gdb: cascaded signal. info on outer one - HOWTO - c

Question

Following program generates SIGSEGV from within SIGUSR1 signal handler.
#include <signal.h>
#include <unistd.h>
#include <iostream>
void sigusr_handler(int sig_number)
{
std::cout << "hallo" << "\n";
char *a = (char*)0x0;
char b = *a;
std::cout << b << "\n";
}
struct sigaction my_action;
int main(int argc, char *argv[])
{
my_action.sa_handler = sigusr_handler;
sigaction(SIGUSR1, &my_action, nullptr);
sleep(1000);
return 0;
}
Let's run program in gdb then send SIGUSR1 to the program.
siginfo looks like following
(gdb) p $_siginfo
$8 = {
si_signo = 11,
si_errno = 0,
si_code = 1,
_sifields = {
_pad = {0, 0, -1814214640, 21880, -1601342336, 32766, -1848922720, 21880, -1811928400, 21880, 0, 0, -1811928400, 21880, 176, 0, -1840786848, 21880, -1847783957, 21880, -1811928400, 21880, -1840786848, 21880, -1814214640, 21880, -1845891081, 21880},
_kill = {
si_pid = 0,
si_uid = 0
},
_timer = {
si_tid = 0,
si_overrun = 0,
si_sigval = {
sival_int = -1814214640,
sival_ptr = 0x557893dd4810
}
},
_rt = {
si_pid = 0,
si_uid = 0,
si_sigval = {
sival_int = -1814214640,
sival_ptr = 0x557893dd4810
}
},
_sigchld = {
si_pid = 0,
si_uid = 0,
si_status = -1814214640,
si_utime = -6877712962820221576,
si_stime = -7941062615231332354
},
_sigfault = {
si_addr = 0x0,
_addr_lsb = 93976365189136,
_addr_bnd = {
_lower = 0x7ffea08d7480,
_upper = 0x557891cbada0
}
},
_sigpoll = {
si_band = 0,
si_fd = -1814214640
}
}
}
Question: is it possible to "guess" from gdb session, that outer signal was SIGUSR1?
regards,
Mat
NOTE TO STACKOVEFLOW: I have no idea what usefull notes I need to add to the above in order to have the question more usefull - IMO: more text makes the question blurred, but I'm adding some notes because I'm getting information:
"It looks like your post is mostly code; please add some more details" otherwise.
READERS OF POST: PLEASE FORGIVE ME.
END OF NOTE TO STACKOVERFLOW

Answer 1

is it possible to "guess" from gdb session, that outer signal was SIGUSR1
Yes:
hallo
Program received signal SIGSEGV, Segmentation fault.
0x00005555555551be in sigusr_handler (sig_number=10) at t.c:9
9 char b = *a;
(gdb) bt
#0 0x00005555555551be in sigusr_handler (sig_number=10) at t.c:9
#1 <signal handler called>
#2 0x00007ffff775cff4 in __GI___nanosleep (requested_time=requested_time#entry=0x7fffffffdb70, remaining=remaining#entry=0x7fffffffdb70)
at ../sysdeps/unix/sysv/linux/nanosleep.c:28
#3 0x00007ffff775cf2a in __sleep (seconds=0) at ../sysdeps/posix/sleep.c:55
#4 0x0000555555555225 in main (argc=1, argv=0x7fffffffdca8) at t.c:20
From this, you can clearly see that the program received SIGSEGV while in the handler for signal 10, which is SIGUSR1.
P.S. It's unclear how this question even arises: GDB should have earlier stopped and told you that SIGUSR1 was received, like so:
Program received signal SIGUSR1, User defined signal 1.
0x00007ffff775cff4 in __GI___nanosleep (requested_time=requested_time#entry=0x7fffffffdb70, remaining=remaining#entry=0x7fffffffdb70) at ../sysdeps/unix/sysv/linux/nanosleep.c:28
28 ../sysdeps/unix/sysv/linux/nanosleep.c: No such file or directory.