erl_driver

As of erts version 5.5.3 the driver interface has been extended (see extended marker). The extended interface introduce version management, the possibility to pass capability flags (see driver flags) to the runtime system at driver initialization, and some new driver API functions.

The driver calls back to the emulator, using the API functions declared in erl_driver.h. They are used for outputting data from the driver, using timers, etc.

A driver is a library with a set of function that the emulator calls, in response to Erlang functions and message sending. There may be multiple instances of a driver, each instance is connected to an Erlang port. Every port has a port owner process. Communication with the port is normally done through the port owner process.

Most of the functions take the port handle as an argument. This identifies the driver instance. Note that this port handle must be stored by the driver, it is not given when the driver is called from the emulator (see driver_entry).

Some of the functions take a parameter of type ErlDrvBinary, a driver binary. It should be both allocated and freed by the caller. Using a binary directly avoids one extra copying of data.

Many of the output functions have a "header buffer", with hbuf and hlen parameters. This buffer is sent as a list before the binary (or list, depending on port mode) that is sent. This is convenient when matching on messages received from the port. (Although in the latest versions of Erlang, there is the binary syntax, that enables you to match on the beginning of a binary.)

In the runtime system with SMP support, drivers are locked either on driver level or port level (driver instance level). By default driver level locking will be used, i.e., only one emulator thread will execute code in the driver at a time. If port level locking is used, multiple emulator threads may execute code in the driver at the same time. There will only be one thread at a time calling driver call-backs corresponding to the same port, though. In order to enable port level locking set the ERL_DRV_FLAG_USE_PORT_LOCKING driver flag in the driver_entry used by the driver. When port level locking is used it is the responsibility of the driver writer to synchronize all accesses to data shared by the ports (driver instances).

Most drivers written before the runtime system with SMP support existed will be able to run in the runtime system with SMP support without being rewritten if driver level locking is used.

Previously, in the runtime system without SMP support, specific driver call-backs were always called from the same thread. This is not the case in the runtime system with SMP support. Regardless of locking scheme used, calls to driver call-backs may be made from different threads, e.g., two consecutive calls to exactly the same call-back for exactly the same port may be made from two different threads. This will for most drivers not be a problem, but it might. Drivers that depend on all call-backs being called in the same thread, have to be rewritten before being used in the runtime system with SMP support.

Most functions in this API are not thread-safe, i.e., they may not be called from an arbitrary thread. Functions that are not documented as thread-safe may only be called from driver call-backs or function calls descending from a driver call-back call. Note that driver call-backs may be called from different threads. This, however, is not a problem for any function in this API, since the emulator has control over these threads.

All functions that a driver needs to do with Erlang are performed through driver API functions. There are functions for the following functionality:

Timer functions

Timer functions are used to control the timer that a driver may use. The timer will have the emulator call the timeout entry function after a specified time. Only one timer is available for each driver instance.

Queue handling

Every driver instance has an associated queue. This queue is a SysIOVec that works as a buffer. It's mostly used for the driver to buffer data that should be written to a device, it is a byte stream. If the port owner process closes the driver, and the queue is not empty, the driver will not be closed. This enables the driver to flush its buffers before closing.

The queue can be manipulated from arbitrary threads if a port data lock is used. See documentation of the ErlDrvPDL type for more information.

Output functions

With the output functions, the driver sends data back to the emulator. They will be received as messages by the port owner process, see open_port/2. The vector function and the function taking a driver binary are faster, because they avoid copying the data buffer. There is also a fast way of sending terms from the driver, without going through the binary term format.

Failure

The driver can exit and signal errors up to Erlang. This is only for severe errors, when the driver can't possibly keep open.

Asynchronous calls

The latest Erlang versions (R7B and later) has provision for asynchronous function calls, using a thread pool provided by Erlang. There is also a select call, that can be used for asynchronous drivers.

Multi-threading

A POSIX thread like API for multi-threading is provided. The Erlang driver thread API only provide a subset of the functionality provided by the POSIX thread API. The subset provided is more or less the basic functionality needed for multi-threaded programming:

• Threads
• Mutexes
• Condition variables
• Read/Write locks
• Thread specific data

The Erlang driver thread API can be used in conjunction with the POSIX thread API on UN-ices and with the Windows native thread API on Windows. The Erlang driver thread API has the advantage of being portable, but there might exist situations where you want to use functionality from the POSIX thread API or the Windows native thread API.

The Erlang driver thread API only returns error codes when it is reasonable to recover from an error condition. If it isn't reasonable to recover from an error condition, the whole runtime system is terminated. For example, if a create mutex operation fails, an error code is returned, but if a lock operation on a mutex fails, the whole runtime system is terminated.

Note that there exists no "condition variable wait with timeout" in the Erlang driver thread API. This is due to issues with pthread_cond_timedwait(). When the system clock suddenly is changed, it isn't always guaranteed that you will wake up from the call as expected. An Erlang runtime system has to be able to cope with sudden changes of the system clock. Therefore, we have omitted it from the Erlang driver thread API. In the Erlang driver case, timeouts can and should be handled with the timer functionality of the Erlang driver API.

In order for the Erlang driver thread API to function, thread support has to be enabled in the runtime system. An Erlang driver can check if thread support is enabled by use of driver_system_info(). Note that some functions in the Erlang driver API are thread-safe only when the runtime system has SMP support, also this information can be retrieved via driver_system_info(). Also note that a lot of functions in the Erlang driver API are not thread-safe regardless of whether SMP support is enabled or not. If a function isn't documented as thread-safe it is not thread-safe.

NOTE: When executing in an emulator thread, it is very important that you unlock all locks you have locked before letting the thread out of your control; otherwise, you are very likely to deadlock the whole emulator. If you need to use thread specific data in an emulator thread, only have the thread specific data set while the thread is under your control, and clear the thread specific data before you let the thread out of your control.

In the future there will probably be debug functionality integrated with the Erlang driver thread API. All functions that create entities take a name argument. Currently the name argument is unused, but it will be used when the debug functionality has been implemented. If you name all entities created well, the debug functionality will be able to give you better error reports.

Adding / removing drivers

A driver can add and later remove drivers.

Monitoring processes

A driver can monitor a process that does not own a port.

