ACCEPT(2) System Calls Manual ACCEPT(2)

accept, accept4, paccept — accept a connection on a socket

Standard C Library (libc, -lc)

#include <sys/socket.h>

int
accept(int s, struct sockaddr * restrict addr, socklen_t * restrict addrlen);

int
accept4(int s, struct sockaddr * restrict addr, socklen_t * restrict addrlen, int flags);

int
paccept(int s, struct sockaddr * restrict addr, socklen_t * restrict addrlen, const sigset_t * restrict sigmask, int flags);

The argument s is a socket that has been created with socket(2), bound to an address with bind(2), and is listening for connections after a listen(2). The () function extracts the first connection request on the queue of pending connections, creates a new socket with the same properties of s and allocates a new file descriptor for the socket. If no pending connections are present on the queue, and the socket is not marked as non-blocking, accept() blocks the caller until a connection is present. If the socket is marked non-blocking and no pending connections are present on the queue, accept() returns an error as described below. The accepted socket may not be used to accept more connections. The original socket s remains open.

The argument addr is a result parameter that is filled in with the address of the connecting entity, as known to the communications layer. The exact format of the addr parameter is determined by the domain in which the communication is occurring. The addrlen is a value-result parameter; it should initially contain the amount of space pointed to by addr; on return it will contain the actual length (in bytes) of the address returned. This call is used with connection-based socket types, currently with SOCK_STREAM.

It is possible to select(2) or poll(2) a socket for the purposes of doing an () by selecting or polling it for read.

For certain protocols which require an explicit confirmation, such as ISO or DATAKIT, () 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.

One can obtain user connection request data without confirming the connection by issuing a recvmsg(2) call with an msg_iovlen of 0 and a non-zero msg_controllen, or by issuing a getsockopt(2) request. Similarly, one can provide user connection rejection information by issuing a sendmsg(2) call with providing only the control information, or by calling setsockopt(2).

The socket returned by () inherits the O_NONBLOCK setting of s. This is a nonstandard guarantee; portable applications should not rely it.

The () function behaves exactly like accept(), but the socket it returns does not inherit the O_NONBLOCK flag of s; instead, it applies the following bits set in flags to the returned file descriptor:

Set the close-on-exec property.
Set the close-on-fork property.
Sets non-blocking I/O.
Return EPIPE instead of raising SIGPIPE.

The () function behaves exactly like accept4(), but it also accepts a signal mask sigmask. If sigmask is non-NULL, paccept() replaces the signal mask of the calling thread by it while paccept() is waiting for a connection, and then restores the signal mask on return.

The () function is equivalent to paccept() with NULL as the argument for sigmask.

The accept(), accept4(), and paccept() functions return -1 and set errno(2) on error. If they succeed, they return a non-negative integer that is a descriptor for the accepted socket.

The accept() implementation makes the new file descriptor inherit file flags (like O_NONBLOCK) from the listening socket. It's a traditional behaviour for BSD derivative systems. On the other hand, there are implementations which don't do so. Linux is an example of such implementations. Portable programs should not rely on either of the behaviours.

The accept() and accept4() functions conform to IEEE Std 1003.1-2024 (“POSIX.1”).

The non-standard paccept() function is compatible with the Linux implementation.

The accept() function will fail if:

[]
The socket is marked non-blocking and no connections are present to be accepted.
[]
The descriptor is invalid.
[]
A connection has been aborted.
[]
The addr parameter is not in a writable part of the user address space.
[]
The accept() call has been interrupted by a signal.
[]
The socket has not been set up to accept connections (using bind(2) and listen(2)).
[]
The per-process descriptor table is full.
[]
The system file table is full.
[]
The descriptor references a file, not a socket.
[]
The referenced socket is not of type SOCK_STREAM.

bind(2), connect(2), listen(2), poll(2), select(2), socket(2)

The accept() function appeared in 4.2BSD. The paccept() function appeared in NetBSD 6.0. The accept4() function appeared in NetBSD 8.0.

July 8, 2025 NetBSD 11.0