IABSD.fr/src/lib/libc/sys/accept.2

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• Author : guenther
  Date : 2025-08-04 04:59:30
  Hash : 42a7be81
  Message : Implement the POSIX-2024 close-on-fork flag, but modified to be reset on exec as preserving it across exec is not necessary for its original purpose and has security and usability concerns. Many thanks to Ricardo Branco (rbranco (at) suse.de) who did an independent implementation, caught that /dev/fd/* needed to be handled, and provided a port of the illumos test suite. Thanks to tb@ for assistance with that. ok deraadt@

• lib/libc/sys/accept.2

• .\"	$OpenBSD: accept.2,v 1.31 2025/08/04 04:59:31 guenther Exp $
  .\"	$NetBSD: accept.2,v 1.7 1996/01/31 20:14:42 mycroft Exp $
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  .\"     @(#)accept.2	8.2 (Berkeley) 12/11/93
  .\"
  .Dd $Mdocdate: August 4 2025 $
  .Dt ACCEPT 2
  .Os
  .Sh NAME
  .Nm accept ,
  .Nm accept4
  .Nd accept a connection on a socket
  .Sh SYNOPSIS
  .In sys/socket.h
  .Ft int
  .Fn accept "int s" "struct sockaddr *addr" "socklen_t *addrlen"
  .Ft int
  .Fn accept4 "int s" "struct sockaddr *addr" "socklen_t *addrlen" "int flags"
  .Sh DESCRIPTION
  The argument
  .Fa s
  is a socket that has been created with
  .Xr socket 2 ,
  bound to an address with
  .Xr bind 2 ,
  and is listening for connections after a
  .Xr listen 2 .
  The
  .Fn accept
  call extracts the first connection request on the queue of pending
  connections, creates a new socket with the same non-blocking I/O mode as
  .Fa s ,
  and allocates a new file descriptor for the socket with the
  close-on-exec and close-on-fork flags clear.
  .Pp
  The
  .Fn accept4
  system call is similar, however the non-blocking I/O mode,
  close-on-exec flag,
  and close-on-fork flag on the new file descriptor are
  determined by the
  .Dv SOCK_NONBLOCK , SOCK_CLOEXEC ,
  and
  .Dv SOCK_CLOFORK
  flags, respectively, in the
  .Fa flags
  argument.
  .Pp
  If no pending connections are present on the queue,
  and the socket is not marked as non-blocking,
  .Fn 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,
  .Fn accept
  returns an error as described below.
  The accepted socket may not be used to accept more connections.
  The original socket
  .Fa s
  remains open.
  .Pp
  The argument
  .Fa 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
  .Fa addr
  parameter is determined by the domain in which the communication
  is occurring.
  The structure
  .Vt sockaddr_storage
  exists for greater portability.
  It is large enough to hold any of the types that may be returned in the
  .Fa addr
  parameter.
  .Pp
  The
  .Fa addrlen
  is a value-result parameter; it should initially contain the
  amount of space pointed to by
  .Fa addr ;
  on return it will contain the actual length (in bytes) of the
  address returned.
  If
  .Fa addrlen
  does not point to enough space to hold the entire socket address, the
  result will be truncated to the initial value of
  .Fa addrlen
  (in bytes).
  This call is used with connection-based socket types, currently with
  .Dv SOCK_STREAM .
  .Pp
  It is possible to
  .Xr select 2
  or
  .Xr poll 2
  a socket for the purposes of doing an
  .Fn accept
  by selecting it for read.
  .Sh RETURN VALUES
  If successful,
  .Fn accept
  and
  .Fn accept4
  return a non-negative integer, the accepted socket file descriptor.
  Otherwise, a value of \-1 is returned and
  .Va errno
  is set to indicate the error.
  .Sh EXAMPLES
  The following code uses struct
  .Vt sockaddr_storage
  to allocate enough space for the returned address:
  .Bd -literal -offset indent
  #include <sys/types.h>
  #include <sys/socket.h>
  
  struct sockaddr_storage addr;
  socklen_t len = sizeof(addr);
  int retcode;
  
  retcode = accept(s, (struct sockaddr *)&addr, &len);
  if (retcode == -1)
  	err(1, "accept");
  .Ed
  .Sh ERRORS
  .Fn accept
  and
  .Fn accept4
  will fail if:
  .Bl -tag -width Er
  .It Bq Er EBADF
  The descriptor is invalid.
  .It Bq Er ENOTSOCK
  The descriptor doesn't reference a socket.
  .It Bq Er EOPNOTSUPP
  The referenced socket is not of type
  .Dv SOCK_STREAM .
  .It Bq Er EINTR
  A signal was caught before a connection arrived.
  .It Bq Er EINVAL
  The referenced socket is not listening for connections (that is,
  .Xr listen 2
  has not yet been called).
  .It Bq Er EFAULT
  The
  .Fa addr
  or
  .Fa addrlen
  parameter is not in a valid part of the process address space.
  .It Bq Er EWOULDBLOCK
  The socket is marked non-blocking and no connections
  are present to be accepted.
  .It Bq Er EMFILE
  The per-process descriptor table is full.
  .It Bq Er ENFILE
  The system file table is full.
  .It Bq Er ECONNABORTED
  A connection has been aborted.
  .El
  .Pp
  In addition,
  .Fn accept4
  will fail if
  .Bl -tag -width Er
  .It Bq Er EINVAL
  .Fa flags
  is invalid.
  .El
  .Sh SEE ALSO
  .Xr bind 2 ,
  .Xr connect 2 ,
  .Xr listen 2 ,
  .Xr poll 2 ,
  .Xr select 2 ,
  .Xr socket 2
  .Sh STANDARDS
  The
  .Fn accept
  and
  .Fn accept4
  functions conform to
  .St -p1003.1-2024 .
  .Sh HISTORY
  The
  .Fn accept
  system call first appeared in
  .Bx 4.1c
  and
  .Fn accept4
  in
  .Ox 5.7 .
  .Sh CAVEATS
  When
  .Er EMFILE
  or
  .Er ENFILE
  is returned,
  new connections are neither dequeued nor discarded.
  Thus considerable care is required in
  .Xr select 2
  and
  .Xr poll 2
  loops.