How to use precompiled headers in Android NDK project?

Introduction

Precompiled headers (PCHs) can significantly speed up the compilation process in C/C++ projects. In Android NDK projects, PCHs can help reduce build times, especially for large and complex projects. This article will guide you on how to effectively use precompiled headers in your Android NDK projects.

Understanding Precompiled Headers

What are Precompiled Headers?

Precompiled headers store the compiled results of commonly used header files into a single file. When compiling your source code, the compiler can directly use this precompiled header file instead of parsing the individual header files again, leading to faster compilation times.

Benefits of Using Precompiled Headers

• Faster Build Times: PCHs eliminate the need for the compiler to repeatedly parse common header files, leading to substantial build time reductions.
• Improved Developer Productivity: Faster build times mean quicker feedback cycles, improving developer productivity and efficiency.

Setting up Precompiled Headers in Android NDK

Step 1: Choose a Header File for Precompilation

Select a header file that is included in almost all your source files and contains frequently used declarations. For example, you might choose , , or your project’s main header file containing common definitions.

Step 2: Configure the Precompiled Header

The following code demonstrates how to configure the precompiled header in your Android.mk file.

LOCAL_PATH := $(call my-dir)
include $(CLEAR_VARS)
LOCAL_MODULE := my_module
LOCAL_C_INCLUDES := $(LOCAL_PATH)/include
LOCAL_CFLAGS += -include $(LOCAL_PATH)/include/precompiled_header.h
LOCAL_PCH_NAME := precompiled_header.h
LOCAL_PCH_SOURCE := $(LOCAL_PATH)/include/precompiled_header.h
LOCAL_SRC_FILES := src/main.cpp
include $(BUILD_SHARED_LIBRARY)

Here’s a breakdown of the key elements:

• LOCAL_PCH_NAME: Specifies the name of the precompiled header file (without the .pch extension).
• LOCAL_PCH_SOURCE: Defines the path to the header file to be precompiled.

Step 3: Generate the Precompiled Header

When building your project, the NDK will automatically create the precompiled header file (precompiled_header.pch in our example). This file will be stored in the obj directory under your project’s build path.

Step 4: Use the Precompiled Header in Your Source Files

Ensure that the header file designated for precompilation (precompiled_header.h in our example) is included in all your source files. This step is crucial for the compiler to utilize the precompiled header during the compilation process.

#include "precompiled_header.h"

Important Considerations

Header File Selection

• Choose a header file that is included in most source files and contains common definitions to maximize the benefits of precompiled headers.
• Avoid including unstable header files (those frequently updated) in the precompiled header. This prevents unnecessary regenerations of the precompiled header file.

Precompilation Time

Precompiled header generation can take some time. For smaller projects, the time spent precompiling might outweigh the compilation time saved.

Conclusion

Precompiled headers can significantly enhance the build performance of your Android NDK projects, particularly for large and complex codebases. By following the steps outlined in this article, you can efficiently leverage PCHs to reduce build times and boost your development efficiency.