Using long data inside a signal handler.

Question

How to set a variable of type long (on a 64-bit machine = 8 bytes) inside the signal handler? I read that you can only use variables of type sig_atomic_t , which are actually implemented as volatile int inside the signal handler, and it is unsafe to change data types that exceed int .

Answer 1

You can use long inside the signal handler, you can use anything, actually. The only thing you should take care of is proper synchronization to avoid race conditions.

sig_atomic_t should be used for variables shared between the signal handler and the rest of the code. Any "private" variable for a signal handler can be of any type, any size.

Code example:

 #include <signal.h> static volatile long badShared; // NOT OK: shared not sig_atomic_t static volatile sig_atomic_t goodShared; // OK: shared sig_atomic_t void handler(int signum) { int localInt = 17; long localLong = 23; // OK: not shared if (badShared == 0) // NOT OK: shared not sig_atomic_t ++badShared; if (goodShared == 0) // OK: shared sig_atomic_t ++goodShared; } int main() { signal(SOMESIGNAL, handler); badShared++; // NOT OK: shared not sig_atomic_t goodShared++; // OK: shared sig_atomic_t return 0; } 

If you want to use a shared variable other than sig_atomic_t , use atomics ( atomic_long_read , atomic_long_set ).