accept, accept4 - accept a connection on a socket
#include <sys/types.h> /* See NOTES */
int accept(int sockfd, struct sockaddr *addr, socklen_t *addrlen);
#define _GNU_SOURCE /* See feature_test_macros(7) */
int accept4(int sockfd, struct sockaddr *addr,
socklen_t *addrlen, int flags);
() system call is used with connection-based socket types
). It extracts the first connection
request on the queue of pending connections for the listening socket,
, 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
, 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 is filled in with the address of the peer socket, as known to the
communications layer. The exact format of the address returned addr
determined by the socket's address family (see socket(2)
respective protocol man pages). When addr
is NULL, nothing is filled
in; in this case, addrlen
is not used, and should also be NULL.
argument is a value-result argument: the caller must
initialize it to contain the size (in bytes) of the structure pointed to by
; 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
will return a value greater than was supplied to the
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
In order to be notified of incoming connections on a socket, you can use
. 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
when activity occurs on a socket; see socket(7)
For certain protocols which require an explicit confirmation, such as DECNet,
() 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.
is 0, then accept4
() is the same as accept
following values can be bitwise ORed in flags
to obtain different
- 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.
- 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
() (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
() and treat them like EAGAIN
by retrying. In the case of
TCP/IP, these are ENETDOWN
- 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.
- The descriptor is invalid.
- A connection has been aborted.
- The addr argument is not in a writable part of the
user address space.
- The system call was interrupted by a signal that was caught
before a valid connection arrived; see signal(7).
- Socket is not listening for connections, or addrlen
is invalid (e.g., is negative).
- (accept4()) invalid value in flags.
- The per-process limit of open file descriptors has been
- The system limit on the total number of open files has been
- ENOBUFS, ENOMEM
- Not enough free memory. This often means that the memory
allocation is limited by the socket buffer limits, not by the system
- The descriptor references a file, not a socket.
- The referenced socket is not of type
- Protocol error.
In addition, Linux accept
() may fail if:
- 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
. The value ERESTARTSYS
may be seen during a trace.
() system call is available starting with Linux 2.6.28;
support in glibc is available starting with version 2.10.
(): POSIX.1-2001, SVr4, 4.4BSD, (accept
() first appeared in
() is a nonstandard Linux extension.
On Linux, the new socket returned by accept
() does not
file status flags such as O_NONBLOCK
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>
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
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
() never blocks, the passed socket sockfd
needs to have
flag set (see socket(7)
The socklen_t type¶
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
, 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
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
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)."
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