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ACCEPT(2)                     System Calls Manual                    ACCEPT(2)

NAME
     accept, accept4, paccept - accept a connection on a socket

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     #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);

DESCRIPTION
     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 accept() 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 accept() by selecting or polling it for read.

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

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

     The accept4() 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:

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

     The paccept() 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 accept4() function is equivalent to paccept() with NULL as the
     argument for sigmask.

RETURN VALUES
     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.

COMPATIBILITY
     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.

ERRORS
     The accept() function will fail if:

     [EAGAIN]           The socket is marked non-blocking and no connections
                        are present to be accepted.

     [EBADF]            The descriptor is invalid.

     [ECONNABORTED]     A connection has been aborted.

     [EFAULT]           The addr parameter is not in a writable part of the
                        user address space.

     [EINTR]            The accept() call has been interrupted by a signal.

     [EINVAL]           The socket has not been set up to accept connections
                        (using bind(2) and listen(2)).

     [EMFILE]           The per-process descriptor table is full.

     [ENFILE]           The system file table is full.

     [ENOTSOCK]         The descriptor references a file, not a socket.

     [EOPNOTSUPP]       The referenced socket is not of type SOCK_STREAM.

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

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

NetBSD 11.99                     July 8, 2025                     NetBSD 11.99