Mojo C bindings library pt1: MojomBuffer

mojo/public/c/bindings/lib/buffer.c

Index: mojo/public/c/bindings/lib/buffer.c
diff --git a/mojo/public/c/bindings/lib/buffer.c b/mojo/public/c/bindings/lib/buffer.c
new file mode 100644
index 0000000000000000000000000000000000000000..c35fbb3a98a156b4e1154a4b04d67dd3d0b828dc
--- /dev/null
+++ b/mojo/public/c/bindings/lib/buffer.c
@@ -0,0 +1,24 @@
+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "mojo/public/c/bindings/buffer.h"
+
+#include <assert.h>
+#include <stddef.h>  // for NULL
+
+void* MojomBuffer_Allocate(struct MojomBuffer* buf, uint32_t num_bytes) {
+  assert(buf);
+
+  static const size_t kAlignment = 8;
+  const uint32_t bytes_used = buf->num_bytes_used;
+
+  // size = num_bytes is rounded to 8 bytes:
+  const uint64_t size = ((uint64_t)num_bytes + (kAlignment-1))
+                        & ~(kAlignment-1);
+  if (bytes_used + size > buf->buf_size || size == 0)
+    return NULL;
+
+  buf->num_bytes_used += size;
+  return buf->buf + bytes_used;
+}