The C library. Stuff like
The math library. Traditionally Unix systems kept stuff like
cos(3) in a separate library to save space in the days before shared libraries.
The dynamic linker interface library. This is actually just a bunch of stubs that the dynamic linker replaces with pointers to its own implementation at runtime. This is where stuff like
The C++ ABI support functions. The C++ compiler doesn't know how to implement thread-safe static initialization and the like, so it just calls functions that are supplied by the system. Stuff like
__cxa_pure_virtual live here.
The dynamic linker. When you run a dynamically-linked executable, its ELF file has a
DT_INTERP entry that says “use the following program to start me”. On Android, that‘s either
linker64 (depending on whether it’s a 32-bit or 64-bit executable). It's responsible for loading the ELF executable into memory and resolving references to symbols (so that when your code tries to jump to
fopen(3), say, it lands in the right place).
tests/ directory contains unit tests. Roughly arranged as one file per publicly-exported header file.
benchmarks/ directory contains benchmarks.
Adding a system call usually involves:
As mentioned above, this is currently a two-step process:
Note that if you‘re actually just trying to expose device-specific headers to build your device drivers, you shouldn’t modify bionic. Instead use
TARGET_DEVICE_KERNEL_HEADERS and friends described in config.mk.
This is fully automated (and these days handled by the libcore team, because they own icu, and that needs to be updated in sync with bionic):
If you make a change that is likely to have a wide effect on the tree (such as a libc header change), you should run
make checkbuild. A regular
make will not build the entire tree; just the minimum number of projects that are required for the device. Tests, additional developer tools, and various other modules will not be built. Note that
make checkbuild will not be complete either, as
make tests covers a few additional modules, but generally speaking
make checkbuild is enough.
The tests are all built from the tests/ directory.
$ mma # In $ANDROID_ROOT/bionic. $ adb root && adb remount && adb sync $ adb shell /data/nativetest/bionic-unit-tests/bionic-unit-tests32 $ adb shell \ /data/nativetest/bionic-unit-tests-static/bionic-unit-tests-static32 # Only for 64-bit targets $ adb shell /data/nativetest64/bionic-unit-tests/bionic-unit-tests64 $ adb shell \ /data/nativetest64/bionic-unit-tests-static/bionic-unit-tests-static64
Note that we use our own custom gtest runner that offers a superset of the options documented at https://github.com/google/googletest/blob/master/googletest/docs/AdvancedGuide.md#running-test-programs-advanced-options, in particular for test isolation and parallelism (both on by default).
Most of the unit tests are executed by CTS. By default, CTS runs as a non-root user, so the unit tests must also pass when not run as root. Some tests cannot do any useful work unless run as root. In this case, the test should check
getuid() == 0 and do nothing otherwise (typically we log in this case to prevent accidents!). Obviously, if the test can be rewritten to not require root, that's an even better solution.
Currently, the list of bionic CTS tests is generated at build time by running a host version of the test executable and dumping the list of all tests. In order for this to continue to work, all architectures must have the same number of tests, and the host version of the executable must also have the same number of tests.
Running the gtests directly is orders of magnitude faster than using CTS, but in cases where you really have to run CTS:
$ make cts # In $ANDROID_ROOT. $ adb unroot # Because real CTS doesn't run as root. # This will sync any *test* changes, but not *code* changes: $ cts-tradefed \ run singleCommand cts --skip-preconditions -m CtsBionicTestCases
The host tests require that you have
lunched either an x86 or x86_64 target. Note that due to ABI limitations (specifically, the size of pthread_mutex_t), 32-bit bionic requires PIDs less than 65536. To enforce this, set /proc/sys/kernel/pid_max to 65536.
$ ./tests/run-on-host.sh 32 $ ./tests/run-on-host.sh 64 # For x86_64-bit *targets* only.
You can supply gtest flags as extra arguments to this script.
As a way to check that our tests do in fact test the correct behavior (and not just the behavior we think is correct), it is possible to run the tests against the host's glibc.
$ ./tests/run-on-host.sh glibc
For either host or target coverage, you must first:
$ export NATIVE_COVERAGE=true
$ mma $ adb sync $ adb shell \ GCOV_PREFIX=/data/local/tmp/gcov \ GCOV_PREFIX_STRIP=`echo $ANDROID_BUILD_TOP | grep -o / | wc -l` \ /data/nativetest/bionic-unit-tests/bionic-unit-tests32 $ acov
acov will pull all coverage information from the device, push it to the right directories, run
lcov, and open the coverage report in your browser.
First, build and run the host tests as usual (see above).
$ croot $ lcov -c -d $ANDROID_PRODUCT_OUT -o coverage.info $ genhtml -o covreport coverage.info # or lcov --list coverage.info
The coverage report is now available at
$ mma $ adb remount $ adb sync $ adb shell /data/nativetest/bionic-benchmarks/bionic-benchmarks $ adb shell /data/nativetest64/bionic-benchmarks/bionic-benchmarks
You can use
--benchmark_filter=getpid to just run benchmarks with “getpid” in their name.
See the “Host tests” section of “Running the tests” above.
Bionic's test runner will run each test in its own process by default to prevent tests failures from impacting other tests. This also has the added benefit of running them in parallel, so they are much faster.
However, this also makes it difficult to run the tests under GDB. To prevent each test from being forked, run the tests with the flag
On 32-bit Android,
off_t is a signed 32-bit integer. This limits functions that use
off_t to working on files no larger than 2GiB.
Android does not require the
_LARGEFILE_SOURCE macro to be used to make
ftello available. Instead they're always available from API level 24 where they were introduced, and never available before then.
Android also does not require the
_LARGEFILE64_SOURCE macro to be used to make
off64_t and corresponding functions such as
ftruncate64 available. Instead, whatever subset of those functions was available at your target API level will be visible.
There are a couple of exceptions to note. Firstly,
off64_t and the single function
lseek64 were available right from the beginning in API 3. Secondly, Android has always silently inserted
O_LARGEFILE into any open call, so if all you need are functions like
read that don't take/return
off_t, large files have always worked.
Android support for
_FILE_OFFSET_BITS=64 (which turns
off64_t and replaces each
off_t function with its
off64_t counterpart, such as
lseek in the source becoming
lseek64 at runtime) was added late. Even when it became available for the platform, it wasn‘t available from the NDK until r15. Before NDK r15,
_FILE_OFFSET_BITS=64 silently did nothing: all code compiled with that was actually using a 32-bit
off_t. With a new enough NDK, the situation becomes complicated. If you’re targeting an API before 21, almost all functions that take an
off_t become unavailable. You‘ve asked for their 64-bit equivalents, and none of them (except
lseek64) exist. As you increase your target API level, you’ll have more and more of the functions available. API 12 adds some of the
<unistd.h> functions, API 21 adds
mmap, and by API 24 you have everything including
<stdio.h>. See the linker map for full details.
In the 64-bit ABI,
off_t is always 64-bit.
On 32-bit Android,
sigset_t is too small for ARM and x86 (but correct for MIPS). This means that there is no support for real-time signals in 32-bit code.
In the 64-bit ABI,
sigset_t is the correct size for every architecture.
On 32-bit Android,
time_t is 32-bit. The header
<time64.h> and type
time64_t exist as a workaround, but the kernel interfaces exposed on 32-bit Android all use the 32-bit
In the 64-bit ABI,
time_t is 64-bit.