C Programming, Part 4: Debugging

The Hitchhiker's Guide to Debugging C Programs

Feel free to add anything that you found helpful in debugging C programs including but not limited to, debugger usage, recognizing common error types, gotchas, and effective googling tips.

1. Make your code modular using helper functions. If there is a repeated task (getting the pointers to contiguous blocks in MP2 for example), make them helper functions. And make sure each function does one thing very well, so that you don't have to debug twice.

2. Use assertions to make sure your code works up to a certain point -- and importantly, to make sure you don't break it later. For example, if your data structure is a doubly linked list, you can do something like, assert(node->size == node->next->prev->size) to assert that the next node has a pointer to the current node. You can also check the pointer is pointing to an expected range of memory address, not null, ->size is reasonable etc. The NDEBUG macro will disable all assertions, so don't forget to set that once you finish debugging. http://www.cplusplus.com/reference/cassert/assert/

GDB

Introduction: http://www.cs.cmu.edu/~gilpin/tutorial/

Setting breakpoints programmatically

A very useful trick when debugging complex C programs with GDB is setting breakpoints in the source code.

int main() {
    int val = 1;
    val = 42;
    asm("int $3"); // set a breakpoint here
    val = 7;
}

$ gcc main.c -g -o main && ./main
(gdb) r
[...]
Program received signal SIGTRAP, Trace/breakpoint trap.
main () at main.c:6
6	    val = 7;
(gdb) p val
$1 = 42

Checking memory content

http://www.delorie.com/gnu/docs/gdb/gdb_56.html

For example,

int main() {
    char bad_string[3] = {'C', 'a', 't'};
    printf("%s", bad_string);
}

$ gcc main.c -g -o main && ./main
$ Cat ZVQ�� $

(gdb) l
1	#include <stdio.h>
2	int main() {
3	    char bad_string[3] = {'C', 'a', 't'};
4	    printf("%s", bad_string);
5	}
(gdb) b 4
Breakpoint 1 at 0x100000f57: file main.c, line 4.
(gdb) r
[...]
Breakpoint 1, main () at main.c:4
4	    printf("%s", bad_string);
(gdb) x/16xb bad_string
0x7fff5fbff9cd:	0x63	0x61	0x74	0xe0	0xf9	0xbf	0x5f	0xff
0x7fff5fbff9d5:	0x7f	0x00	0x00	0xfd	0xb5	0x23	0x89	0xff

Here, by using the x command with parameters 16xb, we can see that starting at memory address 0x7fff5fbff9c (value of bad_string), printf would actually see the following sequence of bytes as a string because we provided a malformed string without a null terminator.

0x43 0x61 0x74 0xe0 0xf9 0xbf 0x5f 0xff 0x7f 0x00