OSF DCE Application Development Guide--Introduction and Style Guide

Threads
Application developers must be aware of significant differences in the handling of
signals between DCE threads and typical single-threaded environments. In DCE threads,
some signals are handled on a per-process basis, and some are handled on a per-thread
basis. This section explains the semantic details of DCE thread signal handling.
A signal is said to be generated for a process or thread when the event that causes the
signal first occurs. Each process or thread has an action to be taken in response to each
signal supported by the system or implementation. A signal is said to be delivered when
the appropriate signal action for the process or thread is taken. A signal can be blocked
or (masked) by a thread or process by establishing a signal mask containing the signal to
be blocked.
The delivery of a blocked signal is deferred until it is unblocked. (Note that if the action
specified for a signal is to ignore it, the signal effectively remains blocked.) During the
time between its generation and delivery a signal is said to be pending.
Signals can be classified into the following two types, with differing semantics:
• Synchronous signals are generated by a specific thread and delivered to the same
thread. Threads can establish nondefault per-thread signal handlers for sychronous
signals by calling sigaction(). Synchronous signals can be blocked on a per-thread
basis by establishing per-thread signal masks.
• Asynchronous signals are generated by external events, not identifiable with a single
thread. Asynchronous signals are handled on a per-process basis. An asynchronous
signal is delivered exactly once to some thread in a process. All threads in a process
share the same signal mask. Per-process handling of asynchronous signals can be
established by calling sigwait( ).
DCE threads applications must handle synchronous and asynchronous signals
differently.
2.3.4.1 Signal Masking
Signal masks can be examined and changed with the sigprocmask( ) function. When a
synchronous signal is masked via a call to sigprocmask( ) it is masked for the calling
thread. When an asynchronous signal is masked via a call to sigprocmask() it is masked
for the entire process.
Care must be taken when a thread unblocks an asynchronous signal. If another thread has
blocked and is, or is will be, waiting for the same signal, the results can be unpredictable
and may result in the other thread waiting forever. This problem can be avoided by
having all handling of asynchronous signals occur in a single thread, as described in
Section 2.3.4.3.
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