Error passing by reference when making os

Question

I am making an os and have booted into a 64 bit kernel made in c. I have made a print function which is working and am trying to make a function to convert hex values to string so I can print them. My code is causing boot loops, yet when I compile the exact same code to run normally in linux it works perfectly. The relevant code:

int logarithm(double value, int base, int* output) {
    int i = 0;

    while (value > 1) {
        value /= base;
        i++;
    }

    *output = i;
}

int power(int value, int power, int* output) {
    if (power == 0) {
        value = 1;
    } else {
        for (int i = 0; i < (power - 1); i++) {
            value *= value;
        }
    }

    *output = value;
}

void hexToStr(unsigned int hex, char** string) {
    int hexLength = 0;
    logarithm((double)hex, 16, &hexLength);
    char output[hexLength];
    output[hexLength] = 0;

    int powerValue = 0;
    for (int i = 0; i < hexLength; i++) {
        power(16, i, &powerValue);
        output[hexLength - i - 1] = (hex & (powerValue * 15)) / powerValue + '0';
    }

    *string = output;
}

If I change the hexToStr() function code to this (removing the need for logarithm() and power() functions by hardcoding values for the string), it works in both linux and my kernel:

void hexToStr(unsigned int hex, char** string) {
    int hexLength = 10;
    char output[hexLength];
    output[hexLength] = 0;

    int powerValue = 0;
    for (int i = 0; i < hexLength; i++) {
        output[hexLength - i - 1] = 'A';
    }

    *string = output;
}

Any suggestions as to why this would happen?

Answer 1

Needed to enable SSE unit to work with floats and doubles. As well as change how values are passed back. Working code:

void log(float value, float base, uint64_t* output) {
    uint64_t i = 0;

    while (value >= 1) {
        value /= base;
        i++;
    }

    *output = i;
}

void pow(uint64_t value, uint64_t exponent, uint64_t* output) {
    uint64_t result = 1;

    for (uint64_t i = 0; i < exponent; i++) {
        result = result * value;
    }

    *output = result;
}

void hexToStr(uint64_t hex, char* output) {
    uint8_t hexLen = 16;
    log((float)hex, (float)16, &hexLen);
    char result[hexLen + 3];
    result[0] = '0';
    result[1] = 'x';
    result[hexLen + 2] = 0;

    uint64_t powerValue = 1;
    for (uint8_t i = 0; i < hexLen; i++) {
        pow(16, i, &powerValue);
        result[hexLen - i + 1] = (hex & (uint64_t)(powerValue * (uint64_t)15)) / powerValue + '0';
    }

    for (uint8_t i = 0; i < hexLen + 3; i++) {
        switch(result[i]) {
            case ':':
                result[i] = 'A';
                break;
            case ';':
                result[i] = 'B';
                break;
            case '<':
                result[i] = 'C';
                break;
            case '=':
                result[i] = 'D';
                break;
            case '>':
                result[i] = 'E';
                break;
            case '?':
                result[i] = 'F';
                break;
        }
        output[i] = result[i];
    }
}