Disable ANDROID compilation flag for one file

Issue

I am trying to compile an executable binary to be packaged with my Java android application. The executable, VMD, has a main file vmdmain.C with the following

#if defined(ANDROID)
int VMDmain(int argc, char **argv) {
# else
int main(int argc, char **argv) {
#endif

It seems that the android ndk with cmake implicitly sets the ANDROID flag via -DANDROID. This causes the main function above to be defined as VMDmain instead. Of course, with no main function, the executable fails to link, giving the error:

/home/username/Android/Sdk/ndk-bundle/toolchains/llvm/prebuilt/linux-x86_64/sysroot/usr/lib/i686-linux-android/24/crtbegin_dynamic.o:crtbegin.c:function _start_main: error: undefined reference to 'main'

---

I therefore want to, for the file vmdmain.C only, unset the ANDROID flag, so that the main function will be named main instead of VMDmain. I have tried in CMakeLists.txt

set_source_files_properties(src/main/vmd/vmd-1.9.3/src/vmdmain.C PROPERTIES COMPILE_FLAGS -DANDROID=0)

where the add_executable directive in the same CMakeLists.txt file looks like

add_executable(
    vmd

    src/main/vmd/vmd-1.9.3/src/vmdmain.C
    # other files...
)

which results in the following entry in the "command" key for vmdmain.c in compile_commands.json (linebreaks added, some unrelated compile flags and includes, libs excluded for clarity):

/home/username/Android/Sdk/ndk-bundle/toolchains/llvm/prebuilt/linux-x86_64/bin/clang++ \
  --target=i686-none-linux-android24 \
  --gcc-toolchain=/home/ning/Android/Sdk/ndk-bundle/toolchains/llvm/prebuilt/linux-x86_64 \
  -DARCH_ANDROIDARMV7A \
  -DTCL_LIBRARY=\\\"/home/username/application/app/.externalNativeBuild/cmake/debug/x86\\\" \
  -DTCL_PACKAGE_PATH=\\\"/home/username/application/app/.externalNativeBuild/cmake/debug/x86\\\" \
  -DVMDMSMS \
  -DVMDNANOSHAPER \
  -DVMDPLUGIN_STATIC \
  -DVMDSURF \
  --sysroot /home/ning/Android/Sdk/ndk-bundle/toolchains/llvm/prebuilt/linux-x86_64/sysroot \
  -g \
  -DANDROID \
  -fdata-sections \
  -ffunction-sections \
  -funwind-tables \
  -fstack-protector-strong \
  -no-canonical-prefixes \
  -fno-addrsig \
  -Wa,--noexecstack \
  -Wformat \
  -Werror=format-security \
  -stdlib=libc++ \
  -O0 \
  -fno-limit-debug-info \
  -fPIE \
  -DANDROID=0 \
  -o CMakeFiles/vmd.dir/src/main/vmd/vmd-1.9.3/src/vmdmain.C.o \
  -c /home/ning/github/palantir/app/src/main/vmd/vmd-1.9.3/src/vmdmain.C

However, this doesn't seem to work. I still get the same undefined reference to main error.

Solution

To disable a definition for C preprocessor, use

-UANDROID

It's true that

#if (ANDROID)

will work the same for -DANDROID=0 and -UANDROID, but the following (equivalent) statements behave differently:

#if defined(ANDROID)

or

#ifdef ANDROID

Answered By - Alex Cohn