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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 (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 the System
       Interfaces volume of IEEE Std 1003.1-2001, Section 2.10.12, Socket Out-
       of-Band  State).  If  the  protocol for the socket supports out-of-band
       data by marking the stream with an out-of-band data mark,  the  sockat-
       mark()  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 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  applica-
       tion 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 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  implementa-
       tions.  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,  Copyright  (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.opengroup.org/unix/online.html .

IEEE/The Open Group                  2003                       SOCKATMARK(3P)