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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 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 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 Section 2.10.11, Socket Receive Queue,
       Section 2.10.12, Socket Out-of-Band Data State, Section  2.10.14,  Sig-
       nals, 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
       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 Electri-
       cal 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.ker-
       nel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group                  2013                       SOCKATMARK(3P)