A signal handler can run while the program is doing something else. An asynchronous signal safe function may be called from that handler without violating C’s signal-handling rules. The restriction matters because a handler must not access or change program state in an uncontrolled way. section 17.6
The C standard guarantees only a small set of calls in this context:
- abort, _Exit, and quick_exit, which terminate the program. section 17.6
- signal, when used for the same signal number that invoked the handler. section 17.6
- Certain functions acting on atomic objects. section 17.6
TAKEAWAY: Unless specified otherwise, a C library function is not asynchronous signal safe. In particular, do not call exit or perform I/O from an asynchronous signal handler. A platform may permit more, but that does not make those calls portable. section 17.6
143 /**
144 ** @brief A minimal signal handler
145 **
146 ** After updating the signal count, for most signals this
147 ** simply stores the signal value in "interrupt" and returns.
148 **/
149 static void signal_handler(int sig) {
150 sh_count(sig);
151 switch (sig) {
152 case SIGTERM: quick_exit(EXIT_FAILURE);
153 case SIGABRT: _Exit(EXIT_FAILURE);
154 #ifdef SIGCONT
155 // continue normal operation
156 case SIGCONT: return;
157 #endif
158 default:
159 /* reset the handling to its default */
160 signal(sig, SIG_DFL);
161 interrupt = sig;
162 return;
163 }
164 }
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The handler demonstrates permitted exits through quick_exit and _Exit, and resets handling with signal for the signal it received. Its call to sh_count relies on the separately qualified atomic-counting implementation. section 17.6
