Programming

$ cat gdb.c

#include <stdio.h>
#include <string.h>

int main(void) {
        char *strings[]={"a","B"}; // What happens when we forget to make element 3 NULL?
        char buf[64];
        int n;

        for(n=0; strings[n] != NULL; n++)
        {
                char *val=strings[n];
                strcpy(buf, "123:");
                strcat(buf, val);
                printf("%s\n", buf);
        }
}

$ gcc -ggdb gdb.c

$ ./a.out

123:a
123:B
123:B
Segmentation fault

$ gdb ./a.out

# It's not running yet.  Set a breakpoint so it doesn't run, crash,
# and quit before we can see anything.
(gdb) break main
Breakpoint 1 at 0x400592: file gdb.c, line 5.

(gdb) run
Starting program: /home/tyler/code/c/1shot/a.out
warning: Could not load shared library symbols for linux-vdso.so.1.
Do you need "set solib-search-path" or "set sysroot"?

Breakpoint 1, main () at gdb.c:5
5               char *strings[]={"a","B"};
# Run it one step at a time, to see line numbers.
(gdb) step
9               for(n=0; strings[n] != NULL; n++)
(gdb) step
11                      char *val=strings[n];

# Let's stop at this line every loop.
(gdb) break gdb.c:11
Breakpoint 2 at 0x4005ab: file gdb.c, line 11.

# What registers the relevant values are in can vary.
# That's why gdb needs debugging info -- so you can ask it for
# VARIABLES.
(gdb) print strings[n]
$1 = 0x4006a4 "a"

# That looks ok.  That won't crash.  Let it run.
(gdb) continue
Continuing.
123:a

Breakpoint 2, main () at gdb.c:11
11                      char *val=strings[n];
(gdb) print strings[n]
$2 = 0x4006a6 "B"

# Still OK.
(gdb) continue
Continuing.
123:B

Breakpoint 2, main () at gdb.c:11
11                      char *val=strings[n];
(gdb) print strings[n]
$3 = 0x4006a6 "B"

# Um.  The array has two elements, but this is the third loop?
# This value "looks ok" but is actually garbage.
# You cannot tell this without knowing the program of course...
(gdb) continue
Continuing.
123:B

Breakpoint 2, main () at gdb.c:11
11                      char *val=strings[n];
(gdb) print strings[n]

# We are now two elements beyond the end of the array.
# The first just "happened" to work by coincidence, but THIS WILL
# crash, and this is why:
$4 = 0x300000000 <error: Cannot access memory at address 0x300000000>
(gdb) continue
Continuing.

Program received signal SIGSEGV, Segmentation fault.
0x00007ffff7ab6360 in ?? () from /lib64/libc.so.6
(gdb) quit
A debugging session is active.

        Inferior 1 [process 9589] will be killed.

Quit anyway? (y or n) y

$