(1).

procs

Returns a binary containing a string of process and port information formatted as in Erlang crash dumps. For more information see the "How to interpret the Erlang crash dumps" chapter in the ERTS User's Guide.

scheduler_id

Returns the scheduler id (a positive integer) of current scheduler thread.

schedulers

Returns the number of scheduler threads.

smp_support

Returns true if the emulator has been compiled with smp support; otherwise, false is returned.

system_version

Returns a string containing the emulator type and version as well as some important properties such as the size of the thread pool, etc.

system_architecture

Returns a string containing the processor and OS architecture the emulator is built for.

threads

Returns true if the emulator has been compiled with thread support; otherwise, false is returned.

thread_pool_size

Returns the number of threads used for driver calls as an integer.

trace_control_word

Returns the value of the node's trace control word. For more information see documentation of the function get_tcw in "Match Specifications in Erlang", ERTS User's Guide.

version

Returns a string containing the version number of the emulator.

wordsize

Returns the word size in bytes as an integer, i.e. on a 32-bit architecture 4 is returned, and on a 64-bit architecture 8 is returned.

The scheduler argument has changed name to scheduler_id. This in order to avoid mixup with the schedulers argument. The scheduler argument was introduced in ERTS version 5.5 and renamed in ERTS version 5.5.1.

Types:

Returns the current system monitoring settings set by erlang:system_monitor/2 as {MonitorPid, Options}, or undefined if there are no settings. The order of the options may be different from the one that was set.

Types:

When called with the argument undefined, all system performance monitoring settings are cleared.

Calling the function with {MonitorPid, Options} as argument, is the same as calling erlang:system_monitor(MonitorPid, Options).

Returns the previous system monitor settings just like erlang:system_monitor/0.

Types:

Sets system performance monitoring options. MonitorPid is a local pid that will receive system monitor messages, and the second argument is a list of monitoring options:

{long_gc, Time}

If a garbage collection in the system takes at least Time wallclock milliseconds, a message {monitor, GcPid, long_gc, Info} is sent to MonitorPid. GcPid is the pid that was garbage collected and Info is a list of two-element tuples describing the result of the garbage collection. One of the tuples is {timeout, GcTime} where GcTime is the actual time for the garbage collection in milliseconds. The other are the tuples tagged with heap_size, stack_size, mbuf_size and heap_block_size from the gc_start trace message (see erlang:trace/3).

{large_heap, Size}

If a garbage collection in the system results in the allocated size of a heap being at least Size words, a message {monitor, GcPid, large_heap, Info} is sent to MonitorPid. GcPid and Info are the same as for long_gc above, except that the tuple tagged with timeout is not present.

busy_port

If a process in the system gets suspended because it sends to a busy port, a message {monitor, SusPid, busy_port, Port} is sent to MonitorPid. SusPid is the pid that got suspended when sending to Port.

busy_dist_port

If a process in the system gets suspended because it sends to a process on a remote node whose inter-node communication was handled by a busy port, a message {monitor, SusPid, busy_dist_port, Port} is sent to MonitorPid. SusPid is the pid that got suspended when sending through the inter-node communication port Port.

Returns the previous system monitor settings just like erlang:system_monitor/0.

If a monitoring process gets so large that it itself starts to cause system monitor messages when garbage collecting, the messages will enlargen the process's message queue and probably make the problem worse. Keep the monitoring process neat and do not set the system monitor limits too tight.

Failure: badarg if MonitorPid does not exist.

Types:

Returns a binary data object which is the result of encoding Term according to the Erlang external term format.

This can be used for a variety of purposes, for example writing a term to a file in an efficient way, or sending an Erlang term to some type of communications channel not supported by distributed Erlang.

See also binary_to_term/1.

Types:

Returns a binary data object which is the result of encoding Term according to the Erlang external term format.

If the option compressed is provided, the external term format will be compressed. The compressed format is automatically recognized by binary_to_term/1 in R7.

See also binary_to_term/1.

Types:

A non-local return from a function. If evaluated within a catch, catch will return the value Any.

> catch throw({hello, there}).
{hello, there}
        

Failure: nocatch if not evaluated within a catch.

