When will Linux kernel reset the signal handler for SIGSEGV to SIG_DFL?

Question

If I have set a signal handler for SIGSEGV, whereas a segmentation fault is generated like:

int *a = NULL;
*a = 1;

The handler will be invoked, but this signal handler will be invoked only once. So, I guess Linux kernel will reset the signal handler to SIG_DFL, but when? I want to know the details, so I checked the Linux kernel source code, but couldn't find the clue yet. Please show me the code if you know the details.

Answer 1

It depends on how you register the signal handler. With sigaction and without the SA_RESETHAND flag, there will be no resetting to SIG_DFL (although returning from a signal handler run in response to a SIGSEGV delivered due to a segmentation fault is technically UB). With SA_RESETHAND it will get reset, and if you register the handler with signal, then whether the handler will be reset or not is unspecified (so don't use signal()).

Example:

#include <signal.h>
#include <unistd.h>

int volatile*a;
void h(int Sig) { write(1,"h\n", 2); }
int main()
{
    //sigaction(SIGSEGV,&(struct sigaction){.sa_handler=h}, 0); //won't reset the handler, will likely loop
    sigaction(SIGSEGV,&(struct sigaction){.sa_handler=h,.sa_flags=SA_RESETHAND}, 0); //will reset the handler
    //signal(SIGSEGV,h); //may or may not reset the handler
    *a=1;
    return 0;
}