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ACCEPT(2)		   Linux Programmer's Manual		     ACCEPT(2)

       accept, accept4 - accept a connection on a socket

       #include <sys/types.h>	       /* See NOTES */
       #include <sys/socket.h>

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

       #define _GNU_SOURCE	       /* See feature_test_macros(7) */
       #include <sys/socket.h>

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

       The  accept()  system  call  is used with connection-based socket types
       (SOCK_STREAM,  SOCK_SEQPACKET).	 It  extracts  the  first   connection
       request	on  the queue of pending connections for the listening socket,
       sockfd, creates a new connected socket, and returns a new file descrip-
       tor  referring  to that socket.	The newly created socket is not in the
       listening state.	 The original socket  sockfd  is  unaffected  by  this

       The  argument  sockfd is a socket that has been created with socket(2),
       bound to a local address with bind(2), and is listening for connections
       after a listen(2).

       The argument addr is a pointer to a sockaddr structure.	This structure
       is filled in with the address of the peer socket, as known to the  com-
       munications  layer.   The  exact format of the address returned addr is
       determined by the  socket's  address  family  (see  socket(2)  and  the
       respective  protocol  man pages).  When addr is NULL, nothing is filled
       in; in this case, addrlen is not used, and should also be NULL.

       The addrlen argument is a value-result argument: the caller  must  ini-
       tialize	it  to contain the size (in bytes) of the structure pointed to
       by addr; on return it will contain the actual size of the peer address.

       The returned address is truncated if the buffer provided is too	small;
       in  this case, addrlen will return a value greater than was supplied to
       the call.

       If no pending connections are present on the queue, and the  socket  is
       not  marked  as nonblocking, accept() blocks the caller until a connec-
       tion is present.	 If the socket is marked nonblocking  and  no  pending
       connections  are	 present  on  the queue, accept() fails with the error

       In order to be notified of incoming connections on a  socket,  you  can
       use  select(2)  or  poll(2).  A readable event will be delivered when a
       new connection is attempted and you may then call  accept()  to	get  a
       socket  for  that connection.  Alternatively, you can set the socket to
       deliver SIGIO when activity occurs  on  a  socket;  see	socket(7)  for

       For  certain  protocols which require an explicit confirmation, such as
       DECNet, accept() can be thought of as merely dequeuing the next connec-
       tion  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.	Currently only
       DECNet has these semantics on Linux.

       If flags is 0, then accept4() is the same as accept().	The  following
       values can be bitwise ORed in flags to obtain different behavior:

       SOCK_NONBLOCK   Set  the	 O_NONBLOCK  file  status flag on the new open
		       file description.  Using this flag saves extra calls to
		       fcntl(2) to achieve the same result.

       SOCK_CLOEXEC    Set the close-on-exec (FD_CLOEXEC) flag on the new file
		       descriptor.  See the description of the O_CLOEXEC  flag
		       in open(2) for reasons why this may be useful.

       On  success,  these system calls return a nonnegative integer that is a
       descriptor for the accepted socket.  On	error,	-1  is	returned,  and
       errno is set appropriately.

   Error handling
       Linux accept() (and accept4()) passes already-pending network errors on
       the new socket as an error code from accept().  This  behavior  differs
       from  other  BSD	 socket	 implementations.   For reliable operation the
       application should detect the network errors defined for	 the  protocol
       after  accept() and treat them like EAGAIN by retrying.	In the case of

	      The  socket is marked nonblocking and no connections are present
	      to be accepted.	POSIX.1-2001  and  POSIX.1-2008	 allow	either
	      error  to	 be  returned  for this case, and do not require these
	      constants to have the same  value,  so  a	 portable  application
	      should check for both possibilities.

       EBADF  The descriptor is invalid.

	      A connection has been aborted.

       EFAULT The  addr argument is not in a writable part of the user address

       EINTR  The system call was interrupted by  a  signal  that  was	caught
	      before a valid connection arrived; see signal(7).

       EINVAL Socket  is  not listening for connections, or addrlen is invalid
	      (e.g., is negative).

       EINVAL (accept4()) invalid value in flags.

       EMFILE The per-process limit on the number of open file descriptors has
	      been reached.

       ENFILE The system-wide limit on the total number of open files has been

	      Not enough free memory.  This often means that the memory	 allo-
	      cation is limited by the socket buffer limits, not by the system

	      The file descriptor sockfd does not refer to a socket.

	      The referenced socket is not of type SOCK_STREAM.

       EPROTO Protocol error.

       In addition, Linux accept() may fail if:

       EPERM  Firewall rules forbid connection.

       In addition, network errors for the new socket and as defined  for  the
       protocol	 may  be  returned.   Various  Linux  kernels can return other
       value ERESTARTSYS may be seen during a trace.

       The accept4() system call is available starting with Linux 2.6.28; sup-
       port in glibc is available starting with version 2.10.

       accept(): POSIX.1-2001,	POSIX.1-2008,  SVr4,  4.4BSD  (accept()	 first
       appeared in 4.2BSD).

       accept4() is a nonstandard Linux extension.

       On  Linux,  the	new  socket returned by accept() does not inherit file
       status flags such as O_NONBLOCK and O_ASYNC from the listening  socket.
       This  behavior  differs	from the canonical BSD sockets implementation.
       Portable programs should not rely on inheritance or  noninheritance  of
       file  status  flags and always explicitly set all required flags on the
       socket returned from accept().

       POSIX.1-2001 does not require the inclusion of <sys/types.h>, and  this
       header  file  is not required on Linux.	However, some historical (BSD)
       implementations required this header file,  and	portable  applications
       are probably wise to include it.

       There may not always be a connection waiting after a SIGIO is delivered
       or select(2) or poll(2) return a readability event because the  connec-
       tion  might  have  been	removed	 by  an	 asynchronous network error or
       another thread before accept() is called.  If this  happens,  then  the
       call  will  block waiting for the next connection to arrive.  To ensure
       that accept() never blocks, the passed socket sockfd needs to have  the
       O_NONBLOCK flag set (see socket(7)).

   The socklen_t type
       The third argument of accept() was originally declared as an int * (and
       is that under libc4 and libc5 and on many other systems like  4.x  BSD,
       SunOS  4,  SGI);	 a  POSIX.1g draft standard wanted to change it into a
       size_t *, and that is what it is for SunOS 5.  Later POSIX drafts  have
       socklen_t *, and so do the Single UNIX Specification and glibc2.	 Quot-
       ing Linus Torvalds:

       "_Any_ sane library _must_ have "socklen_t" be the same	size  as  int.
       Anything	 else  breaks any BSD socket layer stuff.  POSIX initially did
       make it a size_t, and I (and hopefully others, but  obviously  not  too
       many)  complained  to  them  very loudly indeed.	 Making it a size_t is
       completely broken, exactly because size_t very seldom is the same  size
       as  "int"  on  64-bit architectures, for example.  And it has to be the
       same size as "int" because that's what the  BSD	socket	interface  is.
       Anyway,	 the   POSIX   people  eventually  got	a  clue,  and  created
       "socklen_t".  They shouldn't have touched it in the  first  place,  but
       once  they  did	they felt it had to have a named type for some unfath-
       omable reason (probably somebody didn't like losing  face  over	having
       done  the  original  stupid  thing, so they silently just renamed their

       See bind(2).

       bind(2), connect(2), listen(2), select(2), socket(2), socket(7)

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Linux				  2015-12-28			     ACCEPT(2)