Types:

Returns the current time as {Hour, Minute, Second}.

The time zone and daylight saving time correction depend on the underlying OS.

> time().
{9,42,44}
        

Types:

Returns the tail of List1, that is, the list minus the first element.

> tl([geesties, guilies, beasties]).
[guilies, beasties]
        

Allowed in guard tests.

Failure: badarg if List is the empty list [].

Types:

Turns on (if How == true) or off (if How == false) the trace flags in FlagList for the process or processes represented by PidSpec.

PidSpec is either a pid for a local process, or one of the following atoms:

existing

All processes currently existing.

new

All processes that will be created in the future.

all

All currently existing processes and all processes that will be created in the future.

FlagList can contain any number of the following flags (the "message tags" refers to the list of messages following below):

all

Set all trace flags except {tracer, Tracer} and cpu_timestamp that are in their nature different than the others.

send

Trace sending of messages.
Message tags: send, send_to_non_existing_process.

'receive'

Trace receiving of messages.
Message tags: 'receive'.

procs

Trace process related events.
Message tags: spawn, exit, register, unregister, link, unlink, getting_linked, getting_unlinked.

call

Trace certain function calls. Specify which function calls to trace by calling erlang:trace_pattern/3.
Message tags: call, return_from.

silent

Used in conjunction with the call trace flag. The call, return_from and return_to trace messages are inhibited if this flag is set, but if there are match specs they are executed as normal.
Silent mode is inhibited by executing erlang:trace(_, false, [silent|_]), or by a match spec executing the {silent, false} function.
The silent trace flag facilitates setting up a trace on many or even all processes in the system. Then the interesting trace can be activated and deactivated using the {silent,Bool} match spec function, giving a high degree of control of which functions with which arguments that triggers the trace.
Message tags: call, return_from, return_to. Or rather, the absence of.

return_to

Used in conjunction with the call trace flag. Trace the actual return from a traced function back to its caller. Only works for functions traced with the local option to erlang:trace_pattern/3.
The semantics is that a trace message is sent when a call traced function actually returns, that is, when a chain of tail recursive calls is ended. There will be only one trace message sent per chain of tail recursive calls, why the properties of tail recursiveness for function calls are kept while tracing with this flag. Using call and return_to trace together makes it possible to know exactly in which function a process executes at any time.
To get trace messages containing return values from functions, use the {return_trace} match_spec action instead.
Message tags: return_to.

running

Trace scheduling of processes.
Message tags: in, out.

garbage_collection

Trace garbage collections of processes.
Message tags: gc_start, gc_end.

timestamp

Include a time stamp in all trace messages. The time stamp (Ts) is of the same form as returned by erlang:now().

cpu_timestamp

A global trace flag for the Erlang node that makes all trace timestamps be in CPU time, not wallclock. It is only allowed with PidSpec==all. If the host machine operating system does not support high resolution CPU time measurements, trace/3 exits with badarg.

arity

Used in conjunction with the call trace flag. {M, F, Arity} will be specified instead of {M, F, Args} in call trace messages.

set_on_spawn

Makes any process created by a traced process inherit its trace flags, including the set_on_spawn flag.

set_on_first_spawn

Makes the first process created by a traced process inherit its trace flags, excluding the set_on_first_spawn flag.

set_on_link

Makes any process linked by a traced process inherit its trace flags, including the set_on_link flag.

set_on_first_link

Makes the first process linked to by a traced process inherit its trace flags, excluding the set_on_first_link flag.

{tracer, Tracer}

Specify where to send the trace messages. Tracer must be the pid of a local process or the port identifier of a local port. If this flag is not given, trace messages will be sent to the process that called erlang:trace/3.

The effect of combining set_on_first_link with set_on_link is the same as having set_on_first_link alone. Likewise for set_on_spawn and set_on_first_spawn.

If the timestamp flag is not given, the tracing process will receive the trace messages described below. Pid is the pid of the traced process in which the traced event has occurred. The third element of the tuple is the message tag.

If the timestamp flag is given, the first element of the tuple will be trace_ts instead and the timestamp is added last in the tuple.

{trace, Pid, 'receive', Msg}

When Pid receives the message Msg.

