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 determines whether the socket specified by the
descriptor
s
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
sockatmark()
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
sockatmark()
function does not remove the mark from the stream.
X/Open Sockets Only
sockatmark()
is new in
UNIX 03.
RETURN VALUES
Upon successful completion, the
sockatmark()
function returns a value
indicating whether the socket is at an out-of-band data mark:
- 1
If the protocol has marked the data stream and all data preceding the mark has
been read.
- 0
If there is no mark, or if data
precedes the mark in the receive queue.
On failure,
sockatmark()
returns
-1,
and sets
errno
to indicate the error.
ERRORS
If
sockatmark()
fails,
errno
is set to one of the following values:
- EBADF
The
s
argument is not a valid file descriptor.
- ENOTTY
The
s
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
sockatmark()
returns false, and the next read operation silently consumes the mark.
Hence, this function can only be used reliably in one or both ways:
when the application already knows that the out-of-band data has been seen
by the system
and that it knows data is waiting to be read at the socket (by using
SIGURG
or
select()).
AUTHOR
sockatmark()
was developed by HP and IEEE.
STANDARDS CONFORMANCE
sockatmark(): UNIX 03
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