Version management

Version management is enabled for drivers that have set the extended_marker field of their driver_entry to ERL_DRV_EXTENDED_MARKER. erl_driver.h defines ERL_DRV_EXTENDED_MARKER, ERL_DRV_EXTENDED_MAJOR_VERSION, and ERL_DRV_EXTENDED_MINOR_VERSION. ERL_DRV_EXTENDED_MAJOR_VERSION will be incremented when driver incompatible changes are made to the Erlang runtime system. Normally it will suffice to recompile drivers when the ERL_DRV_EXTENDED_MAJOR_VERSION has changed, but it could, under rare circumstances, mean that drivers have to be slightly modified. If so, this will of course be documented. ERL_DRV_EXTENDED_MINOR_VERSION will be incremented when new features are added. The runtime system uses the minor version of the driver to determine what features to use. The runtime system will refuse to load a driver if the major versions differ, or if the major versions are equal and the minor version used by the driver is greater than the one used by the runtime system.

The emulator will refuse to load a driver that does not use the extended driver interface since, to allow for 64-bit capable drivers, incompatible type changes for the callbacks output, control and call were introduced in release R15B. A driver written with the old types would compile with warnings and when called return garbage sizes to the emulator causing it to read random memory and create huge incorrect result blobs.

Therefore it is not enough to just recompile drivers written with version management for pre-R15B types; the types have to be changed in the driver suggesting other rewrites especially regarding size variables. Investigate all warnings when recompiling!

Also, the API driver functions driver_output*, driver_vec_to_buf, driver_alloc/realloc* and the driver_* queue functions were changed to have larger length arguments and return values. This is a lesser problem since code that passes smaller types will get them auto converted in the calls and as long as the driver does not handle sizes that overflow an int all will work as before.

For erts-5.9 two new integer types ErlDrvSizeT and ErlDrvSSizeT were introduced that can hold 64-bit sizes if necessary.

To not update a driver and just recompile it probably works when building for a 32-bit machine creating a false sense of security. Hopefully that will generate many important warnings. But when recompiling the same driver later on for a 64-bit machine there will be warnings and almost certainly crashes. So it is a BAD idea to postpone updating the driver and not fixing the warnings!

When recompiling with gcc use the -Wstrict-prototypes flag to get better warnings. Try to find a similar flag if you are using some other compiler.

Here follows a checklist for rewriting a pre erts-5.9 driver, most important first.

Return types for driver callbacks

Rewrite driver callback control to use return type ErlDrvSSizeT instead of int.

Rewrite driver callback call to use return type ErlDrvSSizeT instead of int.

Note

These changes are essential to not crash the emulator or worse cause malfunction. Without them a driver may return garbage in the high 32 bits to the emulator causing it to build a huge result from random bytes either crashing on memory allocation or succeeding with a random result from the driver call.

Arguments to driver callbacks

Driver callback output now gets ErlDrvSizeT as 3rd argument instead of previously int.

Driver callback control now gets ErlDrvSizeT as 4th and 6th arguments instead of previously int.

Driver callback call now gets ErlDrvSizeT as 4th and 6th arguments instead of previously int.

Sane compiler's calling conventions probably make these changes necessary only for a driver to handle data chunks that require 64-bit size fields (mostly larger than 2 GB since that is what an int of 32 bits can hold). But it is possible to think of non-sane calling conventions that would make the driver callbacks mix up the arguments causing malfunction.

Note

The argument type change is from signed to unsigned which may cause problems for e.g. loop termination conditions or error conditions if you just change the types all over the place.

Larger size field in ErlIOVec

The size field in ErlIOVec has been changed to ErlDrvSizeT from int. Check all code that use that field.

Automatic type casting probably makes these changes necessary only for a driver that encounters sizes larger than 32 bits.

Note

The size field changed from signed to unsigned which may cause problems for e.g. loop termination conditions or error conditions if you just change the types all over the place.

Arguments and return values in the driver API

Many driver API functions have changed argument type and/or return value to ErlDrvSizeT from mostly int. Automatic type casting probably makes these changes necessary only for a driver that encounters sizes larger than 32 bits.

driver_output

3rd argument

driver_output2

3rd and 5th arguments

driver_output_binary

3rd 5th and 6th arguments

driver_outputv

3rd and 5th arguments

driver_vec_to_buf

3rd argument and return value

driver_alloc

1st argument

driver_realloc

2nd argument

driver_alloc_binary

1st argument

driver_realloc_binary

2nd argument

driver_enq

3rd argument

driver_pushq

3rd argument

driver_deq

2nd argument and return value

driver_sizeq

return value

driver_enq_bin

3rd and 4th argument

driver_pushq_bin

3rd and 4th argument

driver_enqv

3rd argument

driver_pushqv

3rd argument

driver_peekqv

return value

Note

This is a change from signed to unsigned which may cause problems for e.g. loop termination conditions and error conditions if you just change the types all over the place.

ErlDrvSizeT

An unsigned integer type to be used as size_t

ErlDrvSSizeT

A signed integer type the size of ErlDrvSizeT

ErlDrvSysInfo

typedef struct ErlDrvSysInfo {
   int driver_major_version;
   int driver_minor_version;
   char *erts_version;
   char *otp_release;
   int thread_support;
   int smp_support;
   int async_threads;
   int scheduler_threads;
   int nif_major_version;
   int nif_minor_version;
} ErlDrvSysInfo;
      

The ErlDrvSysInfo structure is used for storage of information about the Erlang runtime system. driver_system_info() will write the system information when passed a reference to a ErlDrvSysInfo structure. A description of the fields in the structure follows:

driver_major_version

The value of ERL_DRV_EXTENDED_MAJOR_VERSION when the runtime system was compiled. This value is the same as the value of ERL_DRV_EXTENDED_MAJOR_VERSION used when compiling the driver; otherwise, the runtime system would have refused to load the driver.

driver_minor_version

The value of ERL_DRV_EXTENDED_MINOR_VERSION when the runtime system was compiled. This value might differ from the value of ERL_DRV_EXTENDED_MINOR_VERSION used when compiling the driver.

erts_version

A string containing the version number of the runtime system (the same as returned by erlang:system_info(version)).

otp_release

A string containing the OTP release number (the same as returned by erlang:system_info(otp_release)).

thread_support

A value != 0 if the runtime system has thread support; otherwise, 0.

smp_support

A value != 0 if the runtime system has SMP support; otherwise, 0.

async_threads

The number of async threads in the async thread pool used by driver_async() (the same as returned by erlang:system_info(thread_pool_size)).

scheduler_threads

The number of scheduler threads used by the runtime system (the same as returned by erlang:system_info(schedulers)).

nif_major_version

The value of ERL_NIF_MAJOR_VERSION when the runtime system was compiled.

nif_minor_version

The value of ERL_NIF_MINOR_VERSION when the runtime system was compiled.

