Compiling C for Android

Compile and run C programs on Android to debug pieces of code

Android runs on Unix and is very much capable of running programs compiled with C. The only catch is that it uses some specific architectures which means specific compilers should be used to generate the binary. This can be very useful in viewing the output of a C library like compiled JNI functions for example.

Imagine we reverse-engineered a JNI function that uses a seeded random or the filesystem, which we can't easily replicate on a local Linux machine. We could copy Ghidra's decompiled code into a simple C program that prints the output like this one:

#include <stdio.h>
#include <stdlib.h>

int iVar1;
char __s[0x21];

int main() {
    srand(0x1ca3);
    long lVar4 = 0;
    do {
        iVar1 = rand();
        __s[lVar4] = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ"[iVar1 % 0x3e];
        lVar4 = lVar4 + 1;
    } while (lVar4 != 0x20);

    __s[0x20] = '\0';

    printf("Key: %s\n", __s);
}

In the original code, this was a variable, but I've added the printf to extract this value and read it instead. Running this locally might give different results than if it actually ran on the mobile device, so we need to compile it with a specific Android compiler for the device to understand.

From here, you can compile a program using the Android Native Development Kit. It contains many prebuilt compilers for many different architectures and versions. Simply download and extract the zip linked above and look at the toolchains/llvm/prebuilt/linux-x86_64/bin directory to find all the compilers for both C and C++. Here you can choose one of the architectures like armv7a or aarch64 (ARMv8-A), together with the correct Android API version your device uses, see the link below for a translation table:

Android API LevelsEugeneBelinski

Big table of all Android API versions and their corresponding names and numbers

Some devices don't use ARM and should use regular x86_64 instead. In this case, simply use the x86_64-linux-androidXX-clang compiler instead

Use these binaries like you would any other gcc compiler, for example:

$ x86_64-linux-android27-clang program.c -o program

Then you can copy this binary over to the device using ADB, and run it:

$ adb push ./program /data/local/tmp/program
$ adb shell
# cd /data/local/tmp/
# file program
program: ELF 64-bit LSB shared object, ARM aarch64, version 1 (SYSV), dynamically linked, interpreter /system/bin/linker64, not stripped
# ./program
Key: zxzaKk5uLHdoKo9y8osZSnTe5DCdrIX0