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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
       #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  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 3.25 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                             2009-02-23                         ACCEPT(2)
 

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