ErlDrvBinary

typedef struct ErlDrvBinary {
   ErlDrvSint orig_size;
   char orig_bytes[];
} ErlDrvBinary;

The ErlDrvBinary structure is a binary, as sent between the emulator and the driver. All binaries are reference counted; when driver_binary_free is called, the reference count is decremented, when it reaches zero, the binary is deallocated. The orig_size is the size of the binary, and orig_bytes is the buffer. The ErlDrvBinary does not have a fixed size, its size is orig_size + 2 * sizeof(int).

Note

The refc field has been removed. The reference count of an ErlDrvBinary is now stored elsewhere. The reference count of an ErlDrvBinary can be accessed via driver_binary_get_refc(), driver_binary_inc_refc(), and driver_binary_dec_refc().

Some driver calls, such as driver_enq_binary, increment the driver reference count, and others, such as driver_deq decrement it.

Using a driver binary instead of a normal buffer, is often faster, since the emulator doesn't need to copy the data, only the pointer is used.

A driver binary allocated in the driver, with driver_alloc_binary, should be freed in the driver (unless otherwise stated), with driver_free_binary. (Note that this doesn't necessarily deallocate it, if the driver is still referred in the emulator, the ref-count will not go to zero.)

Driver binaries are used in the driver_output2 and driver_outputv calls, and in the queue. Also the driver call-back outputv uses driver binaries.

If the driver for some reason or another, wants to keep a driver binary around, in a static variable for instance, the reference count should be incremented, and the binary can later be freed in the stop call-back, with driver_free_binary.

Note that since a driver binary is shared by the driver and the emulator, a binary received from the emulator or sent to the emulator, must not be changed by the driver.

Since erts version 5.5 (OTP release R11B), orig_bytes is guaranteed to be properly aligned for storage of an array of doubles (usually 8-byte aligned).

ErlDrvData

The ErlDrvData is a handle to driver-specific data, passed to the driver call-backs. It is a pointer, and is most often type casted to a specific pointer in the driver.

SysIOVec

This is a system I/O vector, as used by writev on unix and WSASend on Win32. It is used in ErlIOVec.

ErlIOVec

typedef struct ErlIOVec {
  int vsize;
  ErlDrvSizeT size;
  SysIOVec* iov;
  ErlDrvBinary** binv;
} ErlIOVec;

The I/O vector used by the emulator and drivers, is a list of binaries, with a SysIOVec pointing to the buffers of the binaries. It is used in driver_outputv and the outputv driver call-back. Also, the driver queue is an ErlIOVec.

ErlDrvMonitor

When a driver creates a monitor for a process, a ErlDrvMonitor is filled in. This is an opaque data-type which can be assigned to but not compared without using the supplied compare function (i.e. it behaves like a struct).

The driver writer should provide the memory for storing the monitor when calling driver_monitor_process. The address of the data is not stored outside of the driver, so the ErlDrvMonitor can be used as any other datum, it can be copied, moved in memory, forgotten etc.

ErlDrvNowData

The ErlDrvNowData structure holds a timestamp consisting of three values measured from some arbitrary point in the past. The three structure members are:

megasecs

The number of whole megaseconds elapsed since the arbitrary point in time

secs

The number of whole seconds elapsed since the arbitrary point in time

microsecs

The number of whole microseconds elapsed since the arbitrary point in time

ErlDrvPDL

If certain port specific data have to be accessed from other threads than those calling the driver call-backs, a port data lock can be used in order to synchronize the operations on the data. Currently, the only port specific data that the emulator associates with the port data lock is the driver queue.

Normally a driver instance does not have a port data lock. If the driver instance wants to use a port data lock, it has to create the port data lock by calling driver_pdl_create(). NOTE: Once the port data lock has been created, every access to data associated with the port data lock has to be done while having the port data lock locked. The port data lock is locked, and unlocked, respectively, by use of driver_pdl_lock(), and driver_pdl_unlock().

A port data lock is reference counted, and when the reference count reaches zero, it will be destroyed. The emulator will at least increment the reference count once when the lock is created and decrement it once when the port associated with the lock terminates. The emulator will also increment the reference count when an async job is enqueued and decrement it after an async job has been invoked, or canceled. Besides this, it is the responsibility of the driver to ensure that the reference count does not reach zero before the last use of the lock by the driver has been made. The reference count can be read, incremented, and decremented, respectively, by use of driver_pdl_get_refc(), driver_pdl_inc_refc(), and driver_pdl_dec_refc().

ErlDrvTid

Thread identifier.

See also: erl_drv_thread_create(), erl_drv_thread_exit(), erl_drv_thread_join(), erl_drv_thread_self(), and erl_drv_equal_tids().

ErlDrvThreadOpts

	int suggested_stack_size;
      

Thread options structure passed to erl_drv_thread_create(). Currently the following fields exist:

suggested_stack_size

A suggestion, in kilo-words, on how large a stack to use. A value less than zero means default size.

See also: erl_drv_thread_opts_create(), erl_drv_thread_opts_destroy(), and erl_drv_thread_create().

ErlDrvMutex

Mutual exclusion lock. Used for synchronizing access to shared data. Only one thread at a time can lock a mutex.

See also: erl_drv_mutex_create(), erl_drv_mutex_destroy(), erl_drv_mutex_lock(), erl_drv_mutex_trylock(), and erl_drv_mutex_unlock().

ErlDrvCond

Condition variable. Used when threads need to wait for a specific condition to appear before continuing execution. Condition variables need to be used with associated mutexes.

See also: erl_drv_cond_create(), erl_drv_cond_destroy(), erl_drv_cond_signal(), erl_drv_cond_broadcast(), and erl_drv_cond_wait().

ErlDrvRWLock

Read/write lock. Used to allow multiple threads to read shared data while only allowing one thread to write the same data. Multiple threads can read lock an rwlock at the same time, while only one thread can read/write lock an rwlock at a time.

See also: erl_drv_rwlock_create(), erl_drv_rwlock_destroy(), erl_drv_rwlock_rlock(), erl_drv_rwlock_tryrlock(), erl_drv_rwlock_runlock(), erl_drv_rwlock_rwlock(), erl_drv_rwlock_tryrwlock(), and erl_drv_rwlock_rwunlock().

ErlDrvTSDKey

Key which thread specific data can be associated with.

