Coprocessing Functions

Advanced Topics: Signal Handling

To understand this section, you should be familiar with the normal operation of signal handling. You may want to review Chapter 5, "Signal-Handling Functions," of SAS/C Library Reference, Volume 1 before proceeding.

Signal handling for programs that use coprocesses is complex because program requirements differ depending on the situation and the signals involved. For instance, for computational signals such as SIGFPE , you may prefer a separate signal handler for each coprocess. On the other hand, for a signal such as SIGINT that can occur at any time, independent of coprocess switches, you may prefer a single handler, which is called regardless of the coprocess active at the time of the signal. The coprocess implementation enables you to define local and global handlers for each signal independently in whatever way is most convenient for your application.

In addition, the library maintains a separate signal-blocking mask for each coprocess. This allows coprocesses to block signals that they are not equipped to handle, while assuring that signals are recognized whenever an appropriate coprocess becomes active. By default, all coprocesses except the main coprocess are created with all signals blocked. For this reason, unless a new coprocess issues sigsetmask or sigprocmask , no asynchronous signals are detected during its execution. Therefore, you can write subroutines that create coprocesses, and you do not need any special code to avoid interference with the asynchronous signal handling of the rest of the program. Note that you can use the second argument to costart to specify a different signal mask for the new coprocess.

When you use coprocesses, there are two functions available to define signal handlers: signal and cosignal . signal defines a local handler, a function that is called only for signals discovered while the coprocess that defined the handler is active. cosignal defines a global handler, a function that is called for signals discovered during the execution of any coprocess. If, at the time of signal discovery, both a local and global handler are defined, the local handler is called unless the local handler is SIG_DFL , in which case the global handler is called.

Usually, you want to define local handlers (using the signal function) for synchronous signals and global handlers (using the cosignal function) for asynchronous signals. For instance, a SIGFPE handler normally is defined with signal because SIGFPE cannot occur while a coprocess is active except as the result of code executed by that coprocess. On the other hand, a SIGINT handler normally is defined with cosignal because the time at which a SIGINT signal is generated has no relationship to coprocess activity.

The local and global signal handlers are maintained independently. A call to signal returns the previous local handler for the calling coprocess; a call to cosignal returns the previous global handler.

You can also use sigaction to establish local or global handlers. Ordinarily, sigaction defines a local handler. You can set the sa_flags bit SA_GLOBAL to define a global handler instead.

Note that when a signal handler is called, no coprocess switch takes place; the handler executes as part of the interrupted coprocess. If you want a signal to be handled by a particular coprocess, you either can have all other coprocesses block the signal or you can define a global handler that cocalls the relevant coprocess, if that coprocess was not the one interrupted. An example of the latter technique is shown in the cosignal function description.

Note that global handlers generally should not use longjmp unless they first verify that the active coprocess is the one that issued the corresponding call to setjmp .

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