commit 8b6ab8f7cbd5882636b72a906c3350aed4f4b2cc
Author: Avril <flanchan@cumallover.me>
Date:   Fri Jun 4 17:00:43 2021 +0100

    working cloneable copy-on-write struct demo

diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..cab3a22
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1 @@
+cow
diff --git a/src/cow.c b/src/cow.c
new file mode 100644
index 0000000..d1a6024
--- /dev/null
+++ b/src/cow.c
@@ -0,0 +1,162 @@
+#define _GNU_SOURCE
+
+#include <stdlib.h>
+#include <stdio.h>
+#include <assert.h>
+#include <string.h>
+#include <unistd.h>
+#include <sys/mman.h>
+#include <limits.h>
+
+#define SIZE 4096
+
+// Copy-on-write mapped memory.
+typedef struct cow {
+	void* origin;
+
+	int fd; // Will be ORd with ~INT_MAX if it's a clone. Will be >0 if it's the original.
+	size_t size;
+} cow_t;
+
+static __attribute__((noreturn)) __attribute__((noinline)) __attribute__((cold)) void die(const char* error)
+{
+	perror(error);
+	exit(1);
+}
+
+static inline int shm_fd(size_t size)
+{
+	_Thread_local static char buffer[12] = {};
+	snprintf(buffer, 11, "0x%08lx", size);
+	//fprintf(stderr, "shm_fd_name: '%s'\n", buffer);
+	int fd = memfd_create(buffer, 0);
+	if(fd<=0) die("cow_create:shm_fd:memfd_create");
+	ftruncate(fd, size);
+	return fd;
+}
+
+static inline int cow_real_fd(const cow_t* cow)
+{
+	return cow->fd & INT_MAX;
+}
+
+int cow_is_fake(const cow_t* cow)
+{
+	return cow->fd<0;
+}
+
+cow_t cow_create(size_t size)
+{
+	cow_t ret;
+	ret.size = size;
+	ret.fd = shm_fd(size);
+	ret.origin = mmap(NULL, size, PROT_READ|PROT_WRITE, MAP_SHARED, ret.fd, 0);
+	if(ret.origin == MAP_FAILED) die("cow_create:mmap");
+
+	return ret;
+}
+
+void cow_free(cow_t cow)
+{
+	munmap(cow.origin, cow.size);
+	if(!cow_is_fake(&cow))
+		close(cow.fd);
+}
+
+cow_t cow_clone(cow_t cow)
+{
+	cow_t clone;
+
+	clone.origin = mmap(cow.origin, cow.size, PROT_READ|PROT_WRITE, MAP_PRIVATE, cow_real_fd(&cow), 0);
+	if(clone.origin == MAP_FAILED) die("cow_clone:mmap");
+	clone.fd = (~INT_MAX) | cow.fd;
+	clone.size = cow.size;
+
+	return clone;
+}
+
+void* cow_ptr(cow_t* restrict cow)
+{
+	return cow->origin;
+}
+
+const void* cow_ptr_const(const cow_t* cow)
+{
+	return cow->origin;
+}
+
+int main()
+{
+	cow_t origin = cow_create(SIZE);
+	
+	strcpy(cow_ptr(&origin), "Hello world");
+	cow_t fake = cow_clone(origin);
+	printf("Fake (pre write): %s\n", (const char*)cow_ptr(&fake));
+	strcpy(cow_ptr(&fake), "Hello fake!");
+
+	printf("Real: %s\n", (const char*)cow_ptr(&origin));
+	printf("Fake: %s\n", (const char*)cow_ptr(&fake));
+	cow_free(fake);
+	cow_free(origin);
+	return 0;
+}
+
+#ifdef DEMO // This code works
+void alter(void* map_ptr)
+{
+	strcpy(map_ptr, "Hello two");
+}
+
+int main()
+{
+	// This works!
+	int graph = shm_fd();
+	void* map_ptr = mmap(NULL, SIZE, PROT_READ|PROT_WRITE, MAP_SHARED, graph, 0);
+	if(map_ptr == MAP_FAILED) die("map_ptr");
+	strcpy(map_ptr, "Hello world");
+	printf("Ptr begins: %s\n", (const char*)map_ptr);
+	{
+		void* map_alter = mmap(map_ptr, SIZE, PROT_READ|PROT_WRITE, MAP_PRIVATE, graph, 0);
+		if(map_alter == MAP_FAILED) die("map_alter");
+		printf("Alter begins: %s\n", (const char*)map_alter);
+		alter(map_alter);
+		printf("Alter ends: %s\n", (const char*)map_alter);
+		munmap(map_alter, SIZE);
+	}
+	printf("Ptr ends: %s\n", (const char*) map_ptr);
+	munmap(map_ptr, SIZE);
+	close(graph);
+	return 0;
+}
+#endif
+// Doesn't work...
+#if 0
+int main()
+{
+	void* map_ptr = mmap(NULL, SIZE, PROT_READ|PROT_WRITE, MAP_ANON | MAP_SHARED, 0, 0);
+	if(map_ptr == MAP_FAILED) die("map_ptr");
+	printf("First ptr is: %p\n  - '", map_ptr);
+	strcpy(map_ptr, "Hello world");
+	printf("%s'\n", (const char*)map_ptr);
+
+#ifdef USE_FORK
+	if(fork()>0) { // Eh... This is ugly.
+		printf("Forked...\n");
+		void* map_alter = mmap(map_ptr, SIZE, PROT_READ|PROT_WRITE, MAP_ANON | MAP_FIXED |MAP_PRIVATE, 0, 0);
+#else
+		//doesnt' work....
+		void* map_alter = mmap(map_ptr, SIZE, PROT_READ|PROT_WRITE, MAP_ANON | MAP_PRIVATE, 0, 0);
+#endif
+		if(map_alter == MAP_FAILED) die("map_alter");
+		printf("Second ptr is: %p\n  - '", map_alter);
+		printf("%s' - '", (const char*)map_alter);
+		strcpy(map_alter, "Hello two");
+		printf("%s'\n", (const char*)map_alter);
+#ifdef USE_FORK
+	} else 
+#endif
+		printf("First is still? '%s'\n", (volatile const char*)map_ptr);
+	
+	return 0;
+}
+#endif