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Requirement details
Beside basic tools like perl and make you'll need to download the Android
NDK. It's available for Linux, macOS and Windows, but only Linux
version was actually tested. There is no reason to believe that macOS
wouldn't work. And as for Windows, it's unclear which "shell" would be
suitable, MSYS2 might have best chances. NDK version should play lesser
role, the goal is to support a range of most recent versions.
Android is a cross-compiled target and you can't rely on `./Configure`
to find out the configuration target for you. You have to name your
target explicitly; there are `android-arm`, `android-arm64`, `android-mips`,
`android-mip64`, `android-x86` and `android-x86_64` (`*MIPS` targets are no
longer supported with NDK R20+).
Do not pass --cross-compile-prefix (as you might be tempted), as it
will be "calculated" automatically based on chosen platform. However,
you still need to know the prefix to extend your PATH, in order to
invoke `$(CROSS_COMPILE)clang` [`*gcc` on NDK 19 and lower] and company.
(`./Configure` will fail and give you a hint if you get it wrong.)
Apart from `PATH` adjustment you need to set `ANDROID_NDK_ROOT` environment
to point at the `NDK` directory. If you're using a side-by-side NDK the path
will look something like `/some/where/android-sdk/ndk/<ver>`, and for a
standalone NDK the path will be something like `/some/where/android-ndk-<ver>`.
Both variables are significant at both configuration and compilation times.
The NDK customarily supports multiple Android API levels, e.g. `android-14`,
`android-21`, etc. By default latest API level is chosen. If you need to target
an older platform pass the argument `-D__ANDROID_API__=N` to `Configure`,
with `N` being the numerical value of the target platform version. For example,
to compile for Android 10 arm64 with a side-by-side NDK r20.0.5594570
export ANDROID_NDK_ROOT=/home/whoever/Android/android-sdk/ndk/20.0.5594570
./Configure android-arm64 -D__ANDROID_API__=29
Older versions of the NDK have GCC under their common prebuilt tools
directory, so the bin path will be slightly different. EG: to compile
for ICS on ARM with NDK 10d:
export ANDROID_NDK_ROOT=/some/where/android-ndk-10d
./Configure android-arm -D__ANDROID_API__=14
Caveat lector! Earlier OpenSSL versions relied on additional `CROSS_SYSROOT`
variable set to `$ANDROID_NDK_ROOT/platforms/android-<api>/arch-<arch>` to
appoint headers-n-libraries' location. It's still recognized in order
to facilitate migration from older projects. However, since API level
appears in `CROSS_SYSROOT` value, passing `-D__ANDROID_API__=N` can be in
conflict, and mixing the two is therefore not supported. Migration to
`CROSS_SYSROOT`-less setup is recommended.
One can engage clang by adjusting PATH to cover same NDK's clang. Just
keep in mind that if you miss it, Configure will try to use gcc...
Also, PATH would need even further adjustment to cover unprefixed, yet
target-specific, ar and ranlib. It's possible that you don't need to
bother, if binutils-multiarch is installed on your Linux system.
Another option is to create so called "standalone toolchain" tailored
for single specific platform including Android API level, and assign its
location to `ANDROID_NDK_ROOT`. In such case you have to pass matching
target name to Configure and shouldn't use `-D__ANDROID_API__=N`. `PATH`
adjustment becomes simpler, `$ANDROID_NDK_ROOT/bin:$PATH` suffices.
Running tests (on Linux)
This is not actually supported. Notes are meant rather as inspiration.
Even though build output targets alien system, it's possible to execute
test suite on Linux system by employing qemu-user. The trick is static
linking. Pass -static to Configure, then edit generated Makefile and
remove occurrences of -ldl and -pie flags. You would also need to pick
API version that comes with usable static libraries, 42/2=21 used to
work. Once built, you should be able to
env EXE_SHELL=qemu-<arch> make test
If you need to pass additional flag to qemu, quotes are your friend, e.g.
env EXE_SHELL="qemu-mips64el -cpu MIPS64R6-generic" make test