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 correspond-
       ing 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 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  implemen-
       tations.  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 Technol-
       ogy -- 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)