See also: erl_drv_tsd_key_create(), erl_drv_tsd_key_destroy(), erl_drv_tsd_set(), and erl_drv_tsd_get().

This function will write information about the Erlang runtime system into the ErlDrvSysInfo structure referred to by the first argument. The second argument should be the size of the ErlDrvSysInfo structure, i.e., sizeof(ErlDrvSysInfo).

See the documentation of the ErlDrvSysInfo structure for information about specific fields.

The driver_output function is used to send data from the driver up to the emulator. The data will be received as terms or binary data, depending on how the driver port was opened.

The data is queued in the port owner process' message queue. Note that this does not yield to the emulator. (Since the driver and the emulator run in the same thread.)

The parameter buf points to the data to send, and len is the number of bytes.

The return value for all output functions is 0. (Unless the driver is used for distribution, in which case it can fail and return -1. For normal use, the output function always returns 0.)

The driver_output2 function first sends hbuf (length in hlen) data as a list, regardless of port settings. Then buf is sent as a binary or list. E.g. if hlen is 3 then the port owner process will receive [H1, H2, H3 | T].

The point of sending data as a list header, is to facilitate matching on the data received.

The return value is 0 for normal use.

This function sends data to port owner process from a driver binary, it has a header buffer (hbuf and hlen) just like driver_output2. The hbuf parameter can be NULL.

The parameter offset is an offset into the binary and len is the number of bytes to send.

Driver binaries are created with driver_alloc_binary.

The data in the header is sent as a list and the binary as an Erlang binary in the tail of the list.

E.g. if hlen is 2, then the port owner process will receive [H1, H2 | <<T>>].

The return value is 0 for normal use.

Note that, using the binary syntax in Erlang, the driver application can match the header directly from the binary, so the header can be put in the binary, and hlen can be set to 0.

This function sends data from an IO vector, ev, to the port owner process. It has a header buffer (hbuf and hlen), just like driver_output2.

The skip parameter is a number of bytes to skip of the ev vector from the head.

You get vectors of ErlIOVec type from the driver queue (see below), and the outputv driver entry function. You can also make them yourself, if you want to send several ErlDrvBinary buffers at once. Often it is faster to use driver_output or driver_output_binary.

E.g. if hlen is 2 and ev points to an array of three binaries, the port owner process will receive [H1, H2, <<B1>>, <<B2>> | <<B3>>].

The return value is 0 for normal use.

The comment for driver_output_binary applies for driver_outputv too.

This function collects several segments of data, referenced by ev, by copying them in order to the buffer buf, of the size len.

If the data is to be sent from the driver to the port owner process, it is faster to use driver_outputv.

The return value is the space left in the buffer, i.e. if the ev contains less than len bytes it's the difference, and if ev contains len bytes or more, it's 0. This is faster if there is more than one header byte, since the binary syntax can construct integers directly from the binary.

This function sets a timer on the driver, which will count down and call the driver when it is timed out. The time parameter is the time in milliseconds before the timer expires.

When the timer reaches 0 and expires, the driver entry function timeout is called.

Note that there is only one timer on each driver instance; setting a new timer will replace an older one.

Return value is 0 (-1 only when the timeout driver function is NULL).

This function cancels a timer set with driver_set_timer.

The return value is 0.

This function reads the current time of a timer, and places the result in time_left. This is the time in milliseconds, before the timeout will occur.

The return value is 0.

This function reads a timestamp into the memory pointed to by the parameter now. See the description of ErlDrvNowData for specification of its fields.

The return value is 0 unless the now pointer is not valid, in which case it is < 0.

This function is used by drivers to provide the emulator with events to check for. This enables the emulator to call the driver when something has happened asynchronously.

The event argument identifies an OS-specific event object. On Unix systems, the functions select/poll are used. The event object must be a socket or pipe (or other object that select/poll can use). On windows, the Win32 API function WaitForMultipleObjects is used. This places other restrictions on the event object. Refer to the Win32 SDK documentation.

The on parameter should be 1 for setting events and 0 for clearing them.

The mode argument is a bitwise-or combination of ERL_DRV_READ, ERL_DRV_WRITE and ERL_DRV_USE. The first two specify whether to wait for read events and/or write events. A fired read event will call ready_input while a fired write event will call ready_output.

ERL_DRV_USE specifies if we are using the event object or if we want to close it. On an emulator with SMP support, it is not safe to clear all events and then close the event object after driver_select has returned. Another thread may still be using the event object internally. To safely close an event object call driver_select with ERL_DRV_USE and on==0. That will clear all events and then call stop_select when it is safe to close the event object. ERL_DRV_USE should be set together with the first event for an event object. It is harmless to set ERL_DRV_USE even though it already has been done. Clearing all events but keeping ERL_DRV_USE set will indicate that we are using the event object and probably will set events for it again.

The return value is 0 (failure, -1, only if the ready_input/ready_output is NULL).

This function allocates a memory block of the size specified in size, and returns it. This only fails on out of memory, in that case NULL is returned. (This is most often a wrapper for malloc).

Memory allocated must be explicitly freed with a corresponding call to driver_free (unless otherwise stated).

This function is thread-safe.

This function resizes a memory block, either in place, or by allocating a new block, copying the data and freeing the old block. A pointer is returned to the reallocated memory. On failure (out of memory), NULL is returned. (This is most often a wrapper for realloc.)

This function is thread-safe.

This function frees the memory pointed to by ptr. The memory should have been allocated with driver_alloc. All allocated memory should be deallocated, just once. There is no garbage collection in drivers.

This function is thread-safe.

This function allocates a driver binary with a memory block of at least size bytes, and returns a pointer to it, or NULL on failure (out of memory). When a driver binary has been sent to the emulator, it must not be altered. Every allocated binary should be freed by a corresponding call to driver_free_binary (unless otherwise stated).

Note that a driver binary has an internal reference counter, this means that calling driver_free_binary it may not actually dispose of it. If it's sent to the emulator, it may be referenced there.

The driver binary has a field, orig_bytes, which marks the start of the data in the binary.

This function is thread-safe.

This function resizes a driver binary, while keeping the data. The resized driver binary is returned. On failure (out of memory), NULL is returned.

This function is only thread-safe when the emulator with SMP support is used.

This function frees a driver binary bin, allocated previously with driver_alloc_binary. Since binaries in Erlang are reference counted, the binary may still be around.

This function is only thread-safe when the emulator with SMP support is used.

Returns current reference count on bin.

This function is only thread-safe when the emulator with SMP support is used.

