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

double hex2double(const char *hex)
{
    char *tmp;
    
    if ((tmp = malloc(strlen(hex) + 3)) == NULL) {
        perror("hex2double()");
        exit(1);
    }
    if (hex[0] != '0' || hex[1] != 'x' || hex[1] != 'X') {
        sprintf(tmp, "0x%s", hex);
    } else {
        strcpy(tmp, hex);
    }
    return atof(tmp);
}

int main()
{
    double floatnum;

    floatnum = hex2double("C203145B");
    printf("Resulting double value = %f\n", floatnum);
    return 0;
}