strlen(key);

was bringt es und wie setzt man es ein:

hier der code irgendwas ist falsch:

#include <stdio.h> 

int main() 
{ 
   char key[255],case_type,idx; 

   // Yes, gets() is sloppy, but I don't care, this is just a neccessary step. 
   // When actually using this, you can input the key any way you want. 
   printf("Enter the string: "); 
   gets(key); 
   for (idx=0; idx < strlen(key); idx++) { 
      // Only process alphabetic characters 
      if (key[idx] < 'A' || (key[idx] > 'Z' && key[idx] < 'a') || key[idx] > 'z') 
         continue; 
      // Determine if the char is upper or lower case 
      if (key[idx] >= 'a') 
         case_type = 'a'; 
      else 
         case_type = 'A'; 
      // Rotate the char's value, ensuring it doesn't accidentally "fall off" the end 
      key[idx] = (key[idx] + 13) % (case_type + 26); 
      if (key[idx] < 26) 
         key[idx] += case_type; 
   } 

   printf("String run through ROT-13: "); 
   puts(key); 
   getchar(); 
   return 0; 
}

gen_keystream(deck,keystream, atoi(key_len));

char str[] = "Super";
size_t len = strlen(str);  //gibt 5 zurück

und gleich dazu warum funktioniert hier das atoi nicht?

gen_keystream(deck,keystream, atoi(key_len));

Was funktioniert nicht?