add forking example

.gitignore

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+zig-out
+zig-cache

README.md

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+# Shared Memory Examples In C
+
+All of the C examples are setup to use the zig build system.
+You can download zig [here](https://ziglang.org/download/) (Use version 0.11.0) and just run `zig build run` in any example.

forking/build.zig

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+const std = @import("std");
+
+pub fn build(b: *std.build.Builder) void {
+    const target = b.standardTargetOptions(.{});
+    const optimize = b.standardOptimizeOption(.{});
+
+    {
+        const exe = b.addExecutable(.{
+            .name = "forking-simple",
+            .root_source_file = .{ .path = "simple.c" },
+            .optimize = optimize,
+            .target = target
+        });
+        exe.linkLibC();
+
+        const run_cmd = b.addRunArtifact(exe);
+        const run_step = b.step("run", "Run simple program");
+        run_step.dependOn(&run_cmd.step);
+
+        b.installArtifact(exe);
+    }
+
+    {
+        const exe = b.addExecutable(.{
+            .name = "forking-mutexes",
+            .root_source_file = .{ .path = "mutexes.c" },
+            .optimize = optimize,
+            .target = target
+        });
+        exe.linkLibC();
+
+        const run_cmd = b.addRunArtifact(exe);
+        const run_step = b.step("run-mutexes", "Run mutexes program");
+        run_step.dependOn(&run_cmd.step);
+
+        b.installArtifact(exe);
+    }
+}

forking/mutexes.c

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+#include <string.h>
+#include <unistd.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <sys/mman.h>
+#include <sys/wait.h>
+#include <pthread.h>
+#include <stdbool.h>
+#include <time.h>
+
+// WARNING: This example does not work, I couldn't figure out why the child process goes into a deadlock on `pthread_mutex_lock`
+
+struct shared_data {
+  pthread_mutex_t mutex;
+  int counter;
+};
+
+void milli_sleep(int milliseconds) {
+  struct timespec ts;
+  ts.tv_sec = milliseconds / 1000;
+  ts.tv_nsec = (milliseconds % 1000) * 1000000;
+  nanosleep(&ts, &ts);
+}
+
+float random_float() {
+  return (float)rand()/(float)RAND_MAX;
+}
+
+int random_range(int from, int to) {
+  return random_float()*(to-from) + from;
+}
+
+void run(const char *name, struct shared_data *data) {
+  printf("[%s] started, mutex: %p\n", name, &data->mutex);
+  for (int i = 0; i < 10; i++) {
+    if (pthread_mutex_lock(&data->mutex) != 0) {
+      perror("pthread_mutex_lock");
+    }
+
+    int counter = data->counter;
+    milli_sleep(random_range(0, 100));
+    data->counter = counter + 1;
+    printf("[%s] counter is %d\n", name, counter + 1);
+
+    if (pthread_mutex_unlock(&data->mutex) != 0) {
+      perror("pthread_mutex_unlock");
+    }
+  }
+  printf("[%s] finished\n", name);
+}
+
+int main() {
+  struct shared_data *shared_data = mmap(NULL, sizeof(struct shared_data), PROT_READ | PROT_WRITE, MAP_SHARED | MAP_ANONYMOUS, -1, 0);
+  if (shared_data == NULL) {
+    perror("mmap");
+    return -1;
+  }
+
+  if (pthread_mutex_init(&shared_data->mutex, NULL) < 0) {
+    printf("Failed to initialise mutex\n");
+    return -1;
+  }
+
+  shared_data->counter = 0;
+
+  int pid = fork();
+  if (pid == 0) {
+    srand(time(NULL)+1);
+    run("Child ", shared_data);
+    return 0;
+  } else {
+    srand(time(NULL)+2);
+    run("Parent", shared_data);
+  }
+
+  int child_status;
+  waitpid(pid, &child_status, 0);
+}
+

forking/simple.c

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+#include <string.h>
+#include <unistd.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <sys/mman.h>
+
+int main() {
+  char parent_message[] = "hello";
+  char child_message[] = "goodbye";
+
+  void* shmem = mmap(NULL, 128, PROT_READ | PROT_WRITE, MAP_SHARED | MAP_ANONYMOUS, -1, 0);
+
+  memcpy(shmem, parent_message, sizeof(parent_message));
+
+  int pid = fork();
+
+  if (pid == 0) {
+  printf("Child read: %s\n", (char*)shmem);
+  memcpy(shmem, child_message, sizeof(child_message));
+  printf("Child wrote: %s\n", (char*)shmem);
+
+  } else {
+  printf("Parent read: %s\n", (char*)shmem);
+  sleep(1);
+  printf("After 1s, parent read: %s\n", (char*)shmem);
+  }
+}