Program outcome changes depending on printf usage

Question

Here is my code that creates this problem;

#include <stdint.h>

#define HASHPRINT(STRING) printf(STRING ": %p\n", simple_hash(STRING))

uint32_t simple_hash(const char *string) {
    uint32_t hash;

    char buffer[4]; /* 4 bytes = 32 bits */
    const char *c;
    int i = 0;

    for(c=string; *c!=0; c++) {
        buffer[i] = *c;
        i++;
        if (i == 3) {
            i=0;
            printf("\nAdding %u to hash\n", *((uint32_t *)buffer));
            hash += *((uint32_t *)buffer);
            hash = hash ^ *((uint32_t *)buffer);
        }
    }

    if (i > 0) {
        hash += *((uint32_t *)buffer);
        hash = hash ^ *((uint32_t *)buffer);
    }

    return hash;
}

void main() {
    HASHPRINT("yasar");
    HASHPRINT("rasay");
    HASHPRINT("arsay");
    HASHPRINT("yasra");
    HASHPRINT("osman");
    HASHPRINT("ali");
    HASHPRINT("veli");
}

Program output changes depending on if I comment out or not the printf function call in line 18.

Without printf, my program outputs this:

yasar: 7D90F834
rasay: 00000005
arsay: 00000003
yasra: 00000001
osman: 00000003
ali: 00000001
veli: 00000005

If I enable the printf function, I get this output instead;

Adding 2004050297 to hash
yasar: 0F2400A0

Adding 7561586 to hash
rasay: 78C921B4

Adding 7565921 to hash
arsay: 78C94C94

Adding 7561593 to hash
yasra: 78C92194

Adding 7172975 to hash
osman: 78DD7FAC

Adding 6909025 to hash
ali: 7842A494

Adding 7103862 to hash
veli: 78C3698C

I would expect that in both cases, calculated hash values (which are printed after word, seperated from word with :) would be the same.

I was wonderin what would be the cause of this problem.

I am using WinXp with MinGW with gcc 4.8.1 version.

score 1 · Answer 1 · answered Sep 02 '15 at 11:30

The function simple_hash() uses uninitialized values of hash and buffer[3] - no wonder that the program's output varies unpredictably.

Note also that the conversion specification %p is inappropriate for uint32_t arguments - correct would be e. g.

#include <inttypes.h>
#define HASHPRINT(STRING) printf(STRING": %08"PRIx32"\n", simple_hash(STRING))

Program outcome changes depending on printf usage

1 Answers1