sockatmark(3N)sockatmark(3N)NAME
sockatmark() - determine whether a socket is at the out-of-band mark
SYNOPSIS DESCRIPTION
The function determines whether the socket specified by the descriptor is at the out-of-band data mark.
If the protocol for the socket supports out-of-band data by marking the stream with an out-of-band mark, the function returns 1 when all
data preceding the mark has been read and the out-of-band mark is the first element in the receive queue. The function does not remove the
mark from the stream.
X/Open Sockets Only
is new in
RETURN VALUES
Upon successful completion, the function returns 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.
If there is no mark, or if data
precedes the mark in the receive queue.
On failure, returns and sets to indicate the error.
ERRORS
If fails, is set to one of the following values:
The argument is not a valid file descriptor.
The argument does not specify a descriptor for a socket.
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 be at
the "mark". However, the system has no way of knowing that the next data segment arriving from the network is carrying the mark. Then
returns false, and the next read operation silently consumes the mark.
Hence, this function can only be used reliably in one or both ways:
o when the application already knows that the out-of-band data has been seen by the system
o and that it knows data is waiting to be read at the socket (by using or
AUTHOR
was developed by HP and IEEE.
SEE ALSO ioctl(2), pselect(2), recv(2), recvmsg(2), socket(7).
STANDARDS CONFORMANCE sockatmark(3N)
Check Out this Related Man Page
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 correspond-
ing Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.
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 implemen-
tations. 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 Technol-
ogy -- 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 Stan-
dard 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.kernel.org/doc/man-pages/reporting_bugs.html .
IEEE /The Open Group 2013 SOCKATMARK(3P)