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NAME

        accept - accept a connection on a socket
 

SYNOPSIS

        #include <sys/types.h>
        #include <sys/socket.h>
 
        int accept(int sockfd, struct sockaddr *addr, socklen_t *addrlen);
 

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, 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.
 
socket(2),
bind(2), and is listening for connections
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
socket(2) and the
        respective protocol man pages).  The addrlen argument is a value-result
        argument: it should initially contain the size of the structure pointed
        to by addr; on return it will contain the actual length (in  bytes)  of
        the address returned. When addr is NULL nothing is filled in.
 
        If  no  pending connections are present on the queue, and the socket is
        not marked as non-blocking, accept() blocks the caller until a  connec‐
        tion  is  present.  If the socket is marked non-blocking and no pending
        connections are present on the queue, accept()  fails  with  the  error
        EAGAIN.
 
        In  order  to  be notified of incoming connections on a socket, you can
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
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 connec‐
        tion 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 DEC‐
        Net has these semantics on Linux.
 

NOTES

        There may not always be a connection waiting after a SIGIO is delivered
poll(2) return a readability event because the  connec‐
        tion  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
socket(7)).
        On  success, accept() returns a non-negative integer that is a descrip‐
        tor for the accepted socket.  On error, -1 is returned,  and  errno  is
        set appropriately.
        Linux  accept() passes already-pending network errors on the new socket
        as an error code from accept().  This behaviour 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 ENET     
        DOWN, EPROTO, ENOPROTOOPT, EHOSTDOWN, ENONET, EHOSTUNREACH, EOPNOTSUPP,
        and ENETUNREACH.
 

ERRORS

        accept() shall fail if:
 
        EAGAIN or EWOULDBLOCK
               The socket is marked non-blocking and no connections are present
               to be accepted.
 
        EBADF  The descriptor is invalid.
 
        ECONNABORTED
               A connection has been aborted.
 
        EINTR  The system call was interrupted by  a  signal  that  was  caught
               before a valid connection arrived.
 
        EINVAL Socket  is  not listening for connections, or addrlen is invalid
               (e.g., is negative).
 
        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.
 
        ENOTSOCK
               The descriptor references a file, not a socket.
 
        EOPNOTSUPP
               The referenced socket is not of type SOCK_STREAM.
 
        accept() may fail if:
 
        EFAULT The addr argument is not in a writable part of the user  address
               space.
 
        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.
 
        EPROTO Protocol error.
 
        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.
        SVr4, 4.4BSD (accept() first appeared in 4.2BSD).
 
        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 behaviour differs from the canonical BSD  sockets  implementation.
        Portable  programs 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().
 

NOTE

        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 unfath‐
        omable reason (probably somebody didn’t like losing  face  over  having
        done  the  original  stupid  thing, so they silently just renamed their
        blunder)."
socket(2)
 

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