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

NAME

       accept, accept4 - accept a connection on a socket

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

       #include <sys/socket.h>

       int accept(int sockfd, struct sockaddr *_Nullable restrict addr,
                  socklen_t *_Nullable restrict addrlen);

       #define _GNU_SOURCE             /* See feature_test_macros(7) */
       #include <sys/socket.h>

       int accept4(int sockfd, struct sockaddr *_Nullable restrict addr,
                  socklen_t *_Nullable restrict 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 to indicate the error, 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.

EPERM Firewall rules forbid connection.

EPROTO Protocol error.

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

       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().

STANDARDS

       accept()
              POSIX.1-2008.

       accept4()
              Linux.

HISTORY

       accept()
              POSIX.1-2001, SVr4, 4.4BSD (accept() first appeared in 4.2BSD).

       accept4()
              Linux 2.6.28, glibc 2.10.

NOTES

       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).