Increments the reference count on bin and returns the reference count reached after the increment.

This function is only thread-safe when the emulator with SMP support is used.

Decrements the reference count on bin and returns the reference count reached after the decrement.

This function is only thread-safe when the emulator with SMP support is used.

This function enqueues data in the driver queue. The data in buf is copied (len bytes) and placed at the end of the driver queue. The driver queue is normally used in a FIFO way.

The driver queue is available to queue output from the emulator to the driver (data from the driver to the emulator is queued by the emulator in normal erlang message queues). This can be useful if the driver has to wait for slow devices etc, and wants to yield back to the emulator. The driver queue is implemented as an ErlIOVec.

When the queue contains data, the driver won't close, until the queue is empty.

The return value is 0.

This function can be called from an arbitrary thread if a port data lock associated with the port is locked by the calling thread during the call.

This function puts data at the head of the driver queue. The data in buf is copied (len bytes) and placed at the beginning of the queue.

The return value is 0.

This function can be called from an arbitrary thread if a port data lock associated with the port is locked by the calling thread during the call.

This function dequeues data by moving the head pointer forward in the driver queue by size bytes. The data in the queue will be deallocated.

The return value is the number of bytes remaining in the queue or -1 on failure.

This function can be called from an arbitrary thread if a port data lock associated with the port is locked by the calling thread during the call.

This function returns the number of bytes currently in the driver queue.

This function can be called from an arbitrary thread if a port data lock associated with the port is locked by the calling thread during the call.

This function enqueues a driver binary in the driver queue. The data in bin at offset with length len is placed at the end of the queue. This function is most often faster than driver_enq, because the data doesn't have to be copied.

This function can be called from an arbitrary thread if a port data lock associated with the port is locked by the calling thread during the call.

The return value is 0.

This function puts data in the binary bin, at offset with length len at the head of the driver queue. It is most often faster than driver_pushq, because the data doesn't have to be copied.

This function can be called from an arbitrary thread if a port data lock associated with the port is locked by the calling thread during the call.

The return value is 0.

This function retrieves the driver queue into a supplied ErlIOVec ev. It also returns the queue size. This is one of two ways to get data out of the queue.

If ev is NULL all ones i.e. -1 type cast to ErlDrvSizeT is returned.

Nothing is removed from the queue by this function, that must be done with driver_deq.

This function can be called from an arbitrary thread if a port data lock associated with the port is locked by the calling thread during the call.

This function retrieves the driver queue as a pointer to an array of SysIOVecs. It also returns the number of elements in vlen. This is one of two ways to get data out of the queue.

Nothing is removed from the queue by this function, that must be done with driver_deq.

The returned array is suitable to use with the Unix system call writev.

This function can be called from an arbitrary thread if a port data lock associated with the port is locked by the calling thread during the call.

This function enqueues the data in ev, skipping the first skip bytes of it, at the end of the driver queue. It is faster than driver_enq, because the data doesn't have to be copied.

The return value is 0.

This function can be called from an arbitrary thread if a port data lock associated with the port is locked by the calling thread during the call.

This function puts the data in ev, skipping the first skip bytes of it, at the head of the driver queue. It is faster than driver_pushq, because the data doesn't have to be copied.

The return value is 0.

This function can be called from an arbitrary thread if a port data lock associated with the port is locked by the calling thread during the call.

This function creates a port data lock associated with the port. NOTE: Once a port data lock has been created, it has to be locked during all operations on the driver queue of the port.

On success a newly created port data lock is returned. On failure NULL is returned. driver_pdl_create() will fail if port is invalid or if a port data lock already has been associated with the port.

This function locks the port data lock passed as argument (pdl).

This function is thread-safe.

This function unlocks the port data lock passed as argument (pdl).

This function is thread-safe.

This function returns the current reference count of the port data lock passed as argument (pdl).

This function is thread-safe.

This function increments the reference count of the port data lock passed as argument (pdl).

The current reference count after the increment has been performed is returned.

This function is thread-safe.

This function decrements the reference count of the port data lock passed as argument (pdl).

The current reference count after the decrement has been performed is returned.

This function is thread-safe.

Start monitoring a process from a driver. When a process is monitored, a process exit will result in a call to the provided process_exit call-back in the ErlDrvEntry structure. The ErlDrvMonitor structure is filled in, for later removal or compare.

The process parameter should be the return value of an earlier call to driver_caller or driver_connected call.

The function returns 0 on success, < 0 if no call-back is provided and > 0 if the process is no longer alive.

This function cancels a monitor created earlier.

The function returns 0 if a monitor was removed and > 0 if the monitor did no longer exist.

The function returns the process id associated with a living monitor. It can be used in the process_exit call-back to get the process identification for the exiting process.

The function returns driver_term_nil if the monitor no longer exists.

This function is used to compare two ErlDrvMonitors. It can also be used to imply some artificial order on monitors, for whatever reason.

The function returns 0 if monitor1 and monitor2 are equal, < 0 if monitor1 is less than monitor2 and > 0 if monitor1 is greater than monitor2.

This function adds a driver entry to the list of drivers known by Erlang. The init function of the de parameter is called.

This function removes a driver entry de previously added with add_driver_entry.

Driver entries added by the erl_ddll erlang interface can not be removed by using this interface.

This function returns the atom name of the erlang error, given the error number in error. Error atoms are: einval, enoent, etc. It can be used to make error terms from the driver.

This function set and resets the busy status of the port. If on is 1, the port is set to busy, if it's 0 the port is set to not busy.

When the port is busy, sending to it with Port ! Data or port_command/2, will block the port owner process, until the port is signaled as not busy.

If the ERL_DRV_FLAG_SOFT_BUSY has been set in the driver_entry, data can be forced into the driver via port_command(Port, Data, [force]) even though the driver has signaled that it is busy.

This function sets flags for how the control driver entry function will return data to the port owner process. (The control function is called from port_control/3 in erlang.)

Currently there are only two meaningful values for flags: 0 means that data is returned in a list, and PORT_CONTROL_FLAG_BINARY means data is returned as a binary from control.

This function signals to erlang that the driver has encountered an EOF and should be closed, unless the port was opened with the eof option, in that case eof is sent to the port. Otherwise, the port is closed and an 'EXIT' message is sent to the port owner process.

The return value is 0.

These functions signal to Erlang that the driver has encountered an error and should be closed. The port is closed and the tuple {'EXIT', error, Err}, is sent to the port owner process, where error is an error atom (driver_failure_atom and driver_failure_posix), or an integer (driver_failure).