{trace, Pid, send, Msg, To}

When Pid sends the message Msg to the process To.

{trace, Pid, send_to_non_existing_process, Msg, To}

When Pid sends the message Msg to the non-existing process To.

{trace, Pid, call, {M, F, Args}}

When Pid calls a traced function. The return values of calls are never supplied, only the call and its arguments.
Note that the trace flag arity can be used to change the contents of this message, so that Arity is specified instead of Args.

{trace, Pid, return_to, {M, F, Args}}

When Pid returns to the specified function. This trace message is sent if both the call and the return_to flags are set, and the function is set to be traced on local function calls. The message is only sent when returning from a chain of tail recursive function calls where at least one call generated a call trace message (that is, the functions match specification matched and {message, false} was not an action).

{trace, Pid, return_from, {M, F, Args}, ReturnValue}

When Pid returns from the specified function. This trace message is sent if the call flag is set, and the function has a match specification with a return_trace or exception_trace action.

{trace, Pid, exception_from, {M, F, Args}, {Class, Value}}

When Pid exits from the specified function due to an exception. This trace message is sent if the call flag is set, and the function has a match specification with an exception_trace action.

{trace, Pid, spawn, Pid2, {M, F, Args}}

When Pid spawns a new process Pid2 with the specified function call as entry point.
Note that Args is supposed to be the argument list, but may be any term in the case of an erroneous spawn.

{trace, Pid, exit, Reason}

When Pid exits with reason Reason.

{trace, Pid, link, Pid2}

When Pid links to a process Pid2.

{trace, Pid, unlink, Pid2}

When Pid removes the link from a process Pid2.

{trace, Pid, getting_linked, Pid2}

When Pid gets linked to a process Pid2.

{trace, Pid, getting_unlinked, Pid2}

When Pid gets unlinked from a process Pid2.

{trace, Pid, register, RegName}

When Pid gets the name RegName registered.

{trace, Pid, unregister, RegName}

When Pid gets the name RegName unregistered. Note that this is done automatically when a registered process exits.

{trace, Pid, in, {M, F, Arity} | 0}

When Pid is scheduled to run. The process will run in function {M, F, Arity}. On some rare occasions the current function cannot be determined, then the last element Arity is 0.

{trace, Pid, out, {M, F, Arity} | 0}

When Pid is scheduled out. The process was running in function {M, F, Arity}. On some rare occasions the current function cannot be determined, then the last element Arity is 0.

{trace, Pid, gc_start, Info}

Sent when garbage collection is about to be started. Info is a list of two-element tuples, where the first element is a key, and the second is the value. You should not depend on the tuples have any defined order. Currently, the following keys are defined.

heap_size

The size of the used part of the heap.

old_heap_size

The size of the used part of the old heap.

stack_size

The actual size of the stack.

recent_size

The size of the data that survived the previous garbage collection.

mbuf_size

The combined size of message buffers associated with the process.

All sizes are in words.

{trace, Pid, gc_end, Info}

Sent when garbage collection is finished. Info contains the same kind of list as in the gc_start message, but the sizes reflect the new sizes after garbage collection.

If the tracing process dies, the flags will be silently removed.

Only one process can trace a particular process. For this reason, attempts to trace an already traced process will fail.

Returns: A number indicating the number of processes that matched PidSpec. If PidSpec is a pid, the return value will be 1. If PidSpec is all or existing the return value will be the number of processes running, excluding tracer processes. If PidSpec is new, the return value will be 0.

Failure: If specified arguments are not supported. For example cpu_timestamp is not supported on all platforms.

Types:

Returns trace information about a process or function.

To get information about a process, PidOrFunc should be a pid or the atom new. The atom new means that the default trace state for processes to be created will be returned. Item must have one of the following values:

flags

Return a list of atoms indicating what kind of traces is enabled for the process. The list will be empty if no traces are enabled, and one or more of the followings atoms if traces are enabled: send, 'receive', set_on_spawn, call, return_to, procs, set_on_first_spawn, set_on_link, running, garbage_collection, timestamp, and arity. The order is arbitrary.

tracer

Return the identifier for process or port tracing this process. If this process is not being traced, the return value will be [].

