accept

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 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).

SEE ALSO
       bind(2), connect(2), listen(2), select(2), socket(2), socket(7)

COLOPHON
       This page is part of release 5.09 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                              2020-04-11                          ACCEPT(2)