Question

I’m trying to implement a function which returns a string of hex values. I print the hex values out using this function:

void print_hex(unsigned char *hash, const hashid type) {

    int i;
    for (i = 0; i < mhash_get_block_size(type); i++) {
        printf("%.2x", hex[i]);
    }
    printf("\n");
}

This outputs some hex value, e.g. 71c092a79cf30c4c7e7baf46a4af3c78cedec9ae3867d1e2600ffc39d58beaf2

How can I modify this function so that it returns a string? i.e.

unsigned char *get_hash_str(unsigned char *hash, const hashid type) { /* ?? */ }

(The goal being to compare the 2 values later)

Answer 1

char * print_hex(const unsigned char *hash, const hashid type)
{
    const char lookupTable[]="0123456789abcdef";
    const size_t hashLength=mhash_get_block_size(type);
    size_t i;
    char * out=malloc(hashLength*2+1);
    if(out==NULL)
        return NULL;
    for (i = 0; i < hashLength; i++)
    {
        out[i*2]=lookupTable[hash[i]>>4];
        out[i*2+1]=lookupTable[hash[i]&0xf];
    }
    out[hashLength*2]=0;
    return out;
}

Obviously the caller is responsible for freeing the returned string.

Still, as @K-Ballo correctly said in his answer, you don’t need to convert to string form two hashes to compare them, all you need in that case is just a memcmp.

int compare_hashes(const unsigned char * hash1, const hashid hash1type, const unsigned char * hash2, const hashid hash2type)
{
    if(hash1type!=hash2type)
        return 0;
    return memcmp(hash1, hash2, mhash_get_block_size(hash1type))==0;
}