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

PROLOG         top

       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         top

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

SYNOPSIS         top

       #include <sys/socket.h>

       int sockatmark(int s);

DESCRIPTION         top

       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         top

       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         top

       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         top



       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         top

       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)

       The use of [ENOTTY] to indicate an incorrect descriptor type matches
       the historical behavior of SIOCATMARK.



SEE ALSO         top

       Section 2.10.12, Socket Out-of-Band Data State, pselect(3p),
       recv(3p), recvmsg(3p)

       The Base Definitions volume of POSIX.1‐2008, sys_socket.h(0p)

COPYRIGHT         top

       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 Electrical 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 .

       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 .

IEEE/The Open Group                 2013                      SOCKATMARK(3P)

Pages that refer to this page: sys_socket.h(0p)