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ACCEPT(2)		       System Calls Manual		       ACCEPT(2)

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

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     #include <sys/types.h>
     #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);

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  lis-
     ten(2).   The accept() system call extracts the first connection request on
     the queue of pending connections, creates a new socket, and allocates a new
     file descriptor for the socket which inherits the state of  the  O_NONBLOCK
     and O_ASYNC properties and the destination of SIGIO and SIGURG signals from
     the original socket s.

     The  accept4()  system  call is similar, but the O_NONBLOCK property of the
     new socket is instead determined by the SOCK_NONBLOCK flag in the flags ar-
     gument, the O_ASYNC property is cleared, the signal destination is  cleared
     and  the  close-on-exec  flag on the new file descriptor can be set via the
     SOCK_CLOEXEC flag in the flags argument.  Similarly, the O_CLOFORK property
     can be set via the SOCK_CLOFORK flag in the flags argument.

     If no pending connections are present on the queue, and the original socket
     is not marked as non-blocking, accept() blocks the caller until  a  connec-
     tion  is  present.   If  the  original 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 con-
     nections.	The original socket s remains open.

     The argument addr is a result argument that is filled-in with  the  address
     of  the connecting entity, as known to the communications layer.  The exact
     format of the addr argument is determined by the domain in which the commu-
     nication is occurring.  A null pointer may be specified for addr if the ad-
     dress information is not desired; in this case, addrlen  is  not  used  and
     should also be null.  Otherwise, the addrlen argument is a value-result ar-
     gument; 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 re-
     turned.  This call is used with connection-based  socket  types,  currently
     with SOCK_STREAM.

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

     For certain protocols which require an explicit confirmation, such  as  ISO
     or  DATAKIT,  accept() can be thought of as merely dequeueing the next con-
     nection 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.

     For  some applications, performance may be enhanced by using an accept_fil-
     ter(9) to pre-process incoming connections.

     When using accept(), portable programs should not rely  on  the  O_NONBLOCK
     and  O_ASYNC  properties  and  the  signal destination being inherited, but
     should set them explicitly using fcntl(2); accept4() sets these  properties
     consistently, but may not be fully portable across Unix platforms.

RETURN VALUES
     These  calls  return -1 on error.	If they succeed, they return a non-nega-
     tive integer that is a descriptor for the accepted socket.

ERRORS
     The accept() and accept4() system calls will fail if:

     [EBADF]		The descriptor is invalid.

     [EINTR]		The accept() operation was interrupted.

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

     [ENFILE]		The system file table is full.

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

     [EINVAL]		listen(2) has not been called on the socket descriptor.

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

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

     [ECONNABORTED]	A connection arrived, but it was closed while waiting on
			the listen queue.

     The accept4() system call will also fail if:

     [EINVAL]		The flags argument is invalid.

SEE ALSO
     bind(2),  connect(2), getpeername(2), getsockname(2), listen(2), select(2),
     socket(2), accept_filter(9)

HISTORY
     The accept() system call appeared in 4.2BSD.

     The accept4() system call appeared in FreeBSD 10.0.

     The SOCK_CLOFORK flag appeared in FreeBSD 15.0.

FreeBSD ports 15.quarterly	  May 17, 2025			       ACCEPT(2)

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