Introduction to C for Android Developers

Introduction

Lately, I’ve spent a lot of time interviewing Android developers. And I was surprised how little amount of Android devs know C language. Most of the developers use only Java and convinced that C is too complex for their needs. But I will try to bust this myth - C code can be much simpler to write and much simpler to run.

Toolchain

The first step is toolchain. For this tutorial, I will use a very basic set of the toolchain. I already have downloaded the latest NDK (r21d) from the official website, added ANDROID_NDK environment variable.

For working with a particular platform you should choose CPU architecture and target API. For example, aarch64-linux-android21 is Android ARM64 architecture with target API 21.

Hello world

If you are not familiar with C I highly recommend starting from “The C Programming Language” book by Kernighan and Ritchie (also known as K&R C). It’s very basic but still very useful in our time. As usual, I will start from Hello world. Here’s an example from this book (slightly modified for modern OS):

#include <stdio.h>

int main() {
    printf("hello, world");
    return 0;
}

Ok, next step building and running as a command line application on Android device.

I use clang compiler (this is a default compiler in NDK 14). Clang is a frontend compiler for different languages including C and C++ that uses LLVM as backend. And a small step into a theory of compilation. In C there are 4 stages of compilation:

1. Preprocessing
2. Actual compilation
3. Assembly
4. Linking

Back to practice, this command compiles .c file to object and then links it to the executable file for toolchain platform:

$ANDROID_NDK/toolchains/llvm/prebuilt/darwin-x86_64/bin/clang -target aarch64-linux-android21 helloworld.c -o main

If you are running on API 21 or higher you need to set a flag for position independent executables.

Starting from Android 4.1 (API level 16), Android’s dynamic linker supports position-independent executables (PIE). From Android 5.0 (API level 21), executables require PIE. To use PIE to build your executables, set the -fPIE flag. This flag makes it harder to exploit memory corruption bugs by randomizing code location. ¹

$ANDROID_NDK/toolchains/llvm/prebuilt/darwin-x86_64/bin/clang -target aarch64-linux-android21 helloworld.o -o main -fPIE -pie

For running helloworld executable you need Android Device or Emulator. Copy executable and run it via adb:

adb push ./main /data/local/tmp/
adb shell /data/local/tmp/main
hello, world%

Congratulations 🎉, you’ve just run your first C command line application for Android. This approach is great for testing some features in C without needing to create an android app, compile Java, package .apk file, etc. Just copy binary and run - very simple.

Libraries

There are 2 types of libraries in C:

• static
• dynamic

In this section, I will create an executable linked against libraries of both types. As an example application, I decide to create a small utility that compares results of two implementations of inverse square root: system math library and hack that was used in Quake III. In the static library, I’ve implemented Quake III version of an inverse square root, in dynamic - math.h version. And this executable will ask a user for an input and print both results.

Let’s start with the static library. Again, a small step in theory:

Static libraries are simply a collection of ordinary object files. This collection is created using the ar (archiver) program. Static libraries permit users to link to programs without having to recompile its code, saving recompilation time. Static libraries are often useful for developers if they wish to permit programmers to link to their library, but don’t want to give the library source code. ²

#include <stdio.h>

float Q_rsqrt( float number )
{
	long i;
	float x2, y;
	const float threehalfs = 1.5F;

	x2 = number * 0.5F;
	y  = number;
	i  = * ( long * ) &y;                     // evil floating point bit level hacking
	i  = 0x5f3759df - ( i >> 1 );             // what the fuck?
	y  = * ( float * ) &i;
	y  = y * ( threehalfs - ( x2 * y * y ) ); // 1st iteration
//	y  = y * ( threehalfs - ( x2 * y * y ) ); // 2nd iteration, this can be removed

	return y;
}

Compile this hacky .c file into object

$ANDROID_NDK/toolchains/llvm/prebuilt/darwin-x86_64/bin/clang -target aarch64-linux-android21 fast-inverse-square-root.c -c

This command will compile .c file into .o file (object). It’s an intermediate object that will be used for creation of a static library. For deeper understanding what objects are we will use nm utility:

The nm command can report the list of symbols in a given library. It works on both static and shared libraries. For a given library nm can list the symbol names defined, each symbol’s value, and the symbol’s type. It can also identify where the symbol was defined in the source code (by filename and line number), if that information is available in the library (see the -l option). The symbol type is displayed as a letter; lowercase means that the symbol is local, while uppercase means that the symbol is global (external). ³

Typical symbol types include:

• T (a normal definition in the code section)
• D (initialized data section)
• B (uninitialized data section)
• U (undefined; the symbol is used by the library but not defined by the library)
• W (weak; if another library also defines this symbol, that definition overrides this one).

