Record a signal number
A signal handler can assign the received signal number to a sig_atomic_t object and return. The assignment remains uninterruptible with respect to signal handling. 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 default case assigns sig to interrupt before returning; the surrounding example uses sig_atomic_t for that object. section 17.6
TAKEAWAY: sig_atomic_t is an integer type whose object evaluations (loads) and assignments (stores) appear indivisible to signal handlers. “Uninterruptible” describes what can be observed, not necessarily how the operation is performed. Its width may be limited, though it is at least 8 bits. section 17.6
Do not use it as a counter: ++ can involve separate load, increment, and store steps, and the value can overflow. When counting signals, a lock-free atomic type is a distinct option where C11 atomics are available. section 17.6
