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 V2_chap02(3)). 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:
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(3)). See V2_chap02(3), V2_chap02(3), V2_chap02(3), and pselect(3) for details.
RATIONALE
The sockatmark () function replaces the
historical SIOCATMARK command to ioctl(3) which
implemented the same functionality on many implementations. Using a wrapper
function follows the adopted conventions to avoid specifying commands to the
ioctl(3) 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
V2_chap02(3), pselect(3), recv(3), recvmsg(3)
XBD <sys/socket.h>
CHANGE HISTORY
First released in Issue 6. Derived from IEEE Std 1003.1g-2000 (“POSIX.1g”).
Issue 7
SD5-XSH-ERN-100 is applied, correcting the definition of the [ENOTTY] error condition.