c - Cesar Cipher out of bounds in Array -

guys found caesar cipher code in site.... when run showing segmentation fault in online compilers..but in c compiler tat i'm using showing processor fault... can pls point out wrong in code

#include <stdio.h> #include <stdlib.h> #include <string.h>  #define caesar(x) rot(13, x) #define decaesar(x) rot(13, x) #define decrypt_rot(x, y) rot((26-x), y)   void rot(int c, char *str) {     int l = strlen(str);     const char *alpha[2] = { "abcdefghijklmnopqrstuvwxyz", "abcdefghijklmnopqrstuvwxyz"};      int i;     (i = 0; < l; i++)     {         if (!isalpha(str[i]))             continue;          str[i] = alpha[isupper(str[i])][((int)(tolower(str[i])-'a')+c)%26];     } }  int main() {     char str[] = "this top secret text message!";      printf("original: %s\n", str);     caesar(str);     printf("encrypted: %s\n", str);     decaesar(str);     printf("decrypted: %s\n", str);      return 0; } 

your code correct except 1 detail, might confusing newbie.

it ok assume function isupper() returns boolean value 1 or 0, if check documentation says a value different 0 (i.e., true) if indeed c uppercase alphabetic letter. 0 (i.e., false) otherwise. , fact isupper() returning int , not _bool causing problem.

int isupper ( int c );

when returning true isupper() might return non_zero value. in case returns 8 ( specific bit-field ). same in yours.

all have cast return of isupper() _bool

printf("%d   %d" ,(_bool)isupper('a') , ((int)(tolower(str[i])-'a')+c)%26 ) ;