We will check all our artifacts (objects, libraries, executables) with nm utility. Let’s check fast-inverse-square-root.o:

$ANDROID_NDK/toolchains/aarch64-linux-android-4.9/prebuilt/darwin-x86_64/bin/aarch64-linux-android-nm fast-inverse-square-root.o
00000000 T Q_rsqrt

As mentioned above T means definition of symbol (in our case method) inside library. Next step archive object file to .a file

The gnu ar program creates, modifies, and extracts from archives. An archive is a single file holding a collection of other files in a structure that makes it possible to retrieve the original individual files (called members of the archive). ⁴

$ANDROID_NDK/toolchains/aarch64-linux-android-4.9/prebuilt/darwin-x86_64/bin/aarch64-linux-android-ar rcs libstatic.a fast-inverse-square-root.o

You should get a libstatic.a file. You can check a list of symbols in archive the same way we do it for .o files.

Shared library is more difficult, let’s start from definition:

Shared libraries are libraries that are loaded by programs when they start. When a shared library is installed properly, all programs that start afterwards automatically use the new shared library. It’s actually much more flexible and sophisticated than this, because the approach used by Linux permits you to: update libraries and still support programs that want to use older, non-backward-compatible versions of those libraries; override specific libraries or even specific functions in a library when executing a particular program; do all this while programs are running using existing libraries. ⁵

Since Android API 23 NDK supports dylib SONAME.

Every shared library has a special name called the soname. The soname has the prefix lib, the name of the library, the phrase .so, followed by a period and a version number that is incremented whenever the interface changes (as a special exception, the lowest-level C libraries don’t start with lib). A fully-qualified soname includes as a prefix the directory it’s in; on a working system a fully-qualified soname is simply a symbolic link to the shared library’s realname. Every shared library also has a realname, which is the filename containing the actual library code. The real name adds to the soname a period, a minor number, another period, and the release number. The last period and release number are optional. The minor number and release number support configuration control by letting you know exactly what version(s) of the library are installed. ⁶

There is a “small” problem with soname in Android: all Android 23+ platforms require soname, but only Android 23+ can read it. That’s why if you current min API lower than 23 you should always set soname == realname. No versioning of soname yet 😔

Code of inverse square root using math.h:

#include <stdio.h>
#include <math.h>

float rsqrt( float number )
{
	return  1 / sqrtf(number);
}

Let’s build shared library:

$ANDROID_NDK/toolchains/llvm/prebuilt/darwin-x86_64/bin/clang -target aarch64-linux-android21 -shared -o libdynamic.so -Xlinker -soname=libdynamic.so standart-inverse-square-root.c

You should get libdynamic.so. Let’s check it with nm:

$ANDROID_NDK/toolchains/aarch64-linux-android-4.9/prebuilt/darwin-x86_64/bin/aarch64-linux-android-nm -D libdynamic.so
0000144c A __bss_start
         U __cxa_atexit
         U __cxa_finalize
0000144c A _edata
0000144c A _end
000002f0 T rsqrt
         U sqrtf

There are 2 interesting rows:

• T rsqrt - it’s method defined in this library
• U sqrtf - system sqrtf defined in math.h

Now check the header of the shared library. For this purpose, we can use readelf utility.

readelf displays information about one or more ELF format object files. ⁷

The biggest advantages of using ELF - it’s independence from CPU, ABI or operation system and can be used anywhere for checking any ELF object. If you have gcc tools preinstalled you can use sysytem readelf. For macOS user I can recommend brew install greadelf

greadelf -d libdynamic.so | grep -E 'NEEDED|SONAME'

 0x00000001 (NEEDED)                     Shared library: [libdl.so]
 0x00000001 (NEEDED)                     Shared library: [libc.so]
 0x0000000e (SONAME)                     Library soname: [libdynamic.so]

As you can see in libdynamic.so header contains soname identical to filename and has 2 dependencies to system libs: libdl and libc. But also we need to link math library here, because of unresolved sqrtf symbol. To do it just add -lm flag

$ANDROID_NDK/toolchains/llvm/prebuilt/darwin-x86_64/bin/clang -target aarch64-linux-android21 -shared -o libdynamic.so -Xlinker -soname=libdynamic.so -lm standart-inverse-square-root.c
greadelf -d libdynamic.so | grep -E 'NEEDED|SONAME'
 0x00000001 (NEEDED)                     Shared library: [libm.so]
 0x00000001 (NEEDED)                     Shared library: [libdl.so]
 0x00000001 (NEEDED)                     Shared library: [libc.so]
 0x0000000e (SONAME)                     Library soname: [libdynamic.so]

