Working on bionic.
What are the big pieces of bionic.
The C library.
Stuff like fopen 3 and kill 2 .
The math library.
Traditionally Unix systems kept stuff like sin 3 and cos 3 in a separate library to save space in the days before shared libraries.
libdl — libdl.
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 dlopen 3 lives.
libstdc — libstdc .
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_guard_acquire and __cxa_pure_virtual live here.
linker — system bin linker and system bin linker64.
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 linker or 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 — unit tests.
The tests directory contains unit tests.
Roughly arranged as one file per publicly-exported header file.
benchmarks — benchmarks.
The benchmarks directory contains benchmarks.
What s in libc .
Adding system calls.
Adding a system call usually involves .
Add entries to SYSCALLS.
TXT itself for documentation on the format.
Run the gensyscalls.
Add constants and perhaps types to the appropriate header file.
Note that you should check to see whether the constants are already in kernel uapi header files, in which case you just need to make sure that the appropriate POSIX header file in libc include includes the relevant file or files.
Add function declarations to the appropriate header file.
Add the function name to the correct section in libc libc.
Add at least basic tests.
Even a test that deliberately supplies an invalid argument helps check that we re generating the right symbol and have the right declaration in the header file, and that you correctly updated the maps in step 5.
You can use strace 1 to confirm that the correct system call is being made.
Updating kernel header files.
As mentioned above, this is currently a two-step process .
sh to go from a Linux source tree to appropriate contents for external kernel-headers .
py to scrub those headers and import them into bionic.
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 .
py in external icu tools .
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.
Running the tests.
The tests are all built from the tests directory.
in particular for test isolation and parallelism both on by кожный покров .
Device tests via CTS.
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 .
The host tests require that you have lunch ed either an x86 or x86_64 target.
Note that due to ABI кожный покров 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.
You кожный покров supply gtest flags as extra arguments to также .
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.
Gathering test coverage.
For either host or target coverage, you must first .
export NATIVE_COVERAGE true.
Note that the build system is ignorant to this flag being toggled, i.
if you change this flag, you will have to manually rebuild bionic.
Set bionic_coverage true in libc Android.
кожный покров and libm Android.
кожный покров from device tests.
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.
Coverage from host tests.
кожный покров, build and run the host tests as usual see above .
The coverage report is now available at covreport index.
Running the 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.
Attaching GDB to the tests.
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 кожный покров parallel, so they are much faster.
However, this also makes it difficult to run the кожный покров under GDB.
To prevent each test from being forked, run кожный покров tests with the flag —no-isolate.
32-bit кожный покров bugs.
This probably belongs in the NDK documentation rather than here, but these are the known ABI bugs in the 32-bit ABI .
time_t is 32-bit.
In the 64-bit ABI, time_t is 64-bit.
off_t is 32-bit.
There is off64_t.
and in newer releases there is almost-complete support for _FILE_OFFSET_BITS.
In the 64-bit ABI, off_t is off64_t.
sigset_t is too small on ARM and x86 but correct on MIPS , so support for real-time signals is broken.