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

NAME
       accept, accept4 - 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 descrip‐
       tor  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 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
       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 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.

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  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
       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 pos‐
	      sibilities.

       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 allo‐
	      cation is limited by the socket buffer limits, not by the system
	      memory.

       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; sup‐
       port 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 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
       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 3.54 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)
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