Next step is building of an executable (helloworld.h):

#include <stdio.h>

float Q_rsqrt( float number );
float rsqrt( float number );

int main() {
	int c;
	scanf("%d", &c);
	printf("Quick inverse square root of %d is %f\n", c, Q_rsqrt(c));
	printf("Standard inverse square root of %d is %f\n", c, rsqrt(c));
	return 0;
}

First, try to compile helloworld.c:

$ANDROID_NDK/toolchains/llvm/prebuilt/darwin-x86_64/bin/clang -target aarch64-linux-android21 -o helloworld helloworld.c
helloworld.c:function main: error: undefined reference to 'Q_rsqrt'
helloworld.c:function main: error: undefined reference to 'rsqrt'
clang38: error: linker command failed with exit code 1 (use -v to see invocation)

Linker said that there are no Q_rsqrt and rsqrt. Try to link against libraries:

$ANDROID_NDK/toolchains/llvm/prebuilt/darwin-x86_64/bin/clang -target aarch64-linux-android21 -L./ -o helloworld helloworld.c libstatic.a libdynamic.so -fPIE -pie

Time to run this on the device. Besides running executable command line application needed to specify LD_LIBRARY_PATH (the path where executable should look for dynamic libraries):

adb push ./libdynamic.so /data/local/tmp
adb push ./helloworld /data/local/tmp
adb shell LD_LIBRARY_PATH=/data/local/tmp /data/local/tmp/helloworld

42
Quick inverse square root of 42 is 0.154036
Standard inverse square root of 42 is 0.154303

Cross-platform compilation and CMake

C is a cross-platform language and in general case C code can be compiled for any platform. Headers that I imported in my sample project refers to libc, it’s the standard library for the C programming language, as specified in the ANSI C standard. In this section, I will show how to build the sample app for host system (in my case macOS) and Android. For this purpose I use CMake:

CMake is an open-source, cross-platform family of tools designed to build, test and package software. CMake is used to control the software compilation process using simple platform and compiler independent configuration files, and generate native makefiles and workspaces that can be used in the compiler environment of your choice. ⁸

Android SDK nowadays contains his own version of CMake, we will use it.

All rules of compilation usually describe in CMakeList.txt. You can check syntaxis of CMake config here. My version of CMakeLists.txt for fast inverse square root sample is:

cmake_minimum_required(VERSION 3.6) # set min version of cmake

project(helloworld) # create project

include_directories ("${PROJECT_SOURCE_DIR}/static-library") # include dir of static lib
include_directories ("${PROJECT_SOURCE_DIR}/dynamic-library") # include dir of shared lib

add_library(sisr SHARED 
 ${PROJECT_SOURCE_DIR}/dynamic-library/standart-inverse-square-root.c) # create shared lib
add_library(fisr STATIC 
 ${PROJECT_SOURCE_DIR}/static-library/fast-inverse-square-root.c) # create static lib

SET( CMAKE_EXE_LINKER_FLAGS  "${CMAKE_EXE_LINKER_FLAGS} -fPIE -pie") # set PIE
add_executable(helloworld helloworld.c) # create executable
target_link_libraries (helloworld fisr) # link static lib
target_link_libraries (helloworld sisr) # link shared lib

CMake building consists of 2 steps: generate makefiles for target platform and run build

Compile the project for mac:

cmake . -Bbuild-mac # -B specify a folder for generated scripts
cd build-mac
cmake --build .
./helloworld

For Android we need to pass android.toolchain.cmake file with parameters for CMake. And for Android scripts will look like:

cmake -DCMAKE_TOOLCHAIN_FILE=$ANDROID_NDK/build/cmake/android.toolchain.cmake . -Bbuild-android
cd build-android
cmake --build .
adb push ./libsisr.so /data/local/tmp
adb push ./helloworld /data/local/tmp
adb shell LD_LIBRARY_PATH=/data/local/tmp /data/local/tmp/helloworld
42
Quick inverse square root of 42 is 0.154036
Standard inverse square root of 42 is 0.154303

As you can see CMake is a very simple tool for building cross-platform C code.

Now you can try something more complicated with one of the IDE that supports CMake build system (e.g. JetBrains CLion).

What’s next?

According to official documentation, Google recommends to use NDK for 2 reasons:

• Squeeze extra performance out of a device to achieve low latency or run computationally intensive applications, such as games or physics simulations.
• Reuse your own or other developers’ C or C++ libraries.

From my point of view, C can be used for any particular code in your project except UI part and communication with a system. Also, it can be useful in the cross-platform development of shared codebase.

I hope my tutorial persuaded you to stop being afraid of C code and inspired to write something on your own. My sample of cross-platform compilation is available on Github.