.\" Copyright (c) 2002 William C. Fenner.  All rights reserved.
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.Dd July 3, 2022
.Dt SOCKATMARK 3
.Os
.Sh NAME
.Nm sockatmark
.Nd determine whether the read pointer is at the OOB mark
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In sys/socket.h
.Ft int
.Fn sockatmark "int s"
.Sh DESCRIPTION
To find out if the read pointer is currently pointing at
the mark in the data stream, the
.Fn sockatmark
function is provided.
If
.Fn sockatmark
returns 1, the next read will return data
after the mark.
Otherwise (assuming out of band data has arrived),
the next read will provide data sent by the client prior
to transmission of the out of band signal.
The routine used
in the remote login process to flush output on receipt of an
interrupt or quit signal is shown below.
It reads the normal data up to the mark (to discard it),
then reads the out-of-band byte.
.Bd -literal -offset indent
#include <sys/socket.h>
\&...
oob()
{
	int out = FWRITE, mark;
	char waste[BUFSIZ];

	/* flush local terminal output */
	ioctl(1, TIOCFLUSH, (char *)&out);
	for (;;) {
		if ((mark = sockatmark(rem)) < 0) {
			perror("sockatmark");
			break;
		}
		if (mark)
			break;
		(void) read(rem, waste, sizeof (waste));
	}
	if (recv(rem, &mark, 1, MSG_OOB) < 0) {
		perror("recv");
		...
	}
	...
}
.Ed
.Sh RETURN VALUES
Upon successful completion, the
.Fn sockatmark
function returns the value 1 if the read pointer is pointing at
the OOB mark, 0 if it is not.
Otherwise the value \-1 is returned
and the global variable
.Va errno
is set to
indicate the error.
.Sh ERRORS
The
.Fn sockatmark
call fails if:
.Bl -tag -width Er
.It Bq Er EBADF
The
.Fa s
argument
is not a valid descriptor.
.It Bq Er ENOTTY
The
.Fa s
argument
is a descriptor for a file, not a socket.
.El
.Sh SEE ALSO
.Xr recv 2 ,
.Xr send 2
.Rs
.%T "An Introductory 4.4BSD Interprocess Communication Tutorial"
.%A Stuart Sechrest
.Re
.%U https://docs.freebsd.org/44doc/psd/20.ipctut/paper.pdf
.Rs
.%T "An Advanced 4.4BSD Interprocess Communication Tutorial"
.%A Samuel J. Leffler
.%A Robert S. Fabry
.%A William N. Joy
.%A Phil Lapsley
.%A Steve Miller
.%A Chris Torek
.%U https://docs.freebsd.org/44doc/psd/21.ipc/paper.pdf
.Re
.Sh HISTORY
The
.Fn sockatmark
function was introduced by
.St -p1003.1-2001 ,
to standardize the historical
.Dv SIOCATMARK
.Xr ioctl 2 .
The
.Er ENOTTY
error instead of the usual
.Er ENOTSOCK
is to match the historical behavior of
.Dv SIOCATMARK .