The driver should fail only when in severe error situations, when the driver cannot possibly keep open, for instance buffer allocation gets out of memory. For normal errors it is more appropriate to send error codes with driver_output.

The return value is 0.

This function returns the port owner process.

This function returns the process id of the process that made the current call to the driver. The process id can be used with driver_send_term to send back data to the caller. driver_caller() only returns valid data when currently executing in one of the following driver callbacks:

start

Called from open_port/2.

output

Called from erlang:send/2, and erlang:port_command/2

outputv

Called from erlang:send/2, and erlang:port_command/2

control

Called from erlang:port_control/3

call

Called from erlang:port_call/3

This functions sends data in the special driver term format. This is a fast way to deliver term data from a driver. It also needs no binary conversion, so the port owner process receives data as normal Erlang terms.

The term parameter points to an array of ErlDrvTermData, with n elements. This array contains terms described in the driver term format. Every term consists of one to four elements in the array. The term first has a term type, and then arguments.

Tuple and lists (with the exception of strings, see below), are built in reverse polish notation, so that to build a tuple, the elements are given first, and then the tuple term, with a count. Likewise for lists.

A tuple must be specified with the number of elements. (The elements precede the ERL_DRV_TUPLE term.)

A list must be specified with the number of elements, including the tail, which is the last term preceding ERL_DRV_LIST.

The special term ERL_DRV_STRING_CONS is used to "splice" in a string in a list, a string given this way is not a list per se, but the elements are elements of the surrounding list.

Term type            Argument(s)
===========================================
ERL_DRV_NIL          
ERL_DRV_ATOM         ErlDrvTermData atom (from driver_mk_atom(char *string))
ERL_DRV_INT          ErlDrvSInt integer
ERL_DRV_UINT         ErlDrvUInt integer
ERL_DRV_INT64        ErlDrvSInt64 *integer_ptr
ERL_DRV_UINT64       ErlDrvUInt64 *integer_ptr
ERL_DRV_PORT         ErlDrvTermData port (from driver_mk_port(ErlDrvPort port))
ERL_DRV_BINARY       ErlDrvBinary *bin, ErlDrvUInt len, ErlDrvUInt offset
ERL_DRV_BUF2BINARY   char *buf, ErlDrvUInt len
ERL_DRV_STRING       char *str, int len
ERL_DRV_TUPLE        int sz
ERL_DRV_LIST         int sz
ERL_DRV_PID          ErlDrvTermData pid (from driver_connected(ErlDrvPort port) or driver_caller(ErlDrvPort port))
ERL_DRV_STRING_CONS  char *str, int len
ERL_DRV_FLOAT        double *dbl
ERL_DRV_EXT2TERM     char *buf, ErlDrvUInt len
        

The unsigned integer data type ErlDrvUInt and the signed integer data type ErlDrvSInt are 64 bits wide on a 64 bit runtime system and 32 bits wide on a 32 bit runtime system. They were introduced in erts version 5.6, and replaced some of the int arguments in the list above.

The unsigned integer data type ErlDrvUInt64 and the signed integer data type ErlDrvSInt64 are always 64 bits wide. They were introduced in erts version 5.7.4.

To build the tuple {tcp, Port, [100 | Binary]}, the following call could be made.

    ErlDrvBinary* bin = ...
    ErlDrvPort port = ...
    ErlDrvTermData spec[] = {
        ERL_DRV_ATOM, driver_mk_atom("tcp"),
        ERL_DRV_PORT, driver_mk_port(port),
            ERL_DRV_INT, 100,
            ERL_DRV_BINARY, bin, 50, 0,
            ERL_DRV_LIST, 2,
        ERL_DRV_TUPLE, 3,
    };
    driver_output_term(port, spec, sizeof(spec) / sizeof(spec[0]));
        
      

Where bin is a driver binary of length at least 50 and port is a port handle. Note that the ERL_DRV_LIST comes after the elements of the list, likewise the ERL_DRV_TUPLE.

The term ERL_DRV_STRING_CONS is a way to construct strings. It works differently from how ERL_DRV_STRING works. ERL_DRV_STRING_CONS builds a string list in reverse order, (as opposed to how ERL_DRV_LIST works), concatenating the strings added to a list. The tail must be given before ERL_DRV_STRING_CONS.

The ERL_DRV_STRING constructs a string, and ends it. (So it's the same as ERL_DRV_NIL followed by ERL_DRV_STRING_CONS.)

    /* to send [x, "abc", y] to the port: */
    ErlDrvTermData spec[] = {
        ERL_DRV_ATOM, driver_mk_atom("x"),
        ERL_DRV_STRING, (ErlDrvTermData)"abc", 3,
        ERL_DRV_ATOM, driver_mk_atom("y"),
        ERL_DRV_NIL,
        ERL_DRV_LIST, 4
    };
    driver_output_term(port, spec, sizeof(spec) / sizeof(spec[0]));
        

    /* to send "abc123" to the port: */
    ErlDrvTermData spec[] = {
        ERL_DRV_NIL,        /* with STRING_CONS, the tail comes first */
        ERL_DRV_STRING_CONS, (ErlDrvTermData)"123", 3,
        ERL_DRV_STRING_CONS, (ErlDrvTermData)"abc", 3,
    };
    driver_output_term(port, spec, sizeof(spec) / sizeof(spec[0]));
        

The ERL_DRV_EXT2TERM term type is used for passing a term encoded with the external format, i.e., a term that has been encoded by erlang:term_to_binary, erl_interface, etc. For example, if binp is a pointer to an ErlDrvBinary that contains the term {17, 4711} encoded with the external format and you want to wrap it in a two tuple with the tag my_tag, i.e., {my_tag, {17, 4711}}, you can do as follows:

    ErlDrvTermData spec[] = {
            ERL_DRV_ATOM, driver_mk_atom("my_tag"),
            ERL_DRV_EXT2TERM, (ErlDrvTermData) binp->orig_bytes, binp->orig_size
        ERL_DRV_TUPLE, 2,
    };
    driver_output_term(port, spec, sizeof(spec) / sizeof(spec[0]));
        

If you want to pass a binary and don't already have the content of the binary in an ErlDrvBinary, you can benefit from using ERL_DRV_BUF2BINARY instead of creating an ErlDrvBinary via driver_alloc_binary() and then pass the binary via ERL_DRV_BINARY. The runtime system will often allocate binaries smarter if ERL_DRV_BUF2BINARY is used. However, if the content of the binary to pass already resides in an ErlDrvBinary, it is normally better to pass the binary using ERL_DRV_BINARY and the ErlDrvBinary in question.

