In a signal handler, how to know where the program is interrupted?

Question

On x86 (either 64-bit or 32-bit) Linux -- for example:

void signal_handler(int) {
   // want to know where the program is interrupted ...
}

int main() {
    ...
    signal(SIGALRM, signal_handler);
    alarm(5);
    ...
    printf(...); <------- at this point, we trigger signal_handler
    ...
}

In signal_handler, how can we know we are interrupted at printf in main()?

Answer 1

Use sigaction with SA_SIGINFO set in sa_flags.

Prototype code:

#define _GNU_SOURCE 1  /* To pick up REG_RIP */
#include <stdio.h>
#include <signal.h>
#include <assert.h>

static void
handler(int signo, siginfo_t *info, void *context)
{
    const ucontext_t *con = (ucontext_t *)context;

    /* I know, never call printf from a signal handler.  Meh. */
    printf("IP: %lx\n", con->uc_mcontext.gregs[REG_RIP]);
}

int
main(int argc, char *argv[])
{
    struct sigaction sa = { };
    sa.sa_flags = SA_SIGINFO;
    sa.sa_sigaction = handler;
    assert(sigaction(SIGINT, &sa, NULL) == 0);
    for (;;);
    return 0;
}

Run it and hit Ctrl-C. (Use Ctrl-\ to terminate...)

This is for x86_64. For 32-bit x86, use REG_EIP instead of REG_RIP.

[edit]

Of course, if you are actually in a library function (like printf) or a system call (like write), the RIP/EIP register might point somewhere funny...

You might want to use libunwind to crawl the stack.