segmentation fault :(

[Moe45673]

I have to create a function that takes a string and detects if it is a valid ISBN number. In my test code, it works fine but in the official test code that my instructor provided, I get a segmentation fault. Checking the core dump, it appears to be an issue with strlen() on line 14 of my code

Here's my code:

#include <iostream>
#include <cstdio>
#include <cstring>
using namespace std;


#include "ISBN.h"
#include "ISBNPrefix.h"

int isValid(const char str[])
{
        int check = 0, i, num[10], val = 0, mod = 1;

        if (strlen(str) == 10) //10 characters in the string
        {
                check = 1;
                for (i = 0; i < 9; i++) //go through first 9 characters, make sure they're a digit. If they are valid, propagate num array with numerical values
                {
                        if (str[i] < '0' || str[i] > '9')
                                check = 0;

                        else
                                num[i] = str[i] - '0';


                }

                if (check != 0) //the first 9 characters are digits, make sure 10th char is either a digit or letter X (10). Propagate 10th num array position with numerical value.
                {
                        if (str[9] < '0' || str[9] > '9' && str[9] != 'X')

                                check = 0;

                        else if (str[9] == 'X')
                                num[9] = 10;



                        else
                                num[9] = str[9] - '0';

                }

                if (check != 0) //do the math to make sure it's all modulo 11
                {

                        for (i = 0; i < 10; i++)
                        {
                                val += num[i] * (10 - i);
                        }

                        mod = val % 11;


                        if (mod != 0)
                                check = 0;
                }

        } // if statement about there being 10 characters

        return check;
}

Here's the part of the main file generating the error:

 // test valid 
        if(isValid(NULL) != 0) {                 
        failed("valid(NULL) did not return false", ++testNumber, &ok,
         totalTested);
    } else
        passed("valid(NULL)", ++testNumber, totalPassed, totalTested);

failed() and passed() are built in functions by the prof that he defined, not me. If I comment out this section, it continues on until I get another segmentation fault later on that also has to do with testing for NULL in another function.

For the heck of it, here's the code for them:

/* passed displays passing message, increments *totalPassed and *total
*  i holds test number
*/
void passed(const char* message, int i, int* totalPassed, int* total) {
    if (DEBUG && message[0] != '\0')
        printf("Passed test %4d (%s)\n", i, message);
    ++*totalPassed;
    ++*total;
}

/* failed displays failure message, resets *ok to false, increments *total
*  i holds test number
*/
void failed(const char* message, int i, bool* ok, int* total) {
    printf("Failed test %4d (%s)\n", i, message);
    *ok = false;
    ++*total;
}

so why is this error occurring?

[Moe45673]

Nvmd! Apparently, all I had to do was add an "if" statement checking if the parameter was null. Forehead slap!