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|Threading, Signals and embedded Prolog|
This section applies to Unix-based environments that have signals or multithreading. The Windows version is compiled for multithreading, and Windows lacks proper signals.
We can distinguish two classes of embedded executables. There are small C/C++ programs that act as an interfacing layer around Prolog. Most of these programs can be replaced using the normal Prolog executable extended with a dynamically loaded foreign extension and in most cases this is the preferred route. In other cases, Prolog is embedded in a complex application that---like Prolog---wants to control the process environment. A good example is Java. Embedding Prolog is generally the only way to get these environments together in one process image. Java VMs, however, are by nature multithreaded and appear to do signal handling (software interrupts).
On Unix systems, SWI-Prolog installs handlers for the following signals:
EINTR, which gives them the opportunity to react to thread-signals.
In some cases the embedded system and SWI-Prolog may both use
SIGUSR2 without conflict. If the embedded system redefines
SIGUSR2 with a handler that runs quickly and no harm is
done in the embedded system due to spurious wakeup when initiated from
Prolog, there is no problem. If SWI-Prolog is initialised
after the embedded system it will call the handler set by the
embedded system and the same conditions as above apply. SWI-Prolog's
handler is a simple function only chaining a possibly previously
registered handler. SWI-Prolog can handle spurious
The --nosignals option can be used to inhibit all
signal processing except for
SIGUSR2. The handling of
is vital for dealing with blocking system call in threads. The used
signal may be changed using the --sigalert=NUM option
or disabled using