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:
- has an empty signal handler. This signal is sent to a thread after
sending a thread-signal (see
It causes blocking system calls to return with
EINTR, which gives them the opportunity to react to thread-signals.
In some cases the embedded system and SWI-Prolog may both use
SIGUSR2without conflict. If the embedded system redefines
SIGUSR2with 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
- is used by the top level to activate the tracer (typically bound to control-C). The first control-C posts a request for starting the tracer in a safe, synchronous fashion. If control-C is hit again before the safe route is executed, it prompts the user whether or not a forced interrupt is desired.
- SIGTERM, SIGABRT and SIGQUIT
- are caught to cleanup before killing the process again using the same signal.
- SIGSEGV, SIGILL, SIGBUS, SIGFPE and SIGSYS
- are caught by to print a backtrace before killing the process again using the same signal.
- is caught and causes the process to exit with status 2 after cleanup.
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