Android rebuilds static library when the binary is compiled

Issue

I have a binary in Android that links to a static library A. Static library libA depends on multiple shared libraries. The binary does not do anything except it imports a class from the static library and executes a simple function.

However, the binary fails to build except I link against the same shared libraries to which the static library A is linked because the compiler tries to recompile libA with the build config of the binary.

Here is my Android.bp of the static library:

cc_library_static {
name: "libA",
relative_install_path: "hw",
vendor: true,
rtti: true,
cflags: [
    "-Wall",
    "-Wextra",
    "-g",
    "-DUNIT_TEST",
    "-fexceptions"
],
srcs: [
    "libA.cpp",
],
shared_libs: [
    "libhidlbase",
    "libhidltransport",
    "libutils",
    "liblog"
],
header_libs: [
    "lib_a_stub_headers",
],
whole_static_libs: [
    "lib_a_stub",
],
export_include_dirs: ["."]
}

Here is my Android.bp for the binary:

cc_binary{
name: "simplebinary",
relative_install_path: "hw",
vendor: true,
cflags: [
    "-fexceptions"
],
whole_static_libs: [
    "libA"
],
shared_libs: [
    "[email protected]",
],
srcs: [
    "simplebinary.cpp",
],
}

The build of the binary fails with:

libA.hpp:4:10: fatal error: 'lib/lib.hpp' file not found

I'm building using the command mm

Solution

According to the error message, the compiler cannot find a header file in its header search path. Header includes are resolved during the preprocessor stage, therefore this is not a linking problem. The preprocessor runs at the beginning of compilation, the linking is done at the end.

From your description, I understand that the code for simplebinary includes the header libA.hpp, provided by libA. I understand that libA.hpp is contained in the same directory as the Android.bp that defines the libA module. Because of the export_include_dirs: ["."], this directory is added to the header search path for the compilation of simplebinary. Therefore, the compiler can find libA.hpp when compiling simplebinary.

Now libA.hpp includes CommonAPI/CommonAPI.hpp. I do not know to which library this header belongs. I assume the header belongs to some library libB, and libA links against libB. I further assume that libB has export_include_dirs set to point to the folder containing CommonAPI/CommonAPI.hpp. You can then make libA re-export this header by adding export_shared_lib_headers: ["libB"] to the module declaration of libA. If libB is not a shared library, you would have to use export_static_lib_headers or export_header_lib_headers instead (reference).

Answered By - f9c69e9781fa194211448473495534