prog.c

 #include <stdio.h>

void affiche(char *message)
{
    int i = 3;
    do{
        printf("%c", (*message)+i);
        } 
    while (*++message);
    printf("\n");
}

int main()
 {
    char * mess1 = NULL;
    char * mess2 = "Hello world.";
    affiche(mess2);
    affiche(mess1);
    return 1;
}