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

NAME

       accept - accept a connection on a socket

SYNOPSIS

       #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);

DESCRIPTION

       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
       descriptor  referring to that socket.  The newly created socket is not in the listening state.  The original socket sockfd is unaffected by
       this call.

       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  connec-
       tions 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
       communications 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 initialize 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
       EAGAIN or EWOULDBLOCK.

       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 details.

       For  certain protocols which require an explicit confirmation, such as DECNet, accept() can be thought of as merely dequeuing 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.  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.

RETURN VALUE

       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  dif-
       fers from other BSD socket implementations.  For reliable operation the application should detect the network errors defined for the proto-
       col after accept() and treat them like EAGAIN by retrying.  In case of TCP/IP these are ENETDOWN, EPROTO, ENOPROTOOPT,  EHOSTDOWN,  ENONET,
       EHOSTUNREACH, EOPNOTSUPP, and ENETUNREACH.

ERRORS

       EAGAIN or EWOULDBLOCK
	      The socket is marked nonblocking and no connections are present to be accepted.  POSIX.1-2001 allows either error to be returned for
	      this case, and does not require these constants to have the same value, so a portable application should check for  both	possibili-
	      ties.

       EBADF  The descriptor is invalid.

       ECONNABORTED
	      A connection has been aborted.

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

       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 of open file descriptors has been reached.

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

       ENOBUFS, ENOMEM
	      Not  enough free memory.	This often means that the memory allocation is limited by the socket buffer limits, not by the system mem-
	      ory.

       ENOTSOCK
	      The descriptor references a file, not a socket.

       EOPNOTSUPP
	      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
       errors such as ENOSR, ESOCKTNOSUPPORT, EPROTONOSUPPORT, ETIMEDOUT.  The value ERESTARTSYS may be seen during a trace.

VERSIONS

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

CONFORMING TO

       accept(): POSIX.1-2001, 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().

NOTES

       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 con-
       nection	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.  Quoting 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
       blunder)."

EXAMPLE

       See bind(2).

SEE ALSO

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

COLOPHON

       This  page is part of release 3.27 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								    2010-09-10								 ACCEPT(2)