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ACCEPT(2) Linux Programmer's Manual ACCEPT(2)
accept - 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) or poll(2). 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.
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.
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
descriptor for the accepted socket. On error, -1 is returned, and errno is
set appropriately.
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 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 allows either error to be returned for this
case, and does not require these constants to have the same value, so a
portable application should check for both possibilities.
EBADF The descriptor is invalid.
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 of open file descriptors has been reached.
ENFILE The system 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 descriptor references a file, not 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, 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) or poll(2) 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)).
The third argument of accept() was originally declared as an int * (and is
that under libc4 and libc5 and on many other systems like 4.x BSD, SunOS 4,
SGI); a POSIX.1g draft standard wanted to change it into a size_t *, and that
is what it is for SunOS 5. Later POSIX drafts have socklen_t *, and so do the
Single UNIX Specification and glibc2. Quoting Linus Torvalds:
"_Any_ sane library _must_ have "socklen_t" be the same size as int. Anything
else breaks any BSD socket layer stuff. POSIX initially did make it a size_t,
and I (and hopefully others, but obviously not too many) complained to them
very loudly indeed. Making it a size_t is completely broken, exactly because
size_t very seldom is the same size as "int" on 64-bit architectures, for
example. And it has to be the same size as "int" because that's what the BSD
socket interface is. Anyway, the POSIX people eventually got a clue, and
created "socklen_t". They shouldn't have touched it in the first place, but
once they did they felt it had to have a named type for some unfathomable
reason (probably somebody didn't like losing face over having done the
original stupid thing, so they silently just renamed their blunder)."
See bind(2).
bind(2), connect(2), listen(2), select(2), socket(2), socket(7)
This page is part of release 3.32 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can be found
at http://www.kernel.org/doc/man-pages/.
Linux 2010-09-10 ACCEPT(2)
HTML rendering created 2010-12-03 by Michael Kerrisk, author of The Linux Programming Interface