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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | STANDARDS | HISTORY | NOTES | EXAMPLES | SEE ALSO | COLOPHON |
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accept(2) System Calls Manual accept(2)
accept, accept4 - accept a connection on a socket
Standard C library (libc, -lc)
#include <sys/socket.h>
int accept(int sockfd, struct sockaddr *_Nullable restrict addr,
socklen_t *_Nullable restrict addrlen);
#define _GNU_SOURCE /* See feature_test_macros(7) */
#include <sys/socket.h>
int accept4(int sockfd, struct sockaddr *_Nullable restrict addr,
socklen_t *_Nullable restrict addrlen, int flags);
The accept() system call is used with connection-based socket
types (SOCK_STREAM, SOCK_SEQPACKET). It extracts the first
connection request on the queue of pending connections for the
listening socket, 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
sockfd is unaffected by this call.
The argument sockfd is a socket that has been created with
socket(2), bound to a local address with bind(2), and is listening
for connections after a listen(2).
The argument addr is a pointer to a 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 addr is determined by the socket's address family
(see socket(2) and the respective protocol man pages). When addr
is NULL, nothing is filled in; in this case, addrlen is not used,
and should also be NULL.
The addrlen argument is a value-result argument: the caller must
initialize it to contain the size (in bytes) of the structure
pointed to by addr; on return it will contain the actual size of
the peer address.
The returned address is truncated if the buffer provided is too
small; in this case, addrlen will return a value greater than was
supplied to the call.
If no pending connections are present on the queue, and the socket
is not marked as nonblocking, accept() blocks the caller until a
connection is present. If the socket is marked nonblocking and no
pending connections are present on the queue, accept() fails with
the error EAGAIN or EWOULDBLOCK.
In order to be notified of incoming connections on a socket, you
can use select(2), poll(2), or epoll(7). A readable event will be
delivered when a new connection is attempted and you may then call
accept() to get a socket for that connection. Alternatively, you
can set the socket to deliver SIGIO when activity occurs on a
socket; see socket(7) for details.
If flags is 0, then accept4() is the same as accept(). The
following values can be bitwise ORed in flags to obtain different
behavior:
SOCK_NONBLOCK
Set the O_NONBLOCK file status flag on the open file
description (see open(2)) referred to by the new file
descriptor. Using this flag saves extra calls to fcntl(2)
to achieve the same result.
SOCK_CLOEXEC
Set the close-on-exec (FD_CLOEXEC) flag on the new file
descriptor. See the description of the O_CLOEXEC flag in
open(2) for reasons why this may be useful.
On success, these system calls return a file descriptor for the
accepted socket (a nonnegative integer). On error, -1 is
returned, errno is set to indicate the error, and addrlen is left
unchanged.
Error handling
Linux accept() (and accept4()) passes already-pending network
errors on the new socket as an error code from accept(). This
behavior differs from other BSD socket implementations. For
reliable operation the application should detect the network
errors defined for the protocol after accept() and treat them like
EAGAIN by retrying. In the case of TCP/IP, these are ENETDOWN,
EPROTO, ENOPROTOOPT, EHOSTDOWN, ENONET, EHOSTUNREACH, EOPNOTSUPP,
and ENETUNREACH.
EAGAIN or EWOULDBLOCK
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.
EBADF sockfd is not an open file descriptor.
ECONNABORTED
A connection has been aborted.
EFAULT The addr argument is not in a writable part of the user
address space.
EINTR The system call was interrupted by a signal that was caught
before a valid connection arrived; see signal(7).
EINVAL Socket is not listening for connections, or addrlen is
invalid (e.g., is negative).
EINVAL (accept4()) invalid value in flags.
EMFILE The per-process limit on the number of open file
descriptors has been reached.
ENFILE The system-wide limit on the total number of open files has
been reached.
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.
ENOTSOCK
The file descriptor sockfd does not refer to a socket.
EOPNOTSUPP
The referenced socket is not of type SOCK_STREAM.
EPERM Firewall rules forbid connection.
EPROTO Protocol error.
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 ENOSR, ESOCKTNOSUPPORT, EPROTONOSUPPORT,
ETIMEDOUT. The value ERESTARTSYS may be seen during a trace.
On Linux, the new socket returned by accept() does not inherit
file status flags such as O_NONBLOCK and O_ASYNC from the
listening socket. This behavior differs from the canonical BSD
sockets implementation. 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
accept().
accept()
POSIX.1-2008.
accept4()
Linux.
accept()
POSIX.1-2001, SVr4, 4.4BSD (accept() first appeared in
4.2BSD).
accept4()
Linux 2.6.28, glibc 2.10.
There may not always be a connection waiting after a SIGIO is
delivered or select(2), poll(2), or epoll(7) return a readability
event because the connection might have been removed by an
asynchronous network error or another thread before accept() is
called. If this happens, then the call will block waiting for the
next connection to arrive. To ensure that accept() never blocks,
the passed socket sockfd needs to have the O_NONBLOCK flag set
(see socket(7)).
For certain protocols which require an explicit confirmation, such
as DECnet, 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.
The socklen_t type
In the original BSD sockets implementation (and on other older
systems) the third argument of accept() was declared as an int *.
A POSIX.1g draft standard wanted to change it into a size_t *;
later POSIX standards and glibc 2.x have socklen_t * .
See bind(2).
bind(2), connect(2), listen(2), select(2), socket(2), socket(7)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
[email protected]
Linux man-pages 6.15 2025-05-17 accept(2)
Pages that refer to this page: bind(2), connect(2), getpeername(2), io_uring_enter2(2), io_uring_enter(2), listen(2), recv(2), seccomp_unotify(2), select(2), select_tut(2), socket(2), socketcall(2), syscalls(2), getaddrinfo(3), gethostbyname(3), getnameinfo(3), io_uring_prep_accept(3), io_uring_prep_accept_direct(3), io_uring_prep_multishot_accept(3), io_uring_prep_multishot_accept_direct(3), sockaddr(3type), capabilities(7), ddp(7), ip(7), sctp(7), signal(7), signal-safety(7), sock_diag(7), socket(7), tcp(7), unix(7)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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