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
   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  nonblocking,  accept()  blocks  the  caller  until   a
   connection  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),  poll(2),  or  epoll(7).   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.

   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
   file 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  sockfd is not an open file descriptor.

   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
          allocation  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;
   support 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), poll(2), or epoll(7) 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)).

   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.

   The socklen_t type
   In the original BSD sockets implementation (and on other older systems)
   the  third  argument  of accept() was declared as an int *.  A POSIX.1g
   draft standard wanted to  change  it  into  a  size_t *C;  later  POSIX
   standards and glibc 2.x have socklen_t * .

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 4.09 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
   https://www.kernel.org/doc/man-pages/.

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