add memory mapped files example

file-mmap/build.zig

@@ -0,0 +1,20 @@
+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 = "mmap-example",
+        .root_source_file = .{ .path = "main.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);
+}

file-mmap/main.c

@@ -0,0 +1,114 @@
+#include <fcntl.h>
+#include <sys/mman.h>
+#include <unistd.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <pthread.h>
+
+struct shared_data {
+  pthread_mutex_t mutex;
+  int counter;
+};
+
+struct shm_shared_data {
+  int file_fd;
+  struct shared_data *shared_data;
+};
+
+void increment_counter(struct shared_data *data) {
+  for (int i = 0; i < 100000; i++) {
+    if (pthread_mutex_lock(&data->mutex) != 0) {
+      perror("pthread_mutex_lock");
+    }
+
+    data->counter++;
+
+    if (pthread_mutex_unlock(&data->mutex) != 0) {
+      perror("pthread_mutex_unlock");
+    }
+  }
+}
+
+struct shm_shared_data shared_data_open() {
+  // Open or create a file
+  int fd = open("/tmp/shared_memory", O_CREAT | O_RDWR, 0666);
+  if (fd == -1) {
+    perror("open");
+    exit(EXIT_FAILURE);
+  }
+
+  int shm_size = sizeof(struct shared_data);
+
+  // Set the size of the file
+  if (ftruncate(fd, shm_size) == -1) {
+    perror("ftruncate");
+    exit(EXIT_FAILURE);
+  }
+
+  // Map the file into the process address space
+  struct shared_data *shared_data = mmap(NULL, shm_size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
+  if (shared_data == MAP_FAILED) {
+    perror("mmap");
+    exit(EXIT_FAILURE);
+  }
+
+  struct shm_shared_data shm_shared_data = {
+    .file_fd = fd,
+    .shared_data = shared_data
+  };
+
+  return shm_shared_data;
+}
+
+void shared_data_close(struct shm_shared_data shm_data) {
+  // Unmap the shared memory
+  if (munmap(shm_data.shared_data, sizeof(struct shared_data)) == -1) {
+    perror("munmap");
+    exit(EXIT_FAILURE);
+  }
+
+  // Close the file
+  if (close(shm_data.file_fd) == -1) {
+    perror("close");
+    exit(EXIT_FAILURE);
+  }
+}
+
+void *thread_callback(void *p) {
+  struct shm_shared_data shm_data = shared_data_open();
+
+  increment_counter(shm_data.shared_data);
+
+  shared_data_close(shm_data);
+
+  return NULL;
+}
+
+int main() {
+  struct shm_shared_data shm_data = shared_data_open();
+
+  pthread_mutexattr_t mutex_attr;
+  pthread_mutexattr_init(&mutex_attr);
+  pthread_mutexattr_setpshared(&mutex_attr, PTHREAD_PROCESS_SHARED);
+
+  shm_data.shared_data->counter = 0;
+  if (pthread_mutex_init(&shm_data.shared_data->mutex, &mutex_attr) < 0) {
+    printf("Failed to initialise mutex\n");
+    return -1;
+  }
+
+  pthread_t thread1;
+  pthread_create(&thread1, NULL, thread_callback, NULL);
+
+  pthread_t thread2;
+  pthread_create(&thread2, NULL, thread_callback, NULL);
+
+  pthread_join(thread1, NULL);
+  pthread_join(thread2, NULL);
+
+  printf("Counter is %d\n", shm_data.shared_data->counter);
+
+  shared_data_close(shm_data);
+
+  return 0;
+}

shm_open/main.c

@@ -67,13 +67,13 @@ struct shm_shared_data shared_data_open() {
 void shared_data_close(struct shm_shared_data shm_data) {
   // Unmap the shared memory and close the file descriptor
   if (munmap(shm_data.shared_data, sizeof(struct shared_data)) == -1) {
-      perror("munmap");
-      exit(EXIT_FAILURE);
+    perror("munmap");
+    exit(EXIT_FAILURE);
   }
 
   if (close(shm_data.shm_fd) == -1) {
-      perror("close");
-      exit(EXIT_FAILURE);
+    perror("close");
+    exit(EXIT_FAILURE);
   }
 }