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 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 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 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 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, 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(P)