multiplication to 10^log[10]N

#include <stdio.h>
#include <math.h>
#include <float.h>
#include <stdlib.h>
#include <string.h>
int main(void) {
    double S_log10=0;
    char formula[500],value[500];
    scanf("%s",formula);
    printf("only '*' or Number\neach value's max:%lf\n%s\n",DBL_MAX,formula);
    int l=0;
    for(int i=0;formula[i]!='\0';i++){
        if(formula[i]=='*'){
            S_log10+=log10(strtod(value,NULL));
            formula[0]='\0';
            l=0;
        }else{
            value[l]=formula[i];
            value[l+1]='\0';
            l++;
        }
    }
    S_log10+=log10(strtod(value,NULL));
    printf("=10^%lf",S_log10);
    return 0;
}