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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),	poll(2), or epoll(7).  A readable event will be deliv-
       ered 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 details.

       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
       file 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  sockfd is not an open file descriptor.

	      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),  poll(2), or epoll(7) return a readability event because
       the connection 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)).

       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.

   The socklen_t type
       In the original BSD sockets implementation (and on other older systems)
       the third argument of accept() was declared as an  int *.   A  POSIX.1g
       draft  standard wanted to change it into a size_t *C; later POSIX stan-
       dards and glibc 2.x have socklen_t * .

       See bind(2).

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

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Linux				  2016-10-08			     ACCEPT(2)