bionic (2) accept.2.gz

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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

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