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).