The ERL_DRV_UINT, ERL_DRV_BUF2BINARY, and ERL_DRV_EXT2TERM term types were introduced in the 5.6 version of erts.

Note that this function is not thread-safe, not even when the emulator with SMP support is used.

This function returns an atom given a name string. The atom is created and won't change, so the return value may be saved and reused, which is faster than looking up the atom several times.

This function converts a port handle to the erlang term format, usable in the driver_output_send function.

This function is the only way for a driver to send data to other processes than the port owner process. The receiver parameter specifies the process to receive the data.

The parameters term and n do the same thing as in driver_output_term.

This function is only thread-safe when the emulator with SMP support is used.

This function performs an asynchronous call. The function async_invoke is invoked in a thread separate from the emulator thread. This enables the driver to perform time-consuming, blocking operations without blocking the emulator.

Erlang is by default started without an async thread pool. The number of async threads that the runtime system should use is specified by the +A command line argument of erl(1). If no async thread pool is available, the call is made synchronously in the thread calling driver_async(). The current number of async threads in the async thread pool can be retrieved via driver_system_info().

If there is a thread pool available, a thread will be used. If the key argument is null, the threads from the pool are used in a round-robin way, each call to driver_async uses the next thread in the pool. With the key argument set, this behaviour is changed. The two same values of *key always get the same thread.

To make sure that a driver instance always uses the same thread, the following call can be used:

    unsigned int myKey = (unsigned int) myPort;

    r = driver_async(myPort, &myKey, myData, myFunc);
        

It is enough to initialize myKey once for each driver instance.

If a thread is already working, the calls will be queued up and executed in order. Using the same thread for each driver instance ensures that the calls will be made in sequence.

The async_data is the argument to the functions async_invoke and async_free. It's typically a pointer to a structure that contains a pipe or event that can be used to signal that the async operation completed. The data should be freed in async_free, because it's called if driver_async_cancel is called.

When the async operation is done, ready_async driver entry function is called. If async_ready is null in the driver entry, the async_free function is called instead.

The return value is a handle to the asynchronous task, which can be used as argument to driver_async_cancel.

This function used to cancel a scheduled asynchronous operation, if it was still in the queue. It returned 1 if it succeeded, and 0 if it failed.

Since it could not guarantee success, it was more or less useless. The user had to implement synchronization of cancellation anyway. It also unnecessarily complicated the implementation. Therefore, as of OTP-R15B driver_async_cancel() is deprecated, and scheduled for removal in OTP-R16. It will currently always fail, and return 0.

This function locks the driver used by the port port in memory for the rest of the emulator process' lifetime. After this call, the driver behaves as one of Erlang's statically linked in drivers.

This function creates a new port executing the same driver code as the port creating the new port. A short description of the arguments:

port

The port handle of the port (driver instance) creating the new port.

owner_pid

The process id of the Erlang process which will be owner of the new port. This process will be linked to the new port. You usually want to use driver_caller(port) as owner_pid.

name

The port name of the new port. You usually want to use the same port name as the driver name (driver_name field of the driver_entry).

drv_data

The driver defined handle that will be passed in subsequent calls to driver call-backs. Note, that the driver start call-back will not be called for this new driver instance. The driver defined handle is normally created in the driver start call-back when a port is created via erlang:open_port/2.

The caller of driver_create_port() is allowed to manipulate the newly created port when driver_create_port() has returned. When port level locking is used, the creating port is, however, only allowed to manipulate the newly created port until the current driver call-back that was called by the emulator returns.

Arguments:

name

A string identifying the created thread. It will be used to identify the thread in planned future debug functionality.

tid

A pointer to a thread identifier variable.

func

A pointer to a function to execute in the created thread.

arg

A pointer to argument to the func function.

opts

A pointer to thread options to use or NULL.

This function creates a new thread. On success 0 is returned; otherwise, an errno value is returned to indicate the error. The newly created thread will begin executing in the function pointed to by func, and func will be passed arg as argument. When erl_drv_thread_create() returns the thread identifier of the newly created thread will be available in *tid. opts can be either a NULL pointer, or a pointer to an ErlDrvThreadOpts structure. If opts is a NULL pointer, default options will be used; otherwise, the passed options will be used.

The created thread will terminate either when func returns or if erl_drv_thread_exit() is called by the thread. The exit value of the thread is either returned from func or passed as argument to erl_drv_thread_exit(). The driver creating the thread has the responsibility of joining the thread, via erl_drv_thread_join(), before the driver is unloaded. It is not possible to create "detached" threads, i.e., threads that don't need to be joined.

This function is thread-safe.

Arguments:

name

A string identifying the created thread options. It will be used to identify the thread options in planned future debug functionality.

This function allocates and initialize a thread option structure. On failure NULL is returned. A thread option structure is used for passing options to erl_drv_thread_create(). If the structure isn't modified before it is passed to erl_drv_thread_create(), the default values will be used.

This function is thread-safe.

Arguments:

opts

A pointer to thread options to destroy.

This function destroys thread options previously created by erl_drv_thread_opts_create().

This function is thread-safe.

Arguments:

exit_value

A pointer to an exit value or NULL.

This function terminates the calling thread with the exit value passed as argument. You are only allowed to terminate threads created with erl_drv_thread_create(). The exit value can later be retrieved by another thread via erl_drv_thread_join().

This function is thread-safe.

Arguments:

tid

The thread identifier of the thread to join.

exit_value

A pointer to a pointer to an exit value, or NULL.

This function joins the calling thread with another thread, i.e., the calling thread is blocked until the thread identified by tid has terminated. On success 0 is returned; otherwise, an errno value is returned to indicate the error. A thread can only be joined once. The behavior of joining more than once is undefined, an emulator crash is likely. If exit_value == NULL, the exit value of the terminated thread will be ignored; otherwise, the exit value of the terminated thread will be stored at *exit_value.

This function is thread-safe.

This function returns the thread identifier of the calling thread.

This function is thread-safe.

Arguments:

tid1

A thread identifier.

tid2

A thread identifier.

This function compares two thread identifiers for equality, and returns 0 it they aren't equal, and a value not equal to 0 if they are equal.

This function is thread-safe.

Arguments:

name

A string identifying the created mutex. It will be used to identify the mutex in planned future debug functionality.

