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



NAME
       accept - 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
       #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  lis-
       tening 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.  Alter-
       natively, 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 dif-
       ferent 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
       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 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 allows either  error
              to  be returned for this case, and does not require these constants to have the same value, so a portable applica-
              tion 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
       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, 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) 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
       See bind(2).

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

COLOPHON
       This  page  is  part of release 3.25 of the Linux man-pages project.  A description of the project, and information about
       reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



Linux                                                      2009-02-23                                                  ACCEPT(2)