Ubuntu Manpage: accept, accept4 - accept a connection on a socket

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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 open file description (see  open(2))  referred
                       to  by the new file descriptor.  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 file descriptor for the accepted socket (a nonnegative integer).
       On error, -1 is returned, errno is set appropriately, and addrlen is left unchanged.

   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 * .

EXAMPLES

       See bind(2).

COLOPHON

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