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


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


cc [ flag ... ] file ... -lxnet [ library ... ]
#include <sys/socket.h>

int sockatmark(int s);


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.


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.


The sockatmark() function will fail if:

The s argument is not a valid file descriptor.

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


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).


See attributes(5) for descriptions of the following attributes:

|Interface Stability | Standard |
|MT-Level | Safe |


recv(3XNET), recvmsg(3XNET), select(3C), attributes(5), standards(5)

October 1, 2003 SOCKATMARK(3XNET)