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       accept - accept a connection on a socket


       #include <sys/types.h>          /* See NOTES */
       #include <sys/socket.h>

       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, 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 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
       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).  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
       connection  is  present.   If  the socket is marked non-blocking and no
       pending connections are present on the queue, accept() fails  with  the
       error EAGAIN.

       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
       connection 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
       descriptor 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


       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.

       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
              allocation is limited by the socket buffer limits,  not  by  the
              system memory.

       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 <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
       connection 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
       unfathomable  reason  (probably  somebody  didn’t like losing face over
       having done the original stupid thing, so they  silently  just  renamed
       their blunder)."


       See bind(2).


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


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