To get information about a function, PidOrFunc should be a three-element tuple: {Module, Function, Arity} or the atom on_load. No wildcards are allowed. Returns undefined if the function does not exist or false if the function is not traced at all. Item must have one of the following values:

traced

Return global if this function is traced on global function calls, local if this function is traced on local function calls (i.e local and global function calls), and false if neither local nor global function calls are traced.

match_spec

Return the match specification for this function, if it has one. If the function is locally or globally traced but has no match specification defined, the returned value is [].

meta

Return the meta trace tracer process or port for this function, if it has one. If the function is not meta traced the returned value is false, and if the function is meta traced but has once detected that the tracer proc is invalid, the returned value is [].

meta_match_spec

Return the meta trace match specification for this function, if it has one. If the function is meta traced but has no match specification defined, the returned value is [].

call_count

Return the call count value for this function or true for the pseudo function on_load if call count tracing is active. Return false otherwise. See also erlang:trace_pattern/3 .

all

Return a list contaning the {Item, Value} tuples for all other items, or return false if no tracing is active for this function.

The actual return value will be {Item, Value}, where Value is the requested information as described above. If a pid for a dead process was given, or the name of a non-existing function, Value will be undefined.

If PidOrFunc is the on_load, the information returned refers to the default value for code that will be loaded.

The same as erlang:trace_pattern(MFA, MatchSpec, []), retained for backward compatibility.

Types:

This BIF is used to enable or disable call tracing for exported functions. It must be combined with erlang:trace/3 to set the call trace flag for one or more processes.

Conceptually, call tracing works like this: Inside the Erlang virtual machine there is a set of processes to be traced and a set of functions to be traced. Tracing will be enabled on the intersection of the set. That is, if a process included in the traced process set calls a function included in the traced function set, the trace action will be taken. Otherwise, nothing will happen.

Use erlang:trace/3 to add or remove one or more processes to the set of traced processes. Use erlang:trace_pattern/2 to add or remove exported functions to the set of traced functions.

The erlang:trace_pattern/3 BIF can also add match specifications to an exported function. A match specification comprises a pattern that the arguments to the function must match, a guard expression which must evaluate to true and an action to be performed. The default action is to send a trace message. If the pattern does not match or the guard fails, the action will not be executed.

The MFA argument should be a tuple like {Module, Function, Arity} or the atom on_load (described below). It can be the module, function, and arity for an exported function (or a BIF in any module). The '_' atom can be used to mean any of that kind. Wildcards can be used in any of the following ways:

{Module,Function,'_'}

All exported functions of any arity named Function in module Module.

{Module,'_','_'}

All exported functions in module Module.

{'_','_','_'}

All exported functions in all loaded modules.

Other combinations, such as {Module,'_',Arity}, are not allowed. Local functions will match wildcards only if the local option is in the FlagList.

If the MFA argument is the atom on_load, the match specification and flag list will be used on all modules that are newly loaded.

The MatchSpec argument can take any of the following forms:

false

Disable tracing for the matching function(s). Any match specification will be removed.

true

Enable tracing for the matching function(s).

MatchSpecList

A list of match specifications. An empty list is equivalent to true. See the ERTS User's Guide for a description of match specifications.

restart

For the FlagList option call_count: restart the existing counters. The behaviour is undefined for other FlagList options.

pause

For the FlagList option call_count: pause the existing counters. The behaviour is undefined for other FlagList options.

The FlagList parameter is a list of options. The following options are allowed:

global

Turn on or off call tracing for global function calls (that is, calls specifying the module explicitly). Only exported functions will match and only global calls will generate trace messages. This is the default.

local

Turn on or off call tracing for all types of function calls. Trace messages will be sent whenever any of the specified functions are called, regardless of how they are called. If the return_to flag is set for the process, a return_to message will also be sent when this function returns to its caller.

meta | {meta, Pid}

Turn on or off meta tracing for all types of function calls. Trace messages will be sent to the tracer process or port Pid whenever any of the specified functions are called, regardless of how they are called. If no Pid is specified, self() is used as a default tracer process.
Meta tracing traces all processes and does not care about the process trace flags set by trace/3, the trace flags are instead fixed to [call, timestamp].
The match spec function {return_trace} works with meta trace and send its trace message to the same tracer process.

call_count

Starts (MatchSpec == true) or stops (MatchSpec == false) call count tracing for all types of function calls. For every function a counter is incremented when the function is called, in any process. No process trace flags need to be activated.
If call count tracing is started while already running, the count is restarted from zero. Running counters can be paused with MatchSpec == pause. Paused and running counters can be restarted from zero with MatchSpec == restart.
The counter value can be read with erlang:trace_info/2 .

