.\" Copyright (c) 1983, 1990, 1991 The Regents of the University of California. .\" All rights reserved. .\" .\" SPDX-License-Identifier: BSD-4-Clause-UC .\" .\" Modified 1993-07-24 by Rik Faith .\" Modified 1996-10-21 by Eric S. Raymond .\" Modified 1998-2000 by Andi Kleen to match Linux 2.2 reality .\" Modified 2002-04-23 by Roger Luethi .\" Modified 2004-06-17 by Michael Kerrisk .\" 2008-12-04, mtk, Add documentation of accept4() .\" .TH accept 2 2022-12-04 "Linux man-pages 6.03" .SH NAME accept, accept4 \- accept a connection on a socket .SH LIBRARY Standard C library .RI ( libc ", " \-lc ) .SH SYNOPSIS .nf .B #include .PP .BI "int accept(int " sockfd ", struct sockaddr *_Nullable restrict " addr , .BI " socklen_t *_Nullable restrict " addrlen ); .PP .BR "#define _GNU_SOURCE" " /* See feature_test_macros(7) */" .B #include .PP .BI "int accept4(int " sockfd ", struct sockaddr *_Nullable restrict " addr , .BI " socklen_t *_Nullable restrict " addrlen ", int " flags ); .fi .SH DESCRIPTION The .BR accept () system call is used with connection-based socket types .RB ( SOCK_STREAM , .BR SOCK_SEQPACKET ). It extracts the first connection request on the queue of pending connections for the listening socket, .IR sockfd , creates a new connected socket, and returns a new file descriptor referring to that socket. The newly created socket is not in the listening state. The original socket .I sockfd is unaffected by this call. .PP The argument .I sockfd is a socket that has been created with .BR socket (2), bound to a local address with .BR bind (2), and is listening for connections after a .BR listen (2). .PP The argument .I addr is a pointer to a .I sockaddr structure. This structure is filled in with the address of the peer socket, as known to the communications layer. The exact format of the address returned .I addr is determined by the socket's address family (see .BR socket (2) and the respective protocol man pages). When .I addr is NULL, nothing is filled in; in this case, .I addrlen is not used, and should also be NULL. .PP The .I addrlen argument is a value-result argument: the caller must initialize it to contain the size (in bytes) of the structure pointed to by .IR addr ; on return it will contain the actual size of the peer address. .PP The returned address is truncated if the buffer provided is too small; in this case, .I addrlen will return a value greater than was supplied to the call. .PP If no pending connections are present on the queue, and the socket is not marked as nonblocking, .BR accept () blocks the caller until a connection is present. If the socket is marked nonblocking and no pending connections are present on the queue, .BR accept () fails with the error .B EAGAIN or .BR EWOULDBLOCK . .PP In order to be notified of incoming connections on a socket, you can use .BR select (2), .BR poll (2), or .BR epoll (7). A readable event will be delivered when a new connection is attempted and you may then call .BR accept () to get a socket for that connection. Alternatively, you can set the socket to deliver .B SIGIO when activity occurs on a socket; see .BR socket (7) for details. .PP If .I flags is 0, then .BR accept4 () is the same as .BR accept (). The following values can be bitwise ORed in .I flags to obtain different behavior: .TP 16 .B SOCK_NONBLOCK Set the .B O_NONBLOCK file status flag on the open file description (see .BR open (2)) referred to by the new file descriptor. Using this flag saves extra calls to .BR fcntl (2) to achieve the same result. .TP .B SOCK_CLOEXEC Set the close-on-exec .RB ( FD_CLOEXEC ) flag on the new file descriptor. See the description of the .B O_CLOEXEC flag in .BR open (2) for reasons why this may be useful. .SH RETURN VALUE On success, these system calls return a file descriptor for the accepted socket (a nonnegative integer). On error, \-1 is returned, .I errno is set to indicate the error, and .I addrlen is left unchanged. .SS Error handling Linux .BR accept () (and .BR accept4 ()) passes already-pending network errors on the new socket as an error code from .BR accept (). This behavior differs from other BSD socket implementations. For reliable operation the application should detect the network errors defined for the protocol after .BR accept () and treat them like .B EAGAIN by retrying. In the case of TCP/IP, these are .BR ENETDOWN , .BR EPROTO , .BR ENOPROTOOPT , .BR EHOSTDOWN , .BR ENONET , .BR EHOSTUNREACH , .BR EOPNOTSUPP , and .BR ENETUNREACH . .SH ERRORS .TP .BR EAGAIN " or " EWOULDBLOCK .