The Open Group Base Specifications Issue 7, 2018 edition
IEEE Std 1003.1-2017 (Revision of IEEE Std 1003.1-2008)
Copyright © 2001-2018 IEEE and The Open Group


accept - accept a new connection on a socket


#include <sys/socket.h>

int accept(int
socket, struct sockaddr *restrict address,
       socklen_t *restrict


The accept() function shall extract the first connection on the queue of pending connections, create a new socket with the same socket type protocol and address family as the specified socket, and allocate a new file descriptor for that socket. The file descriptor shall be allocated as described in File Descriptor Allocation.

The accept() function takes the following arguments:

Specifies a socket that was created with socket(), has been bound to an address with bind(), and has issued a successful call to listen().
Either a null pointer, or a pointer to a sockaddr structure where the address of the connecting socket shall be returned.
Either a null pointer, if address is a null pointer, or a pointer to a socklen_t object which on input specifies the length of the supplied sockaddr structure, and on output specifies the length of the stored address.

If address is not a null pointer, the address of the peer for the accepted connection shall be stored in the sockaddr structure pointed to by address, and the length of this address shall be stored in the object pointed to by address_len.

If the actual length of the address is greater than the length of the supplied sockaddr structure, the stored address shall be truncated.

If the protocol permits connections by unbound clients, and the peer is not bound, then the value stored in the object pointed to by address is unspecified.

If the listen queue is empty of connection requests and O_NONBLOCK is not set on the file descriptor for the socket, accept() shall block until a connection is present. If the listen() queue is empty of connection requests and O_NONBLOCK is set on the file descriptor for the socket, accept() shall fail and set errno to [EAGAIN] or [EWOULDBLOCK].

The accepted socket cannot itself accept more connections. The original socket remains open and can accept more connections.


Upon successful completion, accept() shall return the non-negative file descriptor of the accepted socket. Otherwise, -1 shall be returned, errno shall be set to indicate the error, and any object pointed to by address_len shall remain unchanged.


The accept() function shall fail if:

O_NONBLOCK is set for the socket file descriptor and no connections are present to be accepted.
The socket argument is not a valid file descriptor.
A connection has been aborted.
The accept() function was interrupted by a signal that was caught before a valid connection arrived.
The socket is not accepting connections.
All file descriptors available to the process are currently open.
The maximum number of file descriptors in the system are already open.
No buffer space is available.
There was insufficient memory available to complete the operation.
The socket argument does not refer to a socket.
The socket type of the specified socket does not support accepting connections.

The accept() function may fail if:

A protocol error has occurred; [OB XSR] [Option Start]  for example, the STREAMS protocol stack has not been initialized. [Option End]

The following sections are informative.




When a connection is available, select() indicates that the file descriptor for the socket is ready for reading.






File Descriptor Allocation, bind, connect, listen, socket

XBD <sys/socket.h>


First released in Issue 6. Derived from the XNS, Issue 5.2 specification.

The restrict keyword is added to the accept() prototype for alignment with the ISO/IEC 9899:1999 standard.

Issue 7

SD5-XBD-ERN-4 is applied, changing the definition of the [EMFILE] error.

Austin Group Interpretation 1003.1-2001 #044 is applied, changing the ``may fail'' [ENOBUFS] and [ENOMEM] errors to become ``shall fail'' errors.

Functionality relating to XSI STREAMS is marked obsolescent.

POSIX.1-2008, Technical Corrigendum 1, XSH/TC1-2008/0018 [464] is applied.

POSIX.1-2008, Technical Corrigendum 2, XSH/TC2-2008/0035 [835] and XSH/TC2-2008/0036 [836] are applied.

End of informative text.


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