3[bionic](https://en.wikipedia.org/wiki/Bionic_(software)) is Android's
4C library, math library, and dynamic linker.
6# Using bionic as an app developer
8See the [user documentation](docs/).
10# Working on bionic itself
12This documentation is about making changes to bionic itself.
14## What are the big pieces of bionic?
16#### libc/ --- libc.so, libc.a
18The C library. Stuff like `fopen(3)` and `kill(2)`.
20#### libm/ --- libm.so, libm.a
22The math library. Traditionally Unix systems kept stuff like `sin(3)` and
23`cos(3)` in a separate library to save space in the days before shared
26#### libdl/ --- libdl.so
28The dynamic linker interface library. This is actually just a bunch of stubs
29that the dynamic linker replaces with pointers to its own implementation at
30runtime. This is where stuff like `dlopen(3)` lives.
32#### libstdc++/ --- libstdc++.so
34The C++ ABI support functions. The C++ compiler doesn't know how to implement
35thread-safe static initialization and the like, so it just calls functions that
36are supplied by the system. Stuff like `__cxa_guard_acquire` and
37`__cxa_pure_virtual` live here.
39#### linker/ --- /system/bin/linker and /system/bin/linker64
41The dynamic linker. When you run a dynamically-linked executable, its ELF file
42has a `DT_INTERP` entry that says "use the following program to start me". On
43Android, that's either `linker` or `linker64` (depending on whether it's a
4432-bit or 64-bit executable). It's responsible for loading the ELF executable
45into memory and resolving references to symbols (so that when your code tries to
46jump to `fopen(3)`, say, it lands in the right place).
48#### tests/ --- unit tests
50The `tests/` directory contains unit tests. Roughly arranged as one file per
51publicly-exported header file.
53#### benchmarks/ --- benchmarks
55The `benchmarks/` directory contains benchmarks, with its own [documentation](benchmarks/README.md).
58## What's in libc/?
67 # Each architecture has its own subdirectory for stuff that isn't shared
68 # because it's architecture-specific. There will be a .mk file in here that
69 # drags in all the architecture-specific files.
71 # Every architecture needs a handful of machine-specific assembler files.
72 # They live here.
74 # Most architectures have a handful of optional assembler files
75 # implementing optimized versions of various routines. The <string.h>
76 # functions are particular favorites.
78 # The syscalls directories contain script-generated assembler files.
79 # See 'Adding system calls' later.
82 # The public header files on everyone's include path. These are a mixture of
83 # files written by us and files taken from BSD.
86 # The kernel uapi header files. These are scrubbed copies of the originals
87 # in external/kernel-headers/. These files must not be edited directly. The
88 # generate_uapi_headers.sh script should be used to go from a kernel tree to
89 # external/kernel-headers/ --- this takes care of the architecture-specific
90 # details. The update_all.py script should be used to regenerate bionic's
91 # scrubbed headers from external/kernel-headers/.
94 # These are private header files meant for use within bionic itself.
97 # Contains the DNS resolver (originates from NetBSD code).
102 # These directories contain unmolested upstream source. Any time we can
103 # just use a BSD implementation of something unmodified, we should.
104 # The structure under these directories mimics the upstream tree,
105 # but there's also...
108 # This is where we keep the hacks necessary to build BSD source
109 # in our world. The *-compat.h files are automatically included
110 # using -include, but we also provide equivalents for missing
111 # header/source files needed by the BSD implementation.
114 # This is the biggest mess. The C++ files are files we own, typically
115 # because the Linux kernel interface is sufficiently different that we
116 # can't use any of the BSD implementations. The C files are usually
117 # legacy mess that needs to be sorted out, either by replacing it with
118 # current upstream source in one of the upstream directories or by
119 # switching the file to C++ and cleaning it up.
122 # The code that implements the functionality to enable debugging of
123 # native allocation problems.
126 # These are legacy files of dubious provenance. We're working to clean
127 # this mess up, and this directory should disappear.
130 # Various tools used to maintain bionic.
133 # A modified superset of the IANA tzcode. Most of the modifications relate
134 # to Android's use of a single file (with corresponding index) to contain
135 # time zone data.
137 # Android-format time zone data.
138 # See 'Updating tzdata' later.
142## Adding libc wrappers for system calls
144The first question you should ask is "should I add a libc wrapper for
145this system call?". The answer is usually "no".
147The answer is "yes" if the system call is part of the POSIX standard.
149The answer is probably "yes" if the system call has a wrapper in at
150least one other C library.
152The answer may be "yes" if the system call has three/four distinct
153users in different projects, and there isn't a more specific library
154that would make more sense as the place to add the wrapper.
156In all other cases, you should use
159Adding a system call usually involves:
161 1. Add entries to SYSCALLS.TXT.
162 See SYSCALLS.TXT itself for documentation on the format.
163 2. Add constants (and perhaps types) to the appropriate header file.
164 Note that you should check to see whether the constants are already in
165 kernel uapi header files, in which case you just need to make sure that
166 the appropriate POSIX header file in libc/include/ includes the
167 relevant file or files.
