SOCKATMARK(3P)                                      POSIX Programmer's Manual                                     SOCKATMARK(3P)



PROLOG
       This  manual  page is part of the POSIX Programmer's Manual.  The Linux implementation of this interface may differ (con-
       sult 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 the System Interfaces volume of IEEE Std 1003.1-2001, Section 2.10.12, Socket Out-of-Band State). If the proto-
       col  for the socket supports out-of-band data by marking the stream with an out-of-band data mark, the sockatmark() func-
       tion 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 s argument does not specify a descriptor for 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 loca-
       tion 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
       Socket Receive Queue, Socket Out-of-Band Data State, 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
       pselect(), recv(), recvmsg(), the Base Definitions volume of IEEE Std 1003.1-2001, <sys/socket.h>

COPYRIGHT
       Portions  of  this  text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition, Standard for
       Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 6,  Copy-
       right (C) 2001-2003 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. 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.open-
       group.org/unix/online.html .



IEEE/The Open Group                                           2003                                                SOCKATMARK(3P)