SOCKATMARK(3)              POSIX Programmer's Manual             SOCKATMARK(3)



PROLOG
       This manual page is part of the POSIX Programmer's Manual.  The OS
       implementation of this interface may differ (consult the corresponding
       OS manual page for details of OS behavior), or the interface may not be
       implemented on OS.

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
       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 Section 2.10.11, Socket Receive Queue,
       Section 2.10.12, Socket Out-of-Band Data State, Section 2.10.14,
       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
       Section 2.10.12, Socket Out-of-Band Data State, pselect(), recv(),
       recvmsg()

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

COPYRIGHT
       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(3)
