Signals

• American National Standards Institute (ANSI) C signals are supported with the function signal(). Because this API is built on top of the POSIX.1 sigaction() model, it might behave unexpectedly.
• POSIX.1 signals are supported with the functions sigaction(), sigpending(), sigprocmask(), sigsuspend(), sigemptyset(), sigfillset(), sigaddset(), sigdelset(), and sigismember().
• Berkeley Software Distribution (BSD) 4.3 signals are supported with the functions killpg(), sigsetmask(), sigblock(), and sigvec(). The signal mask for these functions is an int, not a sigset_t. If a future release of the Interix SDK supports more than 32 signals, these functions will become obsolete. Rather than depend on them, convert your code to use the POSIX.1 signal calls. Although sigpause() is provided, it is provided as the System V call, which does not behave in the same way as the BSD call.
• System V signals are supported with the functions sighold(), sigignore(), sigpause(), sigrelse(), and sigset().

The following important information applies when you are using signals with the Interix SDK:

• Signals on a POSIX.1 system are neither BSD nor System V Interface Definition (SVID). POSIX defined a new signal mechanism based on the API sigaction().
• The set of signals available is the set described in POSIX.1 and the Single UNIX Specification. Your code should use the symbolic names instead of the actual values, since some numbers might differ from traditional implementations.

The POSIX.1 committee introduced their new signal semantics because of problems with traditional signal implementations found on BSD and System V systems. When the System V3 signal() catches a signal, the action associated with the signal is reset to the default. In 4.3 BSD, it is not reset. In the International Standards Organization/American Standards Institute (ISO/ANSI) C standard, the signal() function either resets the default or does an implementation-defined blocking of the signal. The POSIX sigaction() call does not reset the default if the handler returns normally. The Interix SDK follows the POSIX signal semantics.

Because an Interix SDK process has a signal mask, it can block certain signals from arriving, except for SIGKILL or SIGQUIT. A process starts with a signal mask inherited from its parent. If any signals are generated and then blocked by the signal mask, they go into the set of pending signals.

If you are using the signal() API, the signal is still masked and remains masked until the mask is cleared. This can be a significant problem if your code does a longjmp() from the handler. Using sigaction() directly and siglongjmp() will correct some of those unexpected behaviors.

This section covers:

• Available signals
• Signal data structures
• Additional signal functions