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.
NAMEsockatmark — 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)
