Add a C++ helper class for working with shared buffers.

This is a bit more comfortable than passing the shared buffer ID manually
everywhere and keeping track of size etc.

LibC/SharedBuffer.cpp

@@ -0,0 +1,55 @@
+#include <LibC/SharedBuffer.h>
+#include <unistd.h>
+#include <stdio.h>
+
+RetainPtr<SharedBuffer> SharedBuffer::create(pid_t peer, int size)
+{
+    void* data;
+    int shared_buffer_id = create_shared_buffer(peer, size, &data);
+    if (shared_buffer_id < 0) {
+        perror("create_shared_buffer");
+        return nullptr;
+    }
+    return adopt(*new SharedBuffer(shared_buffer_id, size, data));
+}
+
+RetainPtr<SharedBuffer> SharedBuffer::create_from_shared_buffer_id(int shared_buffer_id)
+{
+    void* data = get_shared_buffer(shared_buffer_id);
+    if (data == (void*)-1) {
+        perror("get_shared_buffer");
+        return nullptr;
+    }
+    int size = get_shared_buffer_size(shared_buffer_id);
+    if (size < 0) {
+        perror("get_shared_buffer_size");
+        return nullptr;
+    }
+    return adopt(*new SharedBuffer(shared_buffer_id, size, data));
+}
+
+SharedBuffer::SharedBuffer(int shared_buffer_id, int size, void* data)
+    : m_shared_buffer_id(shared_buffer_id)
+    , m_size(size)
+    , m_data(data)
+{
+}
+
+SharedBuffer::~SharedBuffer()
+{
+    if (m_shared_buffer_id >= 0) {
+        int rc = release_shared_buffer(m_shared_buffer_id);
+        if (rc < 0) {
+            perror("release_shared_buffer");
+        }
+    }
+}
+
+void SharedBuffer::seal()
+{
+    int rc = seal_shared_buffer(m_shared_buffer_id);
+    if (rc < 0) {
+        perror("seal_shared_buffer");
+        exit(1);
+    }
+}