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

       accept - accept a connection on a socket

       #include <&lt;sys/types.h>&gt;          /* See NOTES */
       #include <&lt;sys/socket.h>&gt;

       int accept(int sockfd, struct sockaddr *addr, socklen_t *addrlen);

       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).  The addrlen argument is a value-result
       argument: it should initially contain the size of the structure pointed
       to  by  addr; on return it will contain the actual length (in bytes) of
       the address returned.  When addr is NULL nothing is filled in.

       If no pending connections are present on the queue, and the  socket  is
       not  marked as non-blocking, accept() blocks the caller until a connec-
       tion is present.  If the socket is marked non-blocking 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)  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

       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.

       On success, accept() returns a non-negative integer that is a  descrip-
       tor  for  the  accepted socket.  On error, -1 is returned, and errno is
       set appropriately.

   Error Handling
       Linux accept() 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  ENET-
       and ENETUNREACH.

       accept() shall fail if:

              The socket is marked non-blocking and no connections are present
              to be accepted.

       EBADF  The descriptor is invalid.

              A connection has been aborted.

       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).

       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

              The descriptor references a file, not a socket.

              The referenced socket is not of type SOCK_STREAM.

       accept() may fail if:

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

              Not  enough free memory.  This often means that the memory allo-
              cation is limited by the socket buffer limits, not by the system

       EPROTO Protocol error.

       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.

       SVr4, 4.4BSD, (accept() first appeared in 4.2BSD), POSIX.1-2001.

       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 non-inheritance 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 &lt;sys/types.h&gt;, 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

       See bind(2).

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

       This  page  is  part of release 3.05 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                             2004-06-17                         ACCEPT(2)