accept

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  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.

       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 on the number of open file descriptors has
              been reached.

       ENFILE The system-wide 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 file descriptor sockfd does not refer to 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,  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().

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 4.04 of the Linux  man-pages  project.   A
       description  of  the project, information about reporting bugs, and the
       latest    version    of    this    page,    can     be     found     at
       http://www.kernel.org/doc/man-pages/.

Linux                             2015-12-28                         ACCEPT(2)
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