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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 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, noth-
       ing 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  non-blocking,  accept()
       blocks  the  caller until a connection is present.  If the socket is marked non-blocking and no pending connec-
       tions 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.  Cur-
       rently 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
       On  success, these system calls return a non-negative 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  retry-
       ing.   In  case of TCP/IP these are ENETDOWN, EPROTO, ENOPROTOOPT, EHOSTDOWN, ENONET, EHOSTUNREACH, EOPNOTSUPP,
       and ENETUNREACH.

ERRORS
       EAGAIN or EWOULDBLOCK
              The socket is marked non-blocking 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  sig-
              nal(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 non-standard 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 pro-
       grams  should  not  rely  on  inheritance or non-inheritance 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  read-
       ability 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  Specifica-
       tion 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)  com-
       plained  to them very loudly indeed.  Making it a size_t is completely broken, exactly because size_t very sel-
       dom 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 origi-
       nal 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 3.22 of the Linux man-pages project.  A description of the project,  and  informa-
       tion about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



Linux                             2009-02-23                         ACCEPT(2)