\" Actually EAGAIN on Linux The socket is marked nonblocking and no connections are present to be accepted. POSIX.1-2001 and POSIX.1-2008 allow either error to be returned for this case, and do not require these constants to have the same value, so a portable application should check for both possibilities. .TP .B EBADF .I sockfd is not an open file descriptor. .TP .B ECONNABORTED A connection has been aborted. .TP .B EFAULT The .I addr argument is not in a writable part of the user address space. .TP .B EINTR The system call was interrupted by a signal that was caught before a valid connection arrived; see .BR signal (7). .TP .B EINVAL Socket is not listening for connections, or .I addrlen is invalid (e.g., is negative). .TP .B EINVAL .RB ( accept4 ()) invalid value in .IR flags . .TP .B EMFILE The per-process limit on the number of open file descriptors has been reached. .TP .B ENFILE The system-wide limit on the total number of open files has been reached. .TP .BR ENOBUFS ", " ENOMEM Not enough free memory. This often means that the memory allocation is limited by the socket buffer limits, not by the system memory. .TP .B ENOTSOCK The file descriptor .I sockfd does not refer to a socket. .TP .B EOPNOTSUPP The referenced socket is not of type .BR SOCK_STREAM . .TP .B EPERM Firewall rules forbid connection. .TP .B EPROTO Protocol error. .PP In addition, network errors for the new socket and as defined for the protocol may be returned. Various Linux kernels can return other errors such as .BR ENOSR , .BR ESOCKTNOSUPPORT , .BR EPROTONOSUPPORT , .BR ETIMEDOUT . The value .B ERESTARTSYS may be seen during a trace. .SH VERSIONS The .BR accept4 () system call is available starting with Linux 2.6.28; support in glibc is available starting with glibc 2.10. .SH STANDARDS .BR accept (): POSIX.1-2001, POSIX.1-2008, SVr4, 4.4BSD .RB ( accept () first appeared in 4.2BSD). .\" The BSD man page documents five possible error returns .\" (EBADF, ENOTSOCK, EOPNOTSUPP, EWOULDBLOCK, EFAULT). .\" POSIX.1-2001 documents errors .\" EAGAIN, EBADF, ECONNABORTED, EINTR, EINVAL, EMFILE, .\" ENFILE, ENOBUFS, ENOMEM, ENOTSOCK, EOPNOTSUPP, EPROTO, EWOULDBLOCK. .\" In addition, SUSv2 documents EFAULT and ENOSR. .PP .BR accept4 () is a nonstandard Linux extension. .PP On Linux, the new socket returned by .BR accept () does \fInot\fP inherit file status flags such as .B O_NONBLOCK and .B O_ASYNC from the listening socket. This behavior differs from the canonical BSD sockets implementation. .\" Some testing seems to show that Tru64 5.1 and HP-UX 11 also .\" do not inherit file status flags -- MTK Jun 05 Portable programs should not rely on inheritance or noninheritance of file status flags and always explicitly set all required flags on the socket returned from .BR accept (). .SH NOTES There may not always be a connection waiting after a .B SIGIO is delivered or .BR select (2), .BR poll (2), or .BR epoll (7) return a readability event because the connection might have been removed by an asynchronous network error or another thread before .BR accept () is called. If this happens, then the call will block waiting for the next connection to arrive. To ensure that .BR accept () never blocks, the passed socket .I sockfd needs to have the .B O_NONBLOCK flag set (see .BR socket (7)). .PP For certain protocols which require an explicit confirmation, such as DECnet, .BR 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. Currently, only DECnet has these semantics on Linux. .\" .SS The socklen_t type In the original BSD sockets implementation (and on other older systems) .\" such as Linux libc4 and libc5, SunOS 4, SGI the third argument of .BR accept () was declared as an \fIint\ *\fP. A POSIX.1g draft standard wanted to change it into a \fIsize_t\ *\fPC; .\" SunOS 5 has 'size_t *' later POSIX standards and glibc 2.x have .IR "socklen_t\ * ". .SH EXAMPLES See .BR bind (2). .SH SEE ALSO .BR bind (2), .BR connect (2), .BR listen (2), .BR select (2), .BR socket (2), .BR socket (7)