### Alternatives to Using Signals

Avoid using UNIX signals in multithreaded programs. DCE Threads provides alternatives to signal handling. These alternatives are discussed in more detail in Using Synchronization Objects and Signaling a Condition Variable.

Note: In order to implement these alternatives, DCE Threads must install its own signal handlers. These are installed when DCE Threads initializes itself, typically on the first thread-function call. At this time, any existing signal handlers are replaced.

Following are several reasons for avoiding signals:

· They cannot be used in a modular way in a multithreaded program.

· They are unnecessary when used as an asynchronous programming technique in a multithreaded program.

· There are almost no threads services available at signal level.

· There is no reliable, portable way to modify predicates.

· The signal-handler interface is unsuitable for use with threads. (For example, there is one signal action per signal per process, there is one signal mask per process, and sigpause( ) blocks the whole process.)

In a multithreaded program, signals cannot be used in a modular way because, on most current UNIX implementations, signals are inherently a process construct. There is only one instantiation of each signal and of each signal handler routine for all of the threads in an application. If one thread handles a particular signal in one way, and a different thread handles the same signal in a different way, then the thread that installs its signal handler last handles the signal. This applies only to asynchronously generated signals; synchronous signals can be handled on a per-thread basis using the DCE Threads sigaction( ) jacket.

Do not use asynchronous programming techniques in conjunction with threads, particularly those that increase parallelism such as using timer signals and I/O signals. These techniques can be complicated. They are also unnecessary because threads provide a mechanism for parallel execution that is simpler and less prone to error where concurrence can be of value. Furthermore, most of the threads routines are not supported for use in interrupt routines (such as signal handlers), and portions of runtime libraries cannot be used reliably inside a signal handler.

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