Writing Threaded Applications

• All DCE applications are multi-threaded — DCE runtime software is multi-threaded and all DCE applications are multi-threaded; even if the application code itself does not explicitly create threads.

• Using non-thread-safe libraries — When making calls to libraries which are not known to be specifically thread-safe, a locking scheme needs to be provided by the application, For example, non-thread-safe routines 1 and 2 make a call to routine A (also non-thread-safe), if routines 1 and 2 use different mutexes to lock their calls to routine A, then routines 1 and 2 can both get into routine A at the same time (violating the programmer's attempt to make the calls thread-safe).

• Using fork() in a threaded application — fork() is not allowed from a threaded task.

• environ is a process-wide resource — Programmers must coordinate threads that use the putenv() and getenv() interfaces to change and read environ.

• Signal mask: A thread-specific resource — If one thread manipulates the signal mask, it only affects signals that a specific thread is interested in.

• Handling synchronous terminating signals — The default behavior of OSF DCE 1.0.2 is to translate synchronous terminating signals into exceptions. If the exception is not caught, the thread that caused the exception is terminated. Any thread that goes through the terminate code causes the entire task to be terminated.

• Establish synchronous signal handlers using sigaction() — It is used to establish handlers for synchronous signals on an individual thread basis only.

• Asynchronous signals — There is no supported mechanism for establishing signal handlers for asynchronous signals on MPE/iX.

• Cancelling threads blocked on a system call — The HP e3000 Kernel Threads provide a cancellation facility that enables one thread to terminate another. The canceled thread normally terminates at a well-defined point. Terminating a thread that is blocked while executing system code is not possible on MPE/iX; only threads executing non-system code may be canceled.

• Using waitpid() — The waitpid() routine allows the parent thread to specify which child it cares about by specifying its PID. This call only works for the initial thread; because children created by any thread within the task are considered children of the whole task.

• Using setjmp and longjmp — Do not use calls to setjmp and longjmp, these routines save and restore the signal mask and could inadvertently cause a signal that another thread is waiting on to be masked. Instead, use _setjmp and _longjmp; these routines do not manipulate the signal mask.

  When executing _longjmp be aware of the following:

  • Ensure you are returning to a state saved within the context of the same thread.

  • If you _longjmp over a TRY clause, an exception could try to _longjmp to a stack frame that no longer exists; and vice versa.

  • Do not _longjmp out of a signal handler.