bionic (3) sockatmark.3posix.gz

PROLOG

       This  manual  page  is part of the POSIX Programmer's Manual.  The Linux implementation of this interface
       may differ (consult the corresponding Linux manual page for details of Linux behavior), or the  interface
       may not be implemented on Linux.

NAME

       sockatmark — determine whether a socket is at the out-of-band mark

SYNOPSIS

       #include <sys/socket.h>

       int sockatmark(int s);

DESCRIPTION

       The sockatmark() function shall determine whether the socket specified by the descriptor s is at the out-
       of-band data mark (see Section 2.10.12, Socket Out-of-Band Data State).  If the protocol for  the  socket
       supports  out-of-band data by marking the stream with an out-of-band data mark, the sockatmark() function
       shall return 1 when all data preceding the mark has been read and the out-of-band data mark is the  first
       element in the receive queue. The sockatmark() function shall not remove the mark from the stream.

RETURN VALUE

       Upon  successful completion, the sockatmark() function shall return a value indicating whether the socket
       is at an out-of-band data mark. If the protocol has marked the data stream and  all  data  preceding  the
       mark has been read, the return value shall be 1; if there is no mark, or if data precedes the mark in the
       receive queue, the sockatmark() function shall return 0. Otherwise, it shall return a value of −1 and set
       errno to indicate the error.

ERRORS

       The sockatmark() function shall fail if:

       EBADF  The s argument is not a valid file descriptor.

       ENOTTY The file associated with the s argument is not a socket.

       The following sections are informative.

EXAMPLES

       None.

APPLICATION USAGE

       The  use  of  this  function between receive operations allows an application to determine which received
       data precedes the out-of-band data and which follows the out-of-band data.

       There is an inherent race condition in the use of this function. On an empty receive queue,  the  current
       read  of  the  location might well be at the ``mark'', but the system has no way of knowing that the next
       data segment that will arrive from the network will carry the mark, and sockatmark() will  return  false,
       and the next read operation will silently consume the mark.

       Hence,  this  function  can only be used reliably when the application already knows that the out-of-band
       data has been seen by the system or that it is known that there is data waiting to be read at the  socket
       (via SIGURG or select()).  See Section 2.10.11, Socket Receive Queue, Section 2.10.12, Socket Out-of-Band
       Data State, Section 2.10.14, Signals, and pselect() for details.

RATIONALE

       The sockatmark() function replaces the historical SIOCATMARK command to  ioctl()  which  implemented  the
       same  functionality  on many implementations. Using a wrapper function follows the adopted conventions to
       avoid specifying commands to the ioctl() function, other than those now included to support XSI  STREAMS.
       The sockatmark() function could be implemented as follows:

           #include <sys/ioctl.h>

           int sockatmark(int s)
           {
               int val;
               if (ioctl(s,SIOCATMARK,&val)==−1)
                   return(−1);
               return(val);
           }

       The  use  of  [ENOTTY]  to  indicate  an  incorrect  descriptor  type  matches the historical behavior of
       SIOCATMARK.

FUTURE DIRECTIONS

       None.

SEE ALSO

       Section 2.10.12, Socket Out-of-Band Data State, pselect(), recv(), recvmsg()

       The Base Definitions volume of POSIX.1‐2008, <sys_socket.h>

COPYRIGHT

       Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2013 Edition,
       Standard  for  Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base
       Specifications Issue 7, Copyright (C) 2013 by the Institute of Electrical and Electronics Engineers,  Inc
       and  The  Open Group.  (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) In the event
       of any discrepancy between this version and the original IEEE and The Open Group Standard,  the  original
       IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online at
       http://www.unix.org/online.html .

       Any typographical or formatting errors that appear in this page are most likely to have  been  introduced
       during   the   conversion  of  the  source  files  to  man  page  format.  To  report  such  errors,  see
       https://www.kernel.org/doc/man-pages/reporting_bugs.html .