This function creates a mutex and returns a pointer to it. On failure NULL is returned. The driver creating the mutex has the responsibility of destroying it before the driver is unloaded.

This function is thread-safe.

Arguments:

mtx

A pointer to a mutex to destroy.

This function destroys a mutex previously created by erl_drv_mutex_create(). The mutex has to be in an unlocked state before being destroyed.

This function is thread-safe.

Arguments:

mtx

A pointer to a mutex to lock.

This function locks a mutex. The calling thread will be blocked until the mutex has been locked. A thread which currently has locked the mutex may not lock the same mutex again.

This function is thread-safe.

Arguments:

mtx

A pointer to a mutex to try to lock.

This function tries to lock a mutex. If successful 0, is returned; otherwise, EBUSY is returned. A thread which currently has locked the mutex may not try to lock the same mutex again.

This function is thread-safe.

Arguments:

mtx

A pointer to a mutex to unlock.

This function unlocks a mutex. The mutex currently has to be locked by the calling thread.

This function is thread-safe.

Arguments:

name

A string identifying the created condition variable. It will be used to identify the condition variable in planned future debug functionality.

This function creates a condition variable and returns a pointer to it. On failure NULL is returned. The driver creating the condition variable has the responsibility of destroying it before the driver is unloaded.

This function is thread-safe.

Arguments:

cnd

A pointer to a condition variable to destroy.

This function destroys a condition variable previously created by erl_drv_cond_create().

This function is thread-safe.

Arguments:

cnd

A pointer to a condition variable to signal on.

This function signals on a condition variable. That is, if other threads are waiting on the condition variable being signaled, one of them will be woken.

This function is thread-safe.

Arguments:

cnd

A pointer to a condition variable to broadcast on.

This function broadcasts on a condition variable. That is, if other threads are waiting on the condition variable being broadcasted on, all of them will be woken.

This function is thread-safe.

Arguments:

cnd

A pointer to a condition variable to wait on.

mtx

A pointer to a mutex to unlock while waiting.

This function waits on a condition variable. The calling thread is blocked until another thread wakes it by signaling or broadcasting on the condition variable. Before the calling thread is blocked it unlocks the mutex passed as argument, and when the calling thread is woken it locks the same mutex before returning. That is, the mutex currently has to be locked by the calling thread when calling this function.

This function is thread-safe.

Arguments:

name

A string identifying the created rwlock. It will be used to identify the rwlock in planned future debug functionality.

This function creates an rwlock and returns a pointer to it. On failure NULL is returned. The driver creating the rwlock has the responsibility of destroying it before the driver is unloaded.

This function is thread-safe.

Arguments:

rwlck

A pointer to an rwlock to destroy.

This function destroys an rwlock previously created by erl_drv_rwlock_create(). The rwlock has to be in an unlocked state before being destroyed.

This function is thread-safe.

Arguments:

rwlck

A pointer to an rwlock to read lock.

This function read locks an rwlock. The calling thread will be blocked until the rwlock has been read locked. A thread which currently has read or read/write locked the rwlock may not lock the same rwlock again.

This function is thread-safe.

Arguments:

rwlck

A pointer to an rwlock to try to read lock.

This function tries to read lock an rwlock. If successful 0, is returned; otherwise, EBUSY is returned. A thread which currently has read or read/write locked the rwlock may not try to lock the same rwlock again.

This function is thread-safe.

Arguments:

rwlck

A pointer to an rwlock to read unlock.

This function read unlocks an rwlock. The rwlock currently has to be read locked by the calling thread.

This function is thread-safe.

Arguments:

rwlck

A pointer to an rwlock to read/write lock.

This function read/write locks an rwlock. The calling thread will be blocked until the rwlock has been read/write locked. A thread which currently has read or read/write locked the rwlock may not lock the same rwlock again.

This function is thread-safe.

Arguments:

rwlck

A pointer to an rwlock to try to read/write lock.

This function tries to read/write lock an rwlock. If successful 0, is returned; otherwise, EBUSY is returned. A thread which currently has read or read/write locked the rwlock may not try to lock the same rwlock again.

This function is thread-safe.

Arguments:

rwlck

A pointer to an rwlock to read/write unlock.

This function read/write unlocks an rwlock. The rwlock currently has to be read/write locked by the calling thread.

This function is thread-safe.

Arguments:

name

A string identifying the created key. It will be used to identify the key in planned future debug functionality.

key

A pointer to a thread specific data key variable.

This function creates a thread specific data key. On success 0 is returned; otherwise, an errno value is returned to indicate the error. The driver creating the key has the responsibility of destroying it before the driver is unloaded.

This function is thread-safe.

Arguments:

key

A thread specific data key to destroy.

This function destroys a thread specific data key previously created by erl_drv_tsd_key_create(). All thread specific data using this key in all threads have to be cleared (see erl_drv_tsd_set()) prior to the call to erl_drv_tsd_key_destroy().

This function is thread-safe.

Arguments:

key

A thread specific data key.

data

A pointer to data to associate with key in calling thread.

This function sets thread specific data associated with key for the calling thread. You are only allowed to set thread specific data for threads while they are fully under your control. For example, if you set thread specific data in a thread calling a driver call-back function, it has to be cleared, i.e. set to NULL, before returning from the driver call-back function.

This function is thread-safe.

Arguments:

key

A thread specific data key.

This function returns the thread specific data associated with key for the calling thread. If no data has been associated with key for the calling thread, NULL is returned.

This function is thread-safe.

Arguments:

key

A null terminated string containing the name of the environment variable.

value

A null terminated string containing the new value of the environment variable.

This function sets the value of an environment variable. It returns 0 on success, and a value != 0 on failure.

This function is thread-safe.

Arguments:

key

A null terminated string containing the name of the environment variable.

value

A pointer to an output buffer.

value_size

A pointer to an integer. The integer is both used for passing input and output sizes (see below).

This function retrieves the value of an environment variable. When called, *value_size should contain the size of the value buffer. On success 0 is returned, the value of the environment variable has been written to the value buffer, and *value_size contains the string length (excluding the terminating null character) of the value written to the value buffer. On failure, i.e., no such environment variable was found, a value less than 0 is returned. When the size of the value buffer is too small, a value greater than 0 is returned and *value_size has been set to the buffer size needed.

This function is thread-safe.

driver_entry(3), erl_ddll(3), erlang(3)

An Alternative Distribution Driver (ERTS User's Guide Ch. 3)