accept(2)



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  re-
       quest  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  re-
       spective 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 EA-
       GAIN 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  ap-
       plication should detect the network errors defined for the protocol af-
       ter 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 er-
              ror to be returned for this case, and do not require these  con-
              stants  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 be-
              fore 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  er-
       rors  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 ap-
       peared 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  im-
       plied  by a normal read or write on the new file descriptor, and rejec-
       tion 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).

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

COLOPHON
       This page is part of release 4.16 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/.

Linux                             2016-10-08                         ACCEPT(2)

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