Double precision str

#include <stdio.h>
#include <stdlib.h>
#include <math.h>

int main(void) {
    char  *str     = (char *) malloc(sizeof(char *));
    int    offset  = 0;
    double num     = 123456.00001;
    int    num_int = (int) num;
    double decimal = num - num_int;
    int    digits  = 0;

    if (num < 0) {
        str[offset++] = '-';
    }

    while (num_int) {
        digits++;
        num_int /= 10;
    }

    for (int i = digits - 1; i >= 0; i--) {
        str[offset++] = '0' + (((int) (num / pow(10, i))) % 10);
    }

    if (decimal) str[offset++] = '.';

    while (decimal) {
        decimal       *= 10;
        str[offset++]  = '0' + (int) decimal;
        decimal        = decimal - (int) decimal;
    }

    str[offset] = '\0';
    printf("num       : %f\n", num);
    printf("actual_num: %s\n", str);

    return 0;
}