What are the types of the arguments of the function invest?


/*
 * NOTE: don't change the main function!
 * Sample usage:
 * $ gcc -Wall -std=gnu99 -g -o invest invest.c
 * $ ./invest 10000 1.05
 * 10500.00
 */
int main(int argc, char **argv) {
    // Read in the command-line arguments and convert the strings to doubles
    double principal = strtod(argv[1], NULL);
    double rate = strtod(argv[2], NULL);

    // Call invest to make you more money
    invest(&principal, rate);

    printf("%.2f\n", principal);
    return 0;
}