build android app

Hello World C program using Android Toolchain
I had reported earlier how I had gotten a C Hello World statically-linked program running on my Android phone using CodeSourcery's toolchain (on linux). Today I got a dynamically-linked Hello World program running on the phone, compiled using Android's prebuilt toolchain from the source.

It's well known that Android uses a stripped down version of libc, called bionic. When you compile your program with static linking, you don't use bionic because well, you linked statically. This is non-ideal. You want to use the bionic library on the phone, and besides you want to compile using Android's prebuilt cross-compiler arm-eabi-gcc

If you are anxious to get things working, use agcc, a perl wrapper over arm-eabi-gcc that sets up everything for you so that you can just:
$ agcc hello.c -o hello
and the resulting binary (hello) just works. Of course you should have the directory containing arm-eabi-gcc in your PATH for agcc to work.

Let's explore a bit deeper. But let me first simplify my hello program to contain just a main
$ cat hello.c int main(int argc, char* argv[]) {
return 0;
}

The minimal set of flags to pass to arm-eabi-gcc to get this working is:
$ arm-eabi-gcc -o hello hello.c -Wl,-rpath-link=/Users/nirnimesh/NIR/android/mydroid/cupcake/out/target/product/generic/obj/lib -L/Users/nirnimesh/NIR/android/mydroid/cupcake/out/target/product/generic/obj/lib -nostdlib /Users/nirnimesh/NIR/android/mydroid/cupcake/out/target/product/generic/obj/lib/crtbegin_dynamic.o -lc

Here /Users/nirnimesh/NIR/android/mydroid/cupcake is the root of my Android source.

$ file hello
hello: ELF 32-bit LSB executable, ARM, version 1 (SYSV), dynamically linked (uses shared libs), not stripped

Note that the executable binary is dynamically linked.

If you leave out -Wl,-rpath-link=/Users/nirnimesh/NIR/android/mydroid/cupcake/out/target/product/generic/obj/lib you get
/Users/nirnimesh/NIR/android/mydroid/cupcake/prebuilt/darwin-x86/toolchain/arm-eabi-4.2.1/bin/../lib/gcc/arm-eabi/4.2.1/../../../../arm-eabi/bin/ld: warning: libdl.so, needed by /Users/nirnimesh/NIR/android/mydroid/cupcake/out/target/product/generic/obj/lib/libc.so, not found (try using -rpath or -rpath-link) /Users/nirnimesh/NIR/android/mydroid/cupcake/out/target/product/generic/obj/lib/libc.so: undefined reference to `dl_unwind_find_exidx' collect2: ld returned 1 exit status

If you leave out -L/Users/nirnimesh/NIR/android/mydroid/cupcake/out/target/product/generic/obj/lib you get
/Users/nirnimesh/NIR/android/mydroid/cupcake/prebuilt/darwin-x86/toolchain/arm-eabi-4.2.1/bin/../lib/gcc/arm-eabi/4.2.1/../../../../arm-eabi/bin/ld: cannot find -lc collect2: ld returned 1 exit status

If you leave out -nostdlib you get
/Users/nirnimesh/NIR/android/mydroid/cupcake/prebuilt/darwin-x86/toolchain/arm-eabi-4.2.1/bin/../lib/gcc/arm-eabi/4.2.1/../../../../arm-eabi/bin/ld: crt0.o: No such file: No such file or directory collect2: ld returned 1 exit status

If you leave out /Users/nirnimesh/NIR/android/mydroid/cupcake/out/target/product/generic/obj/lib/crtbegin_dynamic.o you get
/Users/nirnimesh/NIR/android/mydroid/cupcake/prebuilt/darwin-x86/toolchain/arm-eabi-4.2.1/bin/../lib/gcc/arm-eabi/4.2.1/../../../../arm-eabi/bin/ld: crt0.o: No such file: No such file or directory collect2: ld returned 1 exit status

And finally, f you leave out -lc your compilation fails with:
/Users/nirnimesh/NIR/android/mydroid/cupcake/out/target/product/generic/obj/lib/crtbegin_dynamic.o: In function `_start': bionic/libc/arch-arm/bionic/crtbegin_dynamic.S:(.text+0x10): undefined reference to `__libc_init' collect2: ld returned 1 exit status

So far we've been able to compile C programs using Androids toolchain and run on the phone.
To summarize, it's the most convenient to use agcc as the compiler so you don't have to bother.
Posted by Nirnimesh at 1:16 AM