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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, Mac OS X and Windows, but only Linux
version was actually tested. There is no reason to believe that Mac OS X
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 naturally cross-compiled target and you can't use ./config.
You have to use ./Configure and 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. Though 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_HOME 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_HOME=/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_HOME=/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_HOME/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_HOME. In such case you have to pass matching
target name to Configure and shouldn't use -D__ANDROID_API__=N. PATH
adjustment becomes simpler, $ANDROID_NDK_HOME/bin:$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