How to use GDB inside giant loops

Question

I have the following loop and my code breaks but I don't know at which iteration it breaks exactly.

int n=1000;
for (i=0; i<n; i++) {
                slot = random() % max_allocs;
                doAlloc = random() % 4;
                doWrite = writeData;

                if (!doAlloc || ptr[slot] != NULL) {
                        if (ptr[slot] == NULL)
                                ;//assert(Mem_Free(ptr[slot]) == -1);
                        else
                        {
                                printf("I got here \n");
                                printf("mem free ptr slot is %d \n",Mem_Free(ptr[slot]));
                        }
                        free(shadow[slot]);
                        ptr[slot] = NULL;
                        shadow[slot] = NULL;
                }

                if (doAlloc) {
                        size[slot] = min_alloc_size +
                                (random() % (max_alloc_size - min_alloc_size + 1));
                        printf("size[slot] :%d\n", size[slot]);
                        ptr[slot] = Mem_Alloc(size[slot], BESTFIT);
                        printf("ptr slot is %p \n",ptr[slot]);
                        assert(ptr[slot] != NULL);
                        if (doWrite) {
                                shadow[slot] = malloc(size[slot]);
                                int j;
                                for (j=0; j<size[slot]; j++) {
                                        char data = random();
                                        *((char*)(ptr[slot] + j)) = data;
                                        *((char*)(shadow[slot] + j)) = data;
                                }
                        }
                }
        }

How can I find at which iteration of n the code breaks and how can I put a breakpoint at that iteration?

P.S.: Is there any other better debugger for this purpose in Linux? (If I don't want to use Eclipse!)

Here's the error I am receiving in gdb:

mymain: mymain.c:104: main: Assertion `ptr[slot] != ((void *)0)' failed.

Program received signal SIGABRT, Aborted.
0x000000368da328e5 in raise (sig=6) at ../nptl/sysdeps/unix/sysv/linux/raise.c:64
64    return INLINE_SYSCALL (tgkill, 3, pid, selftid, sig);
(gdb) backtrace
#0  0x000000368da328e5 in raise (sig=6) at ../nptl/sysdeps/unix/sysv/linux/raise.c:64
#1  0x000000368da340c5 in abort () at abort.c:92
#2  0x000000368da2ba0e in __assert_fail_base (fmt=<value optimized out>, assertion=0x40114b "ptr[slot] != ((void *)0)", file=0x401142 "mymain.c", line=<value optimized out>, function=<value optimized out>)
    at assert.c:96
#3  0x000000368da2bad0 in __assert_fail (assertion=0x40114b "ptr[slot] != ((void *)0)", file=0x401142 "mymain.c", line=104, function=0x401199 "main") at assert.c:105
#4  0x0000000000400e2a in main (argc=4, argv=0x7fffffffdb68) at mymain.c:104
(gdb) frame 1
#1  0x000000368da340c5 in abort () at abort.c:92
92        raise (SIGABRT);
(gdb) frame 3
#3  0x000000368da2bad0 in __assert_fail (assertion=0x40114b "ptr[slot] != ((void *)0)", file=0x401142 "mymain.c", line=104, function=0x401199 "main") at assert.c:105
105   __assert_fail_base (_("%s%s%s:%u: %s%sAssertion `%s' failed.\n%n"),

Answer 1

take a look at this: GDB Tutorial. You can use break (to set a breakpoint) and continue / next to do what you want:

1. Don't forget to compile with -g option: gcc -g source.c
2. gdb ./a.out
3. break linenumber
4. continue or next (to proceed to the next breakpoint)
5. print variable (to print the value of variable)

Hope it helps.