Signals, Part 2: POSIX Signals

The POSIX signal implementation ensures that if a process is already

handling a signal, other incoming signals are suspended until the

handler returns. However, if a signal SIGx is sent while a SIGx signal

is already pending, the kernel delivers only one SIGx signal to the

process. This isn't much of a problem since signals don't carry any

information other than the signal number itself. Thus, sending a signal

multiple times in very short periods is similar to sending it once.

Signal Sets

The POSIX signal functions (declared in

) operate on sets of signals packed in a sigset_t datatype. Here are their prototypes and descriptions: * int sigemptyset(sigset_t * pset); -- Clears all the signals in pset. * int sigfillset(sigset_t * pset); -- Fills pset with all available signals. * int sigaddset(sigset_t * pset, int signum); -- Adds signal signum to pset. * int sigdelset(sigset_t * pset, int signum); -- Removes signal signum from pset. * int sigismember(const sigset_t * pset, int signum); -- Returns a nonzero value if signum is included in pset, 0 otherwise. Registering a Handler To register a handler, call sigaction(): int sigaction(int signum, struct sigaction * act, struct sigaction * prev); sigaction() sets a handler for signal signum. The kernel's handling of signum is described in the argument act, which is of type sigaction: struct sigaction { sighanlder_t sa_hanlder; sigset_t sa_mask; unsigned long sa_flags; void (*sa_restorer)(void); /*never used*/ }; sa_hanlder is a pointer to a function of the type: void handler (int signum); However, it can also take one of two special values: SIG_DFL and SIG_IGN. The sa_mask field contains all the signals that the kernel should block while the handler is running. Note that the signal being handled is always blocked, regardless of sa_mask's value. You can override this behavior by setting the appropriate flags of the sa_flags field. These flags can take one or more of the following values combined by the bitwise OR operator: * SA_NOCLDSTOP -- Ensures a parent doesn't receive a SIGCHLD signal if one of its children is stopped. * SA_NOMASK -- Overrides the default blocking of the signal if its handler is currently executing. This flag enables you to emulate ANSI's unreliable signals. * SA_ONESHOT -- Resets the handler of signum to SIG_DFL. This flag emulates the behavior of the ANSI signal() function * SA_RESTART -- Ensures that a syscall restarts when the handler exits. If sigactions's final argument isn't NULL, it is filled with the disposition of signum before sigaction() was called. To obtain the current signal disposition without altering it, pass NULL as a second argument and a valid sigaction pointer as the third.
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