The global and local options are mutually exclusive and global is the default (if no options are specified). The call_count and meta options perform a kind of local tracing, and can also not be combined with global. A function can be either globally or locally traced. If global tracing is specified for a specified set of functions; local, meta and call count tracing for the matching set of local functions will be disabled, and vice versa.

When disabling trace, the option must match the type of trace that is set on the function, so that local tracing must be disabled with the local option and global tracing with the global option (or no option at all), and so forth.

There is no way to directly change part of a match specification list. If a function has a match specification, you can replace it with a completely new one. If you need to change an existing match specification, use the erlang:trace_info/2 BIF to retrieve the existing match specification.

Returns the number of exported functions that matched the MFA argument. This will be zero if none matched at all.

Types:

Returns an integer by the truncating Number.

> trunc(5.5).
5
        

Allowed in guard tests.

Types:

Returns a list which corresponds to Tuple. Tuple may contain any Erlang terms.

> tuple_to_list({share, {'Ericsson_B', 163}}).
[share, {'Ericsson_B', 163}]
        

Types:

Returns the current date and time according to Universal Time Coordinated (UTC), also called GMT, in the form {{Year, Month, Day}, {Hour, Minute, Second}} if supported by the underlying operating system. If not, erlang:universaltime() is equivalent to erlang:localtime().

> erlang:universaltime().
{{1996,11,6},{14,18,43}}
        

Types:

Converts Universal Time Coordinated (UTC) date and time to local date and time, if this is supported by the underlying OS. Otherwise, no conversion is done, and {Date1, Time1} is returned.

> erlang:universaltime_to_localtime({{1996,11,6},{14,18,43}}).
{{1996,11,7},{15,18,43}}
        

Failure: badarg if Date1 or Time1 do not denote a valid date or time.

Types:

Removes the link, if there is one, between the calling process and the process or port referred to by Id.

Returns true and does not fail, even if there is no link to Id, or if Id does not exist.

Once unlink(Id) has returned it is guaranteed that the link between the caller and the entity referred to by Id has no effect on the caller in the future (unless the link is setup again). If caller is trapping exits, an {'EXIT', Id, _} message due to the link might have been placed in the callers message queue prior to the call, though. Note, the {'EXIT', Id, _} message can be the result of the link, but can also be the result of Id calling exit/2. Therefore, it may be appropriate to cleanup the message queue when trapping exits after the call to unlink(Id), as follow:

    unlink(Id),
    receive
        {'EXIT', Id, _} ->
            true
    after 0 ->
            true
    end

        

Prior to OTP release R11B (erts version 5.5) unlink/1 behaved completely asynchronous, i.e., the link was active until the "unlink signal" reached the linked entity. This had one undesirable effect, though. You could never know when you were guaranteed not to be effected by the link. Current behavior can be viewed as two combined operations: asynchronously send an "unlink signal" to the linked entity and ignore any future results of the link.

Types:

Removes the registered name RegName, associated with a pid or a port identifier.

> unregister(db).
true
        

Users are advised not to unregister system processes.

Failure: badarg if RegName is not a registered name.

Returns the pid or port identifier with the registered name RegName. Returns undefined if the name is not registered.

> whereis(db).
<0.43.0>
        

Voluntarily let other processes (if any) get a chance to execute. Using erlang:yield() is similar to receive after 1 -> ok end, except that yield() is faster.

Joe Armstrong - support@erlang.ericsson.se
Mike Williams - support@erlang.ericsson.se
Robert Virding - support@erlang.ericsson.se
Claes Wikström - support@erlang.ericsson.se
Rickard Green - support@erlang.ericsson.se