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ACCEPT(2) Linux Programmer's Manual 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 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), poll(2), or epoll(7).  A readable event will  be  deliv-
     ered  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  allo-
            cation 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; sup-
     port 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 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.

 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  stan-
     dards and glibc 2.x have socklen_t * .

EXAMPLE

     See bind(2).