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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
       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).  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
       initialize  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  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
       details.

       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.

       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 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() (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 non-blocking and no connections are present
              to be accepted.

       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
              allocation  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;
       support 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 non-standard 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 non-inheritance  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
       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)."

EXAMPLE

       See bind(2).

SEE ALSO

       bind(2), connect(2), listen(2), select(2), socket(2), socket(7)

COLOPHON

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       description  of  the project, and information about reporting bugs, can
       be found at http://www.kernel.org/doc/man-pages/.