Vigenere - can't stop keyword from iterating

Code:

---

// Converts plain text to cipher text
        for (int l = 0, o = strlen(s); l < o; l++)
        {
            if (islower(s[l]))
            {
                int m = (l % strlen(argv[1]));
                char c = ((s[l] + shift(argv[1][m]) - 97) % 26 + 97);
                printf("%c", c);
                m++;
            }
            else if (isupper(s[l]))
            {
                int m = (l % strlen(argv[1]));
                char c = (s[l] + shift(argv[1][m]) - 65) % 26 + 65;
                printf("%c", c);
                m++;
            }
            else
            {
                char c = s[l];
                printf("%c", s[l]);
            }

---

You've got a very bad case of single letter identifier disease.
Eye watering alphabet soup is incredibly hard to follow because it has NO meaning.

What's in s?
What's in argv[1]?
What does shift() do?

Post a complete program we can run.

Code:

---

int main ( ) {
  char *s = "...."; // something, anything, it's just an example
  char *key = "key";  // What would be in argv[1]
  /// any other setup
  // your code
}

---

With something like this, we can just copy/paste/run with ZERO guesswork on our part.

Interesting...