Why is this my code producing a segmentation fault when there is only argc[1]?

Question

I'm taking a intro CS course (CS50x) and came across a slight problem.

Why is this C code producing a segmentation fault when there is only argc[1]?

It should print the statement. Why won't it print the error?

Thanks in advance!

int main(int argc, string argv[])
{
    //HOUSEKEEPING
    //Get/Validate Key
    string key = argv[1];
    int finalKey = atoi(key) % 26;

    while (argc != 2)
    {
        for (int i = 0; i < strlen(key); i++)

            if (!isdigit(key[i]))
            {
                printf("Usage: ./caesar key\n");
                return 1;
            }

            else
            {
                printf("Usage: ./caesar key\n");
                return 1;
            }
            //...

Answer 1

argc is an integer argument that keeps track of the number of arguments in the command line, there is no argc[1], there might be an argv[1] if you provide a second command line argument, otherwise trying to read from a non existing argv[x] amounts to undefined behavior, and is the likely culprit of the segmentation fault you are experiencing.

In light of that information you'll notice that the while statement makes little sense, argc remains unchanged so the loop will either never execute, or will be infinite if argc is not 2 (and the program doesn't crash).

The if-else is also fishy, it will always print the message and return, if the character is a digit or is not a digit alike.

Notwithstanding the goal of the program, a syntactically correct code should look more like this:

int main(int argc, string argv[])
{

    if (argc == 2)
    {
        string key = argv[1];

        int finalKey = atoi(key) % 26; //*

        for (int i = 0; i < strlen(key); i++)
        {
            if (!isdigit(key[i]))
            {
                printf("Usage: ./caesar key\n");
                return 1;
            }
        }
    }
    else
    {
        puts("Wrong number of arguments, usage: ./caesar key\n");
    }
}

^* Note that atoi is quite unsafe, consider using strtol. And you can operate directly in argv[1] i.e. int finalKey = atoi(argv[1]) % 26;

Answer 2

I think you are not clear about the operation of argc and argv. argc indicates the number of parameters passed to the main function. At least it will receive one argument (argv[0], which will be the name of the compiled C code). In addition, it will receive all the parameters that you pass through the command line.

In summary, the correct way to do it would be:

if (argc >=2){
        string key = argv[1];
}

Otherwise, you will have a segmentation fault when you try to access the argv[1] value.