NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | CONFORMING TO | NOTES | EXAMPLE | SEE ALSO | COLOPHON

ACCEPT(2)                 Linux Programmer's Manual                ACCEPT(2)

NAME         top

       accept, accept4 - accept a connection on a socket

SYNOPSIS         top

       #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         top

       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) or poll(2).  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.

       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.

       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 new open
                       file description.  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         top

       On success, these system calls return a nonnegative integer that is a
       descriptor for the accepted socket.  On error, -1 is returned, and
       errno is set appropriately.

   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         top

       EAGAIN or EWOULDBLOCK
              The socket is marked nonblocking and no connections are
              present to be accepted.  POSIX.1-2001 allows either error to
              be returned for this case, and does not require these
              constants to have the same value, so a portable application
              should check for both possibilities.

       EBADF  The descriptor is invalid.

       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 of open file descriptors has been
              reached.

       ENFILE The system 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 descriptor references a file, not 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         top

       The accept4() system call is available starting with Linux 2.6.28;
       support in glibc is available starting with version 2.10.

CONFORMING TO         top

       accept(): POSIX.1-2001, 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         top

       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) or poll(2) 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)).

   The socklen_t type
       The third argument of accept() was originally declared as an int *
       (and is that under libc4 and libc5 and on many other systems like 4.x
       BSD, SunOS 4, SGI); a POSIX.1g draft standard wanted to change it
       into a size_t *, and that is what it is for SunOS 5.  Later POSIX
       drafts have socklen_t *, and so do the Single UNIX Specification and
       glibc2.  Quoting Linus Torvalds:

       "_Any_ sane library _must_ have "socklen_t" be the same size as int.
       Anything else breaks any BSD socket layer stuff.  POSIX initially did
       make it a size_t, and I (and hopefully others, but obviously not too
       many) complained to them very loudly indeed.  Making it a size_t is
       completely broken, exactly because size_t very seldom is the same
       size as "int" on 64-bit architectures, for example.  And it has to be
       the same size as "int" because that's what the BSD socket interface
       is.  Anyway, the POSIX people eventually got a clue, and created
       "socklen_t".  They shouldn't have touched it in the first place, but
       once they did they felt it had to have a named type for some
       unfathomable reason (probably somebody didn't like losing face over
       having done the original stupid thing, so they silently just renamed
       their blunder)."

EXAMPLE         top

       See bind(2).

SEE ALSO         top

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

COLOPHON         top

       This page is part of release 3.72 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
       http://www.kernel.org/doc/man-pages/.

Linux                            2010-09-10                        ACCEPT(2)