valid_null.c

#include <stdio.h>
#include <sys/mman.h>

int main() {
	printf("mmaping zero\n");
	int* pa = mmap(0, sizeof(int), PROT_READ | PROT_WRITE, MAP_FIXED | MAP_PRIVATE | MAP_ANON, -1, 0);
	if (pa == MAP_FAILED) {
		perror("mmap failed: ");
		return 1;
	}
	printf("pointer = %p\n", pa);
	printf("assigning value\n");
	*pa = 5;
	printf("value = %d\n", *pa);
	printf("our pointer is %s!", (pa == NULL ? "null" : "not null"));
	return 0;
}