Any ideas on this? Is there some kind of a signal queue, or does it get dropped?
While we are at this question, is it true that signal handlers should do as minimal work as possible?
I read somewhere that a signal handler should use a pipe and just write one byte to it, indicating what the program should do. Then somewhere else the program periodically checks the pipe, and dispatches based on byte in it. (I might have misunderstood it)
Thanks, Boda Cydo.
To answer the second part of your question, "is it true that signal handlers should do as minimal work as possible?" the answer is yes, because there is a very minimal set of functions that are "async signal safe" and therefore able to be called from signal handlers. Async signal safety is kind of an enhanced form of re-entrancy. If foo() is async signal safe, that means that it's safe to call foo() within a signal handler, even if foo() was already executing when the signal was raised.
You can get the full list of async signal safe functions by looking that the section 7 man page for signal (man 7 signal). Calling any function other than one of these from within a signal handler, directly or indirectly, invokes undefined behavior.
The "write a byte to a pipe" approach is a great way to deal with signals without being restricted to async signal safe functions, especially if your program is already oriented around a select loop.
To answer the first part of your question, the default is as follows: If it's the same signal as the one currently being handled, the new signal is blocked (held in a queue) and delivered when the handler returns. If some other signal arrives, the handler for the new signal is called. c.f. the glibc manual.