t/map-private-dup: Demo program showing that we can't get aliasing with MAP_PRIVATE mappings

t/map-private-dup.c

+// Demo program showing that changes made to one MAP_PRIVATE mapping do not
+// propagate to another MAP_PRIVATE mapping even in the same process.
+#define _GNU_SOURCE
+#include <sys/mman.h>
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <fcntl.h>
+#include <stdlib.h>
+#include <stdint.h>
+#include <stdio.h>
+
+#undef NDEBUG
+#include <assert.h>
+
+
+void die(const char *msg)
+{
+    perror(msg);
+    exit(1);
+}
+
+int main()
+{
+	int fd;
+	uint8_t *P1, *P2;
+
+	fd = open("map-private-dup.c", O_RDONLY);
+	if (fd == -1)
+		die("open");
+
+	P1 = mmap(NULL, 4096, PROT_READ | PROT_WRITE, MAP_PRIVATE, fd, 0);
+	if (P1 == MAP_FAILED)
+		die("mmap P1");
+
+	printf("P1 orig:    %c %c %c ...\n", P1[0], P1[1], P1[2]);
+
+	P1[0] = 'x';
+	P1[1] = 'y';
+	P1[2] = 'z';
+
+	printf("P1 changed: %c %c %c ...\n", P1[0], P1[1], P1[2]);
+
+	P2 = mmap(NULL, 4096, PROT_READ | PROT_WRITE, MAP_PRIVATE, fd, 0);
+	if (P2 == MAP_FAILED)
+		die("mmap P2");
+
+	printf("P2 orig:    %c %c %c ...\n", P2[0], P2[1], P2[2]);
+
+	// below asserts will fail because P2 does not see changes of P1.
+	assert (P2[0] == 'x');
+	assert (P2[1] == 'y');
+	assert (P2[2] == 'z');
+
+	P1[0] = 99;
+	assert (P2[0] == 99);
+
+	printf("OK\n");
+	return 0;
+}