C++ - strcmp() does not work correctly?

[Onur Cobanoglu]

There's something really weird going on: strcmp() returns -1 though both strings are exactly the same. Here is a snippet from the output of the debugger (gdb):

    (gdb) print s[i][0] == grammar->symbols_from_int[107][0]
    $36 = true
    (gdb) print s[i][1] == grammar->symbols_from_int[107][1]
    $37 = true
    (gdb) print s[i][2] == grammar->symbols_from_int[107][2]
    $38 = true
    (gdb) print s[i][3] == grammar->symbols_from_int[107][3]
    $39 = true
    (gdb) print s[i][4] == grammar->symbols_from_int[107][4]
    $40 = true
    (gdb) print s[i][5] == grammar->symbols_from_int[107][5]
    $41 = false
    (gdb) print grammar->symbols_from_int[107][4]
    $42 = 0 '\0'
    (gdb) print s[i]
    $43 = (char * const&) @0x202dc50: 0x202d730 "Does"
    (gdb) print grammar->symbols_from_int[107]
    $44 = (char * const&) @0x1c9fb08: 0x1c9a062 "Does"
    (gdb) print strcmp(s[i],grammar->symbols_from_int[107])
    $45 = -1

Any idea what's going on?

Thanks in advance,
Onur

[CrimsonAngel]

Huh? whats a debugger? give me some code ... im the debugger lol

[helios]

whats a debugger?

Quiet, you.

Hmm... This is indeed strange. What if you use your own strcmp()? Does it produce the same behavior?

[Onur Cobanoglu]

Hmm... This is indeed strange. What if you use your own strcmp()? Does it produce the same behavior?

No, it does not. I just tried it and the string comparison function I wrote worked correctly at the same point. So obviously it's a problem with strcmp(). Thanks for your interest.

[helios]

Weird. It seems really unlikely, nay, impossible, for strcmp() to be bugged. What about using it in a different program?

[firedraco]

Make sure there isn't a stray '\n' or something (it might not show on print, I don't remember).

[helios]

Make sure there isn't a stray '\n' or something (it might not show on print, I don't remember).

Yes, but it would show up when comparing each of the elements. The fifth elements of both arrays are equal, and s[i][4] is zero, so there are no hidden characters.

[chrisname]

Post your strcmp code and your code using it and the libC strcmp. This is weird.

[Onur Cobanoglu]

Well, the strcmp() I first used was the standard one from string.h . The string comparison function I wrote is as follows:

bool mystrcmp(const char *s1, const char *s2) {
        int i = 0;
        do {
                if(s1[i] != s2[i])
                        return false;
        } while(s1[i++] != '\0');
        return true;
}

As I told, the one I wrote works correctly at the point strcmp() fails. Actually I suspect something is wrong with g++, since it's giving nonsense errors to me now. I will reboot my computer now and try to upgrade g++, then I'll report back if the problem goes on. Thanks for your interest.

[chrisname]

IIRC strcmp is more like this:

/* strcmp: compare the C-strings s1 and s2
 * Arguments:    const char* s1: a string
 *               const char* s2: a second string, to be compared with s1
 * Return value: 0 if the strings are exactly the same, non-zero otherwise.
 *               If the return value is non-zero; it is the value of the first character in s1 - the first character in s2 which do not match.
 */
int strcmp(const char* s1, const char* s2) {
    for (; (*s1 == *s2) && (*s1 != '\0'); s1++, s2++);
    return (*s1 == *s2) ? 0 : *s1 - *s2;
}

If you called it with this:
strcmp("string1", "string2");
then it would return -1 as '1' - '2' (the ASCII characters and the numbers) = -1.
You would get the same result from ('1' - '0') - ('2' - '0') except in numeric form.