High Availability Monitors Version A.03.02 Release Notes

Writing NOF Applications
Scheduling Asynchronous Events
Chapter 2 101
application from signal catcher context. The disadvantages of
signal-based scheduling mode are:
• Applications that receive work from multiple sources are difficult to
write.
• Applications are required to use HP-UX V.3 signal calls.
• Applications can use only a subset of system calls made from signal
catcher context and therefore from the API callback context.
• Signaling mode is not thread-safe
Use of Signals To detect work arriving from other components of
SNAplus2, the NOF library makes use of the SIGPOLL signal.
Applications can require the SIGPOLL signal for other purposes.
Therefore, the library daisy-chains to any existing signal catchers. The
first verb issued by the application initializes the library's signal catcher.
When the SIGPOLL signal arrives, either it indicates work for SNAplus2
or it was generated for some other reason. The NOF library's signal
catcher processes the signal and then calls the previous signal catcher to
allow the signal to be used for other purposes.
The following restrictions apply to SIGPOLL usage:
• Applications must not permanently disable this signal. The sighold
and sigrelse signal calls can be used to protect a critical region of
code, provided that no SNA verbs are issued between sighold and
sigrelse.
• Applications must preserve the address of the SNAplus2 signal
catchers returned by the sigset call when adding signal catchers to
applications. Applications must then arrange to call these routines
from within the new signal catchers.
• Applications must not use the POSIX signal mechanism SIGACTN,
which can cause problems in SNAplus2 applications.
HP-UX Sleep Call The HP-UX operating system call sleep can cause
problems if applications or their libraries also use signal catchers.
Because the SNAplus2 NOF library operates using a signal catcher, do
not use the sleep call within a NOF application that uses signal-based
scheduling. If necessary, you can use the alarm with a timer to provide
the same function.