accept4

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  open  file
                       description  (see  open(2)) referred to by the new file
                       descriptor.  Using this flag saves extra calls  to  fc-
                       ntl(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, er-
       rno 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 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 5.05 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                             2019-03-06                         ACCEPT(2)