Question

I am playing with pointers just doing some basic stuff to solidify my understanding of them. When I try debugging and following this example that I found on the web using GDB’s ‘next’ and ‘step’, GDB runs off the end of the function. After it reaches the statement “return 0;” it tells me that it “Cannot access memory at address 0x0 0x0000000100000de4 in start()

This is the code:

#include <cstdio>
#include <ctype.h>


int main()
{
    char my_str[] = "hello world";
    *my_str = toupper(*my_str);
    *(my_str + 6) = toupper(*(my_str + 6));
    printf("%s", my_str); // prints, "Hello World"
    return 0;
}

This is gdb’s output:

Breakpoint 1, main () at pwp.cpp:10
10       return 0;
(gdb) n
Cannot access memory at address 0x0
0x0000000100000de4 in start ()
(gdb) s
Single stepping until exit from function start, 
which has no line number information.
0x0000000100000ed6 in dyld_stub_exit ()
(gdb) n
Single stepping until exit from function dyld_stub_exit, 
which has no line number information.
0x0000000100000f08 in dyld_stub_printf ()
(gdb) n
Cannot find bounds of current function
(gdb) q

What is happening to cause this?

Answer 1

Returning from main() does not immediately exit your program — libc does some cleanup before exiting, which includes flushing output on file descriptors such as stdout (which is necessary here, because you didn’t include a \n in your printf).

The n command in GDB attempts to step by one line of source code by default. Since you are single-stepping into code that you don’t have source available for (libc), and because the code you’re stepping into is somewhat odd (it’s a dynamic library “stub” function), the command doesn’t work correctly. If you really want to step one instruction at a time, use si.