From 15b0378545ba2ed703d792057b4658dc043e8bf6 Mon Sep 17 00:00:00 2001
From: Rokas Puzonas <rokas.puz@gmail.com>
Date: Sat, 6 Jan 2024 21:22:15 +0200
Subject: [PATCH] add shmat example

---
 shm_open/main.c |   3 --
 shmat/build.zig |  20 +++++++++
 shmat/main.c    | 105 ++++++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 125 insertions(+), 3 deletions(-)
 create mode 100644 shmat/build.zig
 create mode 100644 shmat/main.c

diff --git a/shm_open/main.c b/shm_open/main.c
index 8f283a9..fe832a4 100644
--- a/shm_open/main.c
+++ b/shm_open/main.c
@@ -1,12 +1,9 @@
-#include <errno.h>
 #include <sys/mman.h>
 #include <fcntl.h>
 #include <stdio.h>
 #include <stdlib.h>
-#include <string.h>
 #include <unistd.h>
 #include <pthread.h>
-#include <semaphore.h>
 #include <stdbool.h>
 
 #define SHM_NAME "/shared_memory"
diff --git a/shmat/build.zig b/shmat/build.zig
new file mode 100644
index 0000000..b0902c7
--- /dev/null
+++ b/shmat/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 = "shmat-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);
+}
diff --git a/shmat/main.c b/shmat/main.c
new file mode 100644
index 0000000..79418c1
--- /dev/null
+++ b/shmat/main.c
@@ -0,0 +1,105 @@
+#include <sys/shm.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <pthread.h>
+
+struct shared_data {
+  pthread_mutex_t mutex;
+  int counter;
+};
+
+struct shm_shared_data {
+  int shm_id;
+  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() {
+  key_t key = ftok("shmfile", 65); // Generate a key for the shared memory segment
+
+  // Create a shared memory segment
+  int shm_id = shmget(key, sizeof(struct shared_data), 0666 | IPC_CREAT);
+  if (shm_id == -1) {
+    perror("shmget");
+    exit(EXIT_FAILURE);
+  }
+
+  // Attach the shared memory segment to the address space of the process
+  struct shared_data *shared_data = shmat(shm_id, NULL, 0);
+  if (shared_data == (void*)-1) {
+    perror("shmat");
+    exit(EXIT_FAILURE);
+  }
+
+  struct shm_shared_data shm_shared_data = {
+    .shm_id = shm_id,
+    .shared_data = shared_data
+  };
+
+  return shm_shared_data;
+}
+
+void shared_data_close(struct shm_shared_data shm_data) {
+  // Detach the shared memory segment
+  if (shmdt(shm_data.shared_data) == -1) {
+    perror("shmdt");
+    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);
+
+  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);
+
+  // Remove the shared memory segment
+  if (shmctl(shm_data.shm_id, IPC_RMID, NULL) == -1) {
+    perror("shmctl");
+    exit(EXIT_FAILURE);
+  }
+
+  return 0;
+}