sigaction not initialized gcc 7

Question

Let be this C file:

#include <stdio.h>
#include <signal.h>

void handle(){
    return;
}

int main() {
    struct sigaction action;
    action.sa_handler = &handle;
    printf("%d\n", action.sa_flags);
    return 0;
}

Compiling with gcc-7 (Ubuntu 7.4.0-1ubuntu1~18.04.1) throws a warning:

src/flags.c:14:2: warning: ‘action.sa_flags’ is used uninitialized in this function [-Wuninitialized]
  printf("%d\n", action.sa_flags);

However, with gcc-5 (Ubuntu 5.4.0-6ubuntu1~16.04.11), no warning.

With gcc-7, the sa_flags value is really not initialized:

(gdb) x action.sa_flags
0x555545a0:    Cannot access memory at address 0x555545a0

where with gcc-5, it gets a strange value:

(gdb) x action.sa_flags
0x4004a0 <_start>:    0x8949ed31

I don't get why:

• with gcc-7, sa_flags are not initiliazed to 0.
• with gcc-5, sa_flags and the _start function have the same address.

Answer 1

The struct sigaction action isn't initialized in either compiler. You set action.sa_handler but all other fields remain uninitialized. Starting with gcc-7 the compiler is smart enough to notice you only initialized some fields of the struct leaving other uninitialized. Gcc-5 simply isn't that smart but that doesn't mean the struct was ever initialized. What you see in action.sa_flags is simply whatever garbage is left on the stack from the crt0 code before main was called.

Although technically wrong for pointers in structs the general solution to this is to memset() the struct to 0.