signal versus sigaction

the use of the

signal(int signum, void (*handler)(int))

is a smidgin dangerous on various operating systems. Under Solaris, for example once the signal has been delivered to the process the signal handler is reset, so a typical piece of code that wants to reuse the signal handler repeatedly will typically set the signal handler again when receiving the signal. This leads to a minor race condition where upon receipt of the signal and the re-setting of the handler the process receives another copy of the same signal. Some of these signals cause Bad things to happen – such as the stopping of the process (SIGTSTP for example). Under Linux it keeps the signal handler in place, so you have no fear of the event triggering an unwanted event.
The manual page for


under Linux makes it clear that the call is deprecated in favour of the much more functional

sigaction(int sig, const struct sigaction *restrict act, struct sigaction *restrict oact)

call, which keeps signal handlers in place when you don’t pass the SA_RESETHAND parameter as part of the sa_flags parameter of the sigaction structure. So you get to explicitly choose to accept a signal once, and then have the system deal with it in the default manner afterwards.
Signals, are of course a real pain in the ass when dealing with sub-processes. For example the use of ptrace to perform profiling works well until you fork. If another SIGPROF signal arrives before you can create your signal handler then the child process is terminated as that’s the default behaviour in that situation.
Under Solaris (and Leopard) you can make use of dtrace to perform profiling on a set of processes without needing to deal with vagaries of signal handling, making this a non-issue. For those of you stuck in LD_PRELOAD land, probably the only thing that can be done is to set the signal disposition to be ignored before execing the new process. you have a small window where the profiling is missing, but the overall increased stability of the application is improved by preventing it from accidentally being terminated due to a profiling signal being received too soon. I know the accuracy nuts would hate that, but it’s part of the price of dealing with standards.