Question

Some C functions can modify the string value without that I pass the address of it.
For example:

char * s = "foo\0";
tobaa(s); /* Now foo is modified to something like "baa" */
printf("%s", s); /* baa */

How do I this?

Answer 1

Simple:

void tobaa(char *s)
{
    strcpy(s, "baa");
}

Also, the extra \0 character is unnecessary, it is added automatically.
Also, make sure you pass a mutable buffer to tobaa or you invoke UB.