c - Wrong output from tolower fuction -

i have problem tolower function. tried use argv output $0@. what's wrong code?

#include <stdlib.h> #include <stdio.h>  void makelower(char *s) {     int i;     for(i = 0; s[i] != '\0'; i++){         s[i] = tolower(s[i]);         }         printf("%s", s); }  int main(int argc, char *argv[]) {     argv[0]="a";      makelower(argv);     return 0; } 

argv pointer pointer, char**. function takes char*. so, need pass like:

makelower(argv[0]); 

but isn't going work because argv[0] points string literal. modifying string literal undefined.

instead pass modifiable array like:

int main(int argc, char *argv[]) {     char arr[] = "a";      makelower(arr);     return 0; } 

other option make copy of string literal passed (via argv[0]) , able modify it. basically, idea can't legally modify string literals in c.