.\" $OpenBSD: accept.2,v 1.30 2022/09/11 06:38:11 jmc Exp $ .\" $NetBSD: accept.2,v 1.7 1996/01/31 20:14:42 mycroft Exp $ .\" .\" Copyright (c) 1983, 1990, 1991, 1993 .\" The Regents of the University of California. All rights reserved. .\" .\" Redistribution and use in source and binary forms, with or without .\" modification, are permitted provided that the following conditions .\" are met: .\" 1. Redistributions of source code must retain the above copyright .\" notice, this list of conditions and the following disclaimer. .\" 2. Redistributions in binary form must reproduce the above copyright .\" notice, this list of conditions and the following disclaimer in the .\" documentation and/or other materials provided with the distribution. .\" 3. Neither the name of the University nor the names of its contributors .\" may be used to endorse or promote products derived from this software .\" without specific prior written permission. .\" .\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND .\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE .\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE .\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE .\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL .\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS .\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) .\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT .\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF .\" SUCH DAMAGE. .\" .\" @(#)accept.2 8.2 (Berkeley) 12/11/93 .\" .Dd $Mdocdate: September 11 2022 $ .Dt ACCEPT 2 .Os .Sh NAME .Nm accept , .Nm accept4 .Nd accept a connection on a socket .Sh SYNOPSIS .In sys/socket.h .Ft int .Fn accept "int s" "struct sockaddr *addr" "socklen_t *addrlen" .Ft int .Fn accept4 "int s" "struct sockaddr *addr" "socklen_t *addrlen" "int flags" .Sh DESCRIPTION The argument .Fa s 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 . The .Fn accept call extracts the first connection request on the queue of pending connections, creates a new socket with the same non-blocking I/O mode as .Fa s , and allocates a new file descriptor for the socket with the close-on-exec flag clear. .Pp The .Fn accept4 system call is similar, however the non-blocking I/O mode of the new socket is determined by the .Dv SOCK_NONBLOCK flag in the .Fa flags argument and the close-on-exec flag on the new file descriptor is determined by the .Dv SOCK_CLOEXEC flag in the .Fa flags argument. .Pp 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 s remains open. .Pp The argument .Fa addr 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 addr parameter is determined by the domain in which the communication is occurring. The structure .Vt sockaddr_storage exists for greater portability. It is large enough to hold any of the types that may be returned in the .Fa addr parameter. .Pp The .Fa addrlen is a value-result parameter; it should initially contain the amount of space pointed to by .Fa addr ; on return it will contain the actual length (in bytes) of the address returned. If .Fa addrlen does not point to enough space to hold the entire socket address, the result will be truncated to the initial value of .Fa addrlen (in bytes). This call is used with connection-based socket types, currently with .Dv SOCK_STREAM . .Pp It is possible to .Xr select 2 or .Xr poll 2 a socket for the purposes of doing an .Fn accept by selecting it for read. .Sh RETURN VALUES If successful, .Fn accept and .Fn accept4 return a non-negative integer, the accepted socket file descriptor. Otherwise, a value of \-1 is returned and .Va errno is set to indicate the error. .Sh EXAMPLES The following code uses struct .Vt sockaddr_storage to allocate enough space for the returned address: .Bd -literal -offset indent #include #include struct sockaddr_storage addr; socklen_t len = sizeof(addr); int retcode; retcode = accept(s, (struct sockaddr *)&addr, &len); if (retcode == -1) err(1, "accept"); .Ed .Sh ERRORS .Fn accept and .Fn accept4 will fail if: .Bl -tag -width Er .It Bq Er EBADF The descriptor is invalid. .It Bq Er ENOTSOCK The descriptor doesn't reference a socket. .It Bq Er EOPNOTSUPP The referenced socket is not of type .Dv SOCK_STREAM . .It Bq Er EINTR A signal was caught before a connection arrived. .It Bq Er EINVAL The referenced socket is not listening for connections (that is, .Xr listen 2 has not yet been called). .It Bq Er EFAULT The .Fa addr or .Fa addrlen parameter is not in a valid part of the process address space. .It Bq Er EWOULDBLOCK The socket is marked non-blocking and no connections are present to be accepted. .It Bq Er EMFILE The per-process descriptor table is full. .It Bq Er ENFILE The system file table is full. .It Bq Er ECONNABORTED A connection has been aborted. .El .Pp In addition, .Fn accept4 will fail if .Bl -tag -width Er .It Bq Er EINVAL .Fa flags is invalid. .El .Sh SEE ALSO .Xr bind 2 , .Xr connect 2 , .Xr listen 2 , .Xr poll 2 , .Xr select 2 , .Xr socket 2 .Sh STANDARDS The .Fn accept function conforms to .St -p1003.1-2008 . The .Fn accept4 function is expected to conform to a future revision of that standard. .Sh HISTORY The .Fn accept system call first appeared in .Bx 4.1c and .Fn accept4 in .Ox 5.7 . .Sh CAVEATS When .Er EMFILE or .Er ENFILE is returned, new connections are neither dequeued nor discarded. Thus considerable care is required in .Xr select 2 and .Xr poll 2 loops.