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ACCEPT(2)		   Linux Programmers Manual		    ACCEPT(2)

       accept - accept a connection on a socket

       #include 	       /* See NOTES */

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

       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  sockets  address  family  (see  socket(2)  and  the
       respective protocol man pages).	The addrlen argument is a value-result
       argument: it should initially contain the size of the structure pointed
       to  by  addr; on return it will contain the actual length (in bytes) of
       the address returned.  When addr is NULL nothing is filled in.

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

       On success, accept() returns a non-negative integer that is a  descrip
       tor  for  the  accepted socket.	On error, -1 is returned, and errno is
       set appropriately.

   Error Handling
       Linux accept() 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  case	of  TCP/IP  these  are

       accept() shall fail if:

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

       EBADF  The descriptor is invalid.

	      A connection has been aborted.

       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).

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

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

	      The descriptor references a file, not a socket.

	      The referenced socket is not of type SOCK_STREAM.

       accept() may fail if:

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

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

       EPROTO Protocol error.

       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.

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

       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 non-inheritance  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 , 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 thats what the  BSD  socket  interface  is.
       Anyway,	 the   POSIX   people  eventually  got	a  clue,  and  created
       "socklen_t".  They shouldnt 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 didnt 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)

       This page is part of release 3.05 of the Linux  man-pages  project.   A
       description  of	the project, and information about reporting bugs, can
       be found at http://www.kernel.org/doc/man-pages/.

Linux				  2004-06-17			     ACCEPT(2)

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