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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | CONFORMING TO | NOTES | EXAMPLE | SEE ALSO | COLOPHON |
ACCEPT(2) Linux Programmer's Manual ACCEPT(2)
accept, accept4 - accept a connection on a socket
#include <sys/types.h> /* See NOTES */
#include <sys/socket.h>
int accept(int sockfd, struct sockaddr *addr, socklen_t *addrlen);
#define _GNU_SOURCE /* See feature_test_macros(7) */
#include <sys/socket.h>
int accept4(int sockfd, struct sockaddr *addr,
socklen_t *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 new open
file description. 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 nonnegative integer that is a
file descriptor for the accepted socket. On error, -1 is returned,
and errno is set appropriately.
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.
EPROTO Protocol error.
In addition, Linux accept() may fail if:
EPERM Firewall rules forbid connection.
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.
The accept4() system call is available starting with Linux 2.6.28;
support in glibc is available starting with version 2.10.
accept(): POSIX.1-2001, POSIX.1-2008, SVr4, 4.4BSD (accept() first
appeared in 4.2BSD).
accept4() is a nonstandard Linux extension.
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().
POSIX.1-2001 does not require the inclusion of <sys/types.h>, and
this header file is not required on Linux. However, some historical
(BSD) implementations required this header file, and portable
applications are probably wise to include it.
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 *C; 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 release 4.12 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
Linux 2016-10-08 ACCEPT(2)
Pages that refer to this page: bind(2), connect(2), getpeername(2), getsockname(2), getsockopt(2), listen(2), recv(2), select(2), select_tut(2), socket(2), socketcall(2), syscalls(2), getaddrinfo(3), gethostbyname(3), getnameinfo(3), capabilities(7), ddp(7), ip(7), sctp(7), signal(7), signal-safety(7), sock_diag(7), socket(7), tcp(7), unix(7)