.\" Copyright (c) 2001-2003 The Open Group, All Rights Reserved 
.TH "SOCKATMARK" 3P 2003 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\" sockatmark 
.SH 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.
.SH NAME
sockatmark \- determine whether a socket is at the out-of-band mark
.SH SYNOPSIS
.LP
\fB#include <sys/socket.h>
.br
.sp
int sockatmark(int\fP \fIs\fP\fB);
.br
\fP
.SH DESCRIPTION
.LP
The \fIsockatmark\fP() function shall determine whether the socket
specified by the descriptor \fIs\fP is at the out-of-band
data mark (see the System Interfaces volume of IEEE\ Std\ 1003.1-2001,
Section 2.10.12, Socket Out-of-Band State). If the protocol for the
socket
supports out-of-band data by marking the stream with an out-of-band
data mark, the \fIsockatmark\fP() 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
\fIsockatmark\fP() function shall not remove the mark from the stream.
.SH RETURN VALUE
.LP
Upon successful completion, the \fIsockatmark\fP() 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 \fIsockatmark\fP() function shall return
0. Otherwise, it shall return a value of -1 and set \fIerrno\fP to
indicate the error.
.SH ERRORS
.LP
The \fIsockatmark\fP() function shall fail if:
.TP 7
.B EBADF
The \fIs\fP argument is not a valid file descriptor.
.TP 7
.B ENOTTY
The \fIs\fP argument does not specify a descriptor for a socket.
.sp
.LP
\fIThe following sections are informative.\fP
.SH EXAMPLES
.LP
None.
.SH APPLICATION USAGE
.LP
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.
.LP
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 \fIsockatmark\fP() will return false, and
the next read operation will silently consume the mark.
.LP
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 \fIselect\fP()). See \fISocket Receive Queue\fP,
\fISocket Out-of-Band Data State\fP, \fISignals\fP, and \fIpselect\fP()
for
details.
.SH RATIONALE
.LP
The \fIsockatmark\fP() function replaces the historical SIOCATMARK
command to \fIioctl\fP() which implemented the same functionality
on many implementations. Using a wrapper
function follows the adopted conventions to avoid specifying commands
to the \fIioctl\fP()
function, other than those now included to support XSI STREAMS. The
\fIsockatmark\fP() function could be implemented as
follows:
.sp
.RS
.nf

\fB#include <sys/ioctl.h>
.sp

int sockatmark(int s)
{
    int val;
    if (ioctl(s,SIOCATMARK,&val)==-1)
        return(-1);
    return(val);
}
\fP
.fi
.RE
.LP
The use of [ENOTTY] to indicate an incorrect descriptor type matches
the historical behavior of SIOCATMARK.
.SH FUTURE DIRECTIONS
.LP
None.
.SH SEE ALSO
.LP
\fIpselect\fP(), \fIrecv\fP(), \fIrecvmsg\fP(), the Base Definitions
volume of IEEE\ Std\ 1003.1-2001, \fI<sys/socket.h>\fP
.SH COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 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 .