ACCEPT(3P)

PROLOG
This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.

NAME
accept — accept a new connection on a socket

SYNOPSIS

#include <sys/socket.h>

int accept(int socket, struct sockaddr *restrict address,
    socklen_t *restrict address_len);

DESCRIPTION
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 Section 2.14, File Descriptor Allocation.

The accept() function takes the following arguments:

socket
    Specifies a socket that was created with socket(), has been bound to an address with bind(), and has issued a successful call to listen().

address
    Either a null pointer, or a pointer to a sockaddr structure where the address of the connecting socket shall be returned.

address_len
    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.

RETURN VALUE
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.

ERRORS
The accept() function shall fail if:

EAGAIN or EWOULDBLOCK
    O_NONBLOCK is set for the socket file descriptor and no connections are present to be accepted.

EBADF
    The socket argument is not a valid file descriptor.

ECONNABORTED
    A connection has been aborted.

EINTR
    The accept() function was interrupted by a signal that was caught before a valid connection arrived.

EINVAL
    The socket is not accepting connections.

EMFILE
    All file descriptors available to the process are currently open.

ENFILE
    The maximum number of file descriptors in the system are already open.

ENOBUFS
    No buffer space is available.

ENOMEM
    There was insufficient memory available to complete the operation.

ENOTSOCK
    The socket argument does not refer to a socket.

EOPNOTSUPP
    The socket type of the specified socket does not support accepting connections.

The accept() function may fail if:

EPROTO
    A protocol error has occurred; for example, the STREAMS protocol stack has not been initialized.

The following sections are informative.

EXAMPLES
None.

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

RATIONALE
None.

FUTURE DIRECTIONS
None.

SEE ALSO
Section 2.14, File Descriptor Allocation, bind(), connect(), listen(), socket()

The Base Definitions volume of POSIX.1-2017, <sys_socket.h>

COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online at http://www.opengroup.org/unix/online.html .

Any typographical or formatting errors that appear in this page are most likely to have been introduced during the conversion of the source files to man page format. To report such errors, see https://www.kernel.org/doc/man-pages/reporting_bugs.html .