SIGSEGV signal handler not called after DirectFBCreate - c

Question

In my application I want to implement backtrace on segmentation fault according to this post:
How to generate a stacktrace when my gcc C++ app crashes
But I encountered a problem. My application uses DirectFB for graphics. After I initialize DirectFB by calling DirectFBCreate, the signal handler stops to be called.No matter where the signal handler is registered. Please compare main1, main2 and main3 functions in code bellow:
#include <stdio.h>
#include <execinfo.h>
#include <signal.h>
#include <stdlib.h>
#include <unistd.h>
#include <directfb.h>
void handler(int sig) {
void *array[10];
size_t size;
// get void*'s for all entries on the stack
size = backtrace(array, 10);
// print out all the frames to stderr
fprintf(stderr, "Error: signal %d:\n", sig);
backtrace_symbols_fd(array, size, STDERR_FILENO);
exit(1);
}
void baz() {
int *foo = (int*)-1; // make a bad pointer
printf("%d\n", *foo); // causes segfault
}
void bar() { baz(); }
void foo() { bar(); }
int main1(int argc, char **argv) {
signal(SIGSEGV, handler); // install our handler
// if the foo() function is called here,
// everything works as it should
foo();
IDirectFB *dfb = NULL;
DFBCHECK (DirectFBInit (&argc, &argv));
DFBCHECK (DirectFBCreate (&dfb));
}
int main2(int argc, char **argv) {
signal(SIGSEGV, handler); // install our handler
IDirectFB *dfb = NULL;
DFBCHECK (DirectFBInit (&argc, &argv));
DFBCHECK (DirectFBCreate (&dfb));
// but calling the foo() function after DirectFBCreate causes
// that the handler is not called
foo();
}
int main2(int argc, char **argv) {
IDirectFB *dfb = NULL;
DFBCHECK (DirectFBInit (&argc, &argv));
DFBCHECK (DirectFBCreate (&dfb));
signal(SIGSEGV, handler); // install our handler
// calling the foo() function after DirectFBCreate causes,
// that the handler is not called
// no matter the signal handler is registered after DirectFBCreate calling
foo();
}
I have also tried sigaction function instead of signal function, with the same result.
I have also tried using sigprocmask(SIG_SETMASK, &mask, NULL) to unblock the signal. But this also didn't help (which I expected).
Finally I found this post signal handler not working,
which seems to solve similar problem by disabling the library's signal handler by calling zsys_handler_set(NULL);. So I tried signal(SIGSEGV, NULL); and signal(SIGSEGV, SIG_DFL);. Again not succeeded. I didn't find any handler disabling function in DirectFB. Although I found [no-]sighandler argument in DirectFB config and used it, this didn't help neighter (which surprised me a lot).
My question is: If the DirectFB is able to steel my handler, how can I take it back?

Answer 1

I used strace as mentioned in the comment. I found out that the DirectFB doesn't call sigaction syscall but it blocks some signals, SIGSEGV included. Unblock the signal after DirectFB initialization was the soulution.
// DirectFb initialization
IDirectFB *dfb = NULL;
DFBCHECK (DirectFBInit (&argc, &argv));
DFBCHECK (DirectFBCreate (&dfb));
// Unblock the signal
sigset_t sa_mask;
sigemptyset(&sa_mask);
sigaddset(&sa_mask, SIGSEGV);
sigprocmask(SIG_UNBLOCK, &sa_mask, NULL);
// here is important to use SIG_UNBLOCK flag
// not SIG_SETMASK as I did in my question!!!
// Now this causes the handler call
foo();