2 Socket Usage

2.1  Introduction

The socket interface (module) is basically a "thin" layer on top of the OS socket interface. It is assumed that, unless you have special needs, gen_[tcp|udp|sctp] should be sufficient (when they become available).

Note that just because we have a documented and described option, it does not mean that the OS supports it. So its recommended that the user reads the platform specific documentation for the option used.

Asynchronous calls

Some functions allow for an asynchronous call (accept/2, connect/3, recv/3,4, recvfrom/3,4, recvmsg/2,3,5, send/3,4, sendmsg/3,4 and sendto/4,5). This is achieved by setting the Timeout argument to nowait. For instance, if calling the recv/3 function with Timeout set to nowait (i.e. recv(Sock, 0, nowait)) when there is actually nothing to read, it will return with:

On Unix

{select, SelectInfo}

SelectInfo contains the SelectHandle.

On Windows

{completion, CompletionInfo}

CompletionInfo contains the CompletionHandle.

When data eventually arrives a 'select' or 'completion' message will be sent to the caller:

On Unix

{'$socket', socket(), select, SelectHandle}

The caller can then make another call to the recv function and now expect data.

Note that all other users are locked out until the 'current user' has called the function (recv in this case). So either immediately call the function or cancel.

On Windows

{'$socket', socket(), completion, {CompletionHandle, CompletionStatus}}

The CompletionStatus contains the result of the operation (read).

The user must also be prepared to receive an abort message:

{'$socket', socket(), abort, Info}

If the operation is aborted for whatever reason (e.g. if the socket is closed "by someone else"). The Info part contains the abort reason (in this case that the socket has been closed Info = {SelectHandle, closed}).

The general form of the 'socket' message is:

{'$socket', Sock :: socket(), Tag :: atom(), Info :: term()}

Where the format of Info is a function of Tag:

The select_handle() is the same as was returned in the SelectInfo.

The completion_handle() is the same as was returned in the CompletionInfo.

2.2  Socket Registry

The socket registry is how we keep track of sockets. There are two functions that can be used for interaction: socket:number_of/0 and socket:which_sockets/1.

In systems which create and delete many sockets dynamically, it (the socket registry) could become a bottleneck. For such systems, there are a couple of ways to control the use of the socket registry.

Firstly, its possible to effect the global default value when building OTP from source with the two configure options:

--enable-esock-socket-registry (default) | --disable-esock-socket-registry

Second, its possible to effect the global default value by setting the environment variable ESOCK_USE_SOCKET_REGISTRY (boolean) before starting the erlang.

Third, its possible to alter the global default value in runtime by calling the function use_registry/1.

And finally, its possible to override the global default when creating a socket (with open/2 and open/4) by providing the attribute use_registry (boolean) in the their Opts argument (which effects that specific socket).

2.3  Socket Options

Options for level otp:

Options for level socket:

Options for level ip:

Options for level ipv6:

Options for level tcp:

Options for level udp:

Options for level sctp: