1. Function main must be of type int and return a return-value.
2. A double pointer is not needed, ClrR wants the address of the string. Since a is a pointer, you can pass a.
3. It's easier to treat a string as an array of chars in situations like these.
4. You need to pass the last character back to main if you want main to have it.
void ClrR (char*, char *);
char* a = "1234";
printf ("%s %c\n", a, ch);
void ClrR(char* str, char *ch)
pos = strlen (str) - 1;
*ch = str [pos];
str [pos] = '\0';