copy on write example with mmap in the same process

main.c

#include <sys/mman.h>
#include <unistd.h>
#include <sys/types.h>

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main() {
	char *hello = "Hello world!\n";
	int fd = memfd_create("test", 0);
	
	if (ftruncate(fd, 4096)) {
		printf("Failure to truncate in-memory file.\n");
		exit(-1);
	}
	
	int prot_flags = PROT_READ | PROT_WRITE;
	void *shared = mmap(NULL, 4096, prot_flags, MAP_SHARED, fd, 0);
	
	if (shared == MAP_FAILED) {
		printf("Shared mapping failed.\n");
		exit(-1);
	}
	
	strncpy(shared, hello, 64);
	
	void *private = mmap(NULL, 4096, prot_flags, MAP_PRIVATE, fd, 0);
	if (private == MAP_FAILED) {
		printf("Private mapping failed.\n");
		exit(-1);
	}
	
	printf(shared);
	printf(private);
	
	strncpy(shared, "Hello to you too, shared!\n", 64);
	
	printf(shared);
	printf(private);
	
	strncpy(private, "Hello to you too, cow!\n", 64);
	
	printf(shared);
	printf(private);
}

Compile with -D_GNU_SOURCE to have memfd_create defined

$ gcc -D_GNU_SOURCE main.c && ./a.out
Hello world!
Hello world!
Hello to you too, shared!
Hello to you too, shared!
Hello to you too, shared!
Hello to you too, cow!