Out-of-band events, such as RESET INDICATION, INTERRUPT, and
CLEAR INDICATION packets, occur during typical X.25 VC operation.
When an out-of-band event occurs, X.25 indicates the event
by sending a SIGURG signal, and places a description of the event
in the out-of-band queue.
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 | WARNING! Applications using X.25 programmatic access
should be designed to receive the SIGURG signal and process the
information associated with the event. If the process has not attached
a signal handler to receive this signal, the signal is ignored.
Failure to respond to some out-of-band events (a RESET packet for
example) may result in the VC being cleared by the network provider. |
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Out-of-band events are placed in the out-of-band queue regardless
of whether a signal handler has been installed. While it is possible
to program applications to periodically examine the out-of-band
queue to obtain any out-of-band events that have occurred, it is
not recommended.
Signal Reception |
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Signal handling under HP-UX is controlled by the signal(2),
sigsetmask(2), and sigvector(2)
system calls. These system calls are described in your HP-UX
man pages. The sigvector()
system call allows you to specify a signal handler (signal catcher)
to process signals when they arrive.
The following example shows how a signal handler may be installed
to receive the SIGURG signal:
struct sigvec vec;
int onurg();
int pid, s;
/*
** arrange for the onurg() signal handler to be called
when SIGURG is received:
*/
vec.sv_handler = onurg;
vec.sv_mask; = 0
vec.sv_onstack = 0;
if (sigvector(SIGURG, &vec, 0) < 0) {
perror("sigvector(SIGURG)");
} |
In addition to installing the signal handler, you must also
call ioctl(SIOCSPGRP) to
ensure that the SIGURG signal is delivered upon receipt of the out-of-bound
data as shown in the code example below. Refer to the s ocket(7)
man page for more information about the ioctl(SIOCSPGRP)
call.
setsigskt(s)
int s;
{
int pid;
/* enables the current process to receive SIGURG
* when the socket has urgent data;
*/
pid = getpid();
/* Note that specifying the process id in the next ioctl()
* means that only this process shall receive the SIGURG
* signal.
If (-1)*getpid() is used instead,
the entire
*
process group (including
* parents and children) will receive the SIGURG signal.
*/
if (ioctl(s, SIOCSPGRP, (char *) &pid) < 0) {
perror ("ioctl(SIOCSPGRP)");
} |
Once installed, the signal handler is called whenever the
specified signal arrives. While a signal handler is executing, additional
signals of the same type are blocked from arrival. All signals sent
by a socket should be handled by the process which is controlling
the VC. This ensures process continuity.
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