SOCKATMARK(3XNET) X/Open Networking Services Library Functions

NAME


sockatmark - determine whether a socket is at the out-of-band mark

SYNOPSIS


cc [ flag ... ] file ... -lxnet [ library ... ]
#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 data mark, the sockatmark() function returns 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
does not remove the mark from the stream.

RETURN VALUES


Upon successful completion, the sockatmark() 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, the return value is 1. If there is no mark, or if data
precedes the mark in the receive queue, the sockatmark() function
returns 0. Otherwise, it returns -1 and sets errno to indicate the
error.

ERRORS


The sockatmark() function will fail if:

EBADF
The s argument is not a valid file descriptor.


ENOTTY
The s argument does not specify a descriptor for a socket.


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, either by SIGURG or select(3C).

ATTRIBUTES


See attributes(7) for descriptions of the following attributes:


+--------------------+-----------------+
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+--------------------+-----------------+
|Interface Stability | Standard |
+--------------------+-----------------+
|MT-Level | Safe |
+--------------------+-----------------+

SEE ALSO


select(3C), recv(3XNET), recvmsg(3XNET), attributes(7), standards(7)

October 1, 2003 SOCKATMARK(3XNET)

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