Quiero cambiar el valor de la variable en la función. Mi código es así:

void change(char *buf){
    char str = "xxxxxxx";
    *buf = &str;
}
int main(){
    char *xxx = NULL;
    change(xxx);
}

cuando depuro con valgrind, dice:

==3709== Invalid write of size 1
==3709==    at 0x80483CA: change (test.c:5)
==3709==    by 0x80483E5: main (test.c:10)
==3709==  Address 0x0 is not stack'd, malloc'd or (recently) free'd
==3709== 
==3709== 
==3709== Process terminating with default action of signal 11 (SIGSEGV)
==3709==  Access not within mapped region at address 0x0
==3709==    at 0x80483CA: change (test.c:5)
==3709==    by 0x80483E5: main (test.c:10)

¿Alguien puede ayudarme? Soy nuevo en C ...