accept(2) System Calls Manual accept(2)
NAME
accept, accept4 - accept a connection on a socket
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#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);
DESCRIPTION
The accept() system call is used with connection-based socket types
(SOCK_STREAM, SOCK_SEQPACKET). It extracts the first connection re-
quest on the queue of pending connections for the listening socket,
sockfd, creates a new connected socket, and returns a new file descrip-
tor 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 com-
munications layer. The exact format of the address returned addr is
determined by the socket's address family (see socket(2) and the re-
spective 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 ini-
tialize 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 connec-
tion is present. If the socket is marked nonblocking and no pending
connections are present on the queue, accept() fails with the error EA-
GAIN 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 deliv-
ered 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 fc-
ntl(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.
RETURN VALUE
On success, these system calls return a file descriptor for the ac-
cepted 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 ap-
plication should detect the network errors defined for the protocol af-
ter 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.
ERRORS
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 er-
ror to be returned for this case, and do not require these con-
stants 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 be-
fore 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 allo-
cation 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 er-
rors such as ENOSR, ESOCKTNOSUPPORT, EPROTONOSUPPORT, ETIMEDOUT. The
value ERESTARTSYS may be seen during a trace.
VERSIONS
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().
STANDARDS
accept()
POSIX.1-2008.
accept4()
Linux.
HISTORY
accept()
POSIX.1-2001, SVr4, 4.4BSD (accept() first appeared in 4.2BSD).
accept4()
Linux 2.6.28, glibc 2.10.
NOTES
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 connec-
tion request and not implying confirmation. Confirmation can be im-
plied by a normal read or write on the new file descriptor, and rejec-
tion 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 *C; later POSIX stan-
dards and glibc 2.x have socklen_t * .
EXAMPLES
See bind(2).
SEE ALSO
bind(2), connect(2), listen(2), select(2), socket(2), socket(7)
Linux man-pages 6.7 2023-10-31 accept(2)
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