sockatmark(3p) — Linux manual page


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

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

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.1‐2017, sys_socket.h(0p)

COPYRIGHT         top

       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 .

       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               2017                    SOCKATMARK(3P)

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