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@(#)accept.2 8.2 (Berkeley) 12/11/93
.Dd December 11, 1993 .Dt ACCEPT 2 .Os BSD 4.2 .Sh NAME .Nm accept .Nd accept a connection on a socket .Sh SYNOPSIS .Fd #include <sys/socket.h> .Ft int .Fo accept .Fa "int socket" .Fa "struct sockaddr *restrict address" .Fa "socklen_t *restrict address_len" .Fc .Sh DESCRIPTION The argument .Fa socket is a socket that has been created with .Xr socket 2 , bound to an address with .Xr bind 2 , and is listening for connections after a .Xr listen 2 . .Fn accept extracts the first connection request on the queue of pending connections, creates a new socket with the same properties of .Fa socket , and allocates a new file descriptor for the socket. If no pending connections are present on the queue, and the socket is not marked as non-blocking, .Fn accept blocks the caller until a connection is present. If the socket is marked non-blocking and no pending connections are present on the queue, .Fn accept returns an error as described below. The accepted socket may not be used to accept more connections. The original socket .Fa socket, remains open.
p The argument .Fa address is a result parameter that is filled in with the address of the connecting entity, as known to the communications layer. The exact format of the .Fa address parameter is determined by the domain in which the communication is occurring. The .Fa address_len is a value-result parameter; it should initially contain the amount of space pointed to by .Fa address ; on return it will contain the actual length (in bytes) of the address returned. This call is used with connection-based socket types, currently with .Dv SOCK_STREAM .
p It is possible to .Xr select 2 a socket for the purposes of doing an .Fn accept by selecting it for read.
p For certain protocols which require an explicit confirmation, such as .Tn ISO or .Tn DATAKIT , .Fn accept can be thought of as merely dequeuing the next connection request and not implying confirmation. Confirmation can be implied by a normal read or write on the new file descriptor, and rejection can be implied by closing the new socket.
p
One can obtain user connection request data without confirming
the connection by issuing a
.Xr recvmsg 2
call with an
.Fa msg_iovlen
of 0 and a non-zero
.Fa msg_controllen ,
or by issuing a
.Xr getsockopt 2
request.
Similarly, one can provide user connection rejection information
by issuing a
.Xr sendmsg 2
call with providing only the control information,
or by calling
.Xr setsockopt 2 .
.Sh RETURN VALUES
The call returns -1 on error and the global variable
.Va errno
is set to indicate the error.
If it succeeds, it returns a non-negative integer
that is a descriptor for the accepted socket.
.Sh ERRORS
The
.Fn accept
system call will fail if:
l -tag -width Er ==========
t Bq Er EBADF .Fa socket
is not a valid file descriptor.
==========
t Bq Er ECONNABORTED The connection to
.Fa socket
has been aborted.
==========
t Bq Er EFAULT The
.Fa address
parameter is not in a writable part of the
user address space.
==========
t Bq Er EINTR The
.Fn accept
system call was terminated by a signal.
==========
t Bq Er EINVAL .Fa socket
is unwilling to accept connections.
==========
t Bq Er EMFILE The per-process descriptor table is full.
==========
t Bq Er ENFILE The system file table is full.
==========
t Bq Er ENOMEM Insufficient memory was available to complete the operation.
==========
t Bq Er ENOTSOCK .Fa socket
references a file type other than a socket.
==========
t Bq Er EOPNOTSUPP .Fa socket
is not of type
.Dv SOCK_STREAM
and thus does not accept connections.
==========
t Bq Er EWOULDBLOCK .Fa socket
is marked as non-blocking and no connections are present to be accepted.
.El
.Sh LEGACY SYNOPSIS
.Fd #include <sys/types.h>
.Fd #include <sys/socket.h>
p The include file n sys/types.h is necessary. .Sh SEE ALSO .Xr bind 2 , .Xr connect 2 , .Xr listen 2 , .Xr select 2 , .Xr socket 2 , .Xr compat 5 .Sh HISTORY The .Fn accept function appeared in x 4.2 .