This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface

       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 Section2.10.11, SocketReceiveQueue, Section2.10.12, SocketOut-of-BandDataState, Section2.10.14, Signals, and pselect() for details.

       Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard
       for Information  Technology  --  Portable  Operating  System  Interface  (POSIX),  The  Open  Group  Base
       Specifications  Issue  7, 2018 Edition, Copyright (C) 2018 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 .

       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.kernel.org/doc/man-pages/reporting_bugs.html .

IEEE/The Open Group                                   2017                                    SOCKATMARK(3POSIX)

       The sockatmark() function shall determine whether the socket specified by the descriptor s is at the out-
       of-band  data  mark (see Section2.10.12, SocketOut-of-BandDataState).  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.

       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.

       Thefollowingsectionsareinformative.

       None.

       None.

       sockatmark — determine whether a socket is at the out-of-band mark

       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.

       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.

       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.

Section2.10.12, SocketOut-of-BandDataState, pselect(), recv(), recvmsg()

       The Base Definitions volume of POSIX.1‐2017, <sys_socket.h>

       #include <sys/socket.h>

       int sockatmark(int s);