168 3. Add function declarations to the appropriate header file. Don't forget
169 to include the appropriate `__INTRODUCED_IN()`.
170 4. Add the function name to the correct section in libc/libc.map.txt.
171 5. Add at least basic tests. Even a test that deliberately supplies
172 an invalid argument helps check that we're generating the right symbol
173 and have the right declaration in the header file, and that you correctly
174 updated the maps in step 5. (You can use strace(1) to confirm that the
175 correct system call is being made.)
178## Updating kernel header files
180As mentioned above, this is currently a two-step process:
182 1. Use generate_uapi_headers.sh to go from a Linux source tree to appropriate
183 contents for external/kernel-headers/.
184 2. Run update_all.py to scrub those headers and import them into bionic.
186Note that if you're actually just trying to expose device-specific headers to
187build your device drivers, you shouldn't modify bionic. Instead use
188`TARGET_DEVICE_KERNEL_HEADERS` and friends described in [config.mk](https://android.googlesource.com/platform/build/+/master/core/config.mk#186).
191## Updating tzdata
193This is handled by the libcore team, because they own icu, and that needs to be
194updated in sync with bionic). See
198## Verifying changes
200If you make a change that is likely to have a wide effect on the tree (such as a
201libc header change), you should run `make checkbuild`. A regular `make` will
202_not_ build the entire tree; just the minimum number of projects that are
203required for the device. Tests, additional developer tools, and various other
204modules will not be built. Note that `make checkbuild` will not be complete
205either, as `make tests` covers a few additional modules, but generally speaking
206`make checkbuild` is enough.
209## Running the tests
211The tests are all built from the tests/ directory.
213### Device tests
215 $ mma # In $ANDROID_ROOT/bionic.
216 $ adb root && adb remount && adb sync
217 $ adb shell /data/nativetest/bionic-unit-tests/bionic-unit-tests
218 $ adb shell \
220 # Only for 64-bit targets
221 $ adb shell /data/nativetest64/bionic-unit-tests/bionic-unit-tests
222 $ adb shell \
225Note that we use our own custom gtest runner that offers a superset of the
226options documented at
228in particular for test isolation and parallelism (both on by default).
230### Device tests via CTS
232Most of the unit tests are executed by CTS. By default, CTS runs as
233a non-root user, so the unit tests must also pass when not run as root.
234Some tests cannot do any useful work unless run as root. In this case,
235the test should check `getuid() == 0` and do nothing otherwise (typically
236we log in this case to prevent accidents!). Obviously, if the test can be
237rewritten to not require root, that's an even better solution.
239Currently, the list of bionic CTS tests is generated at build time by
240running a host version of the test executable and dumping the list of
241all tests. In order for this to continue to work, all architectures must
242have the same number of tests, and the host version of the executable
243must also have the same number of tests.
245Running the gtests directly is orders of magnitude faster than using CTS,
246but in cases where you really have to run CTS:
248 $ make cts # In $ANDROID_ROOT.
249 $ adb unroot # Because real CTS doesn't run as root.
250 # This will sync any *test* changes, but not *code* changes:
251 $ cts-tradefed \
252 run singleCommand cts --skip-preconditions -m CtsBionicTestCases
254### Host tests
256The host tests require that you have `lunch`ed either an x86 or x86_64 target.
257Note that due to ABI limitations (specifically, the size of pthread_mutex_t),
25832-bit bionic requires PIDs less than 65536. To enforce this, set /proc/sys/kernel/pid_max
261 $ ./tests/run-on-host.sh 32
262 $ ./tests/run-on-host.sh 64 # For x86_64-bit *targets* only.
264You can supply gtest flags as extra arguments to this script.
266### Against glibc
268As a way to check that our tests do in fact test the correct behavior (and not
269just the behavior we think is correct), it is possible to run the tests against
270the host's glibc.
272 $ ./tests/run-on-host.sh glibc
275## Gathering test coverage
277For either host or target coverage, you must first:
279 * `$ export NATIVE_COVERAGE=true`
280 * Note that the build system is ignorant to this flag being toggled, i.e. if
281 you change this flag, you will have to manually rebuild bionic.
282 * Set `bionic_coverage=true` in `libc/Android.mk` and `libm/Android.mk`.
284### Coverage from device tests
286 $ mma
287 $ adb sync
288 $ adb shell \
289 GCOV_PREFIX=/data/local/tmp/gcov \
290 GCOV_PREFIX_STRIP=`echo $ANDROID_BUILD_TOP | grep -o / | wc -l` \
292 $ acov
294`acov` will pull all coverage information from the device, push it to the right
295directories, run `lcov`, and open the coverage report in your browser.
297### Coverage from host tests
299First, build and run the host tests as usual (see above).
301 $ croot
302 $ lcov -c -d $ANDROID_PRODUCT_OUT -o coverage.info
303 $ genhtml -o covreport coverage.info # or lcov --list coverage.info
305The coverage report is now available at `covreport/index.html`.
308## Attaching GDB to the tests
310Bionic's test runner will run each test in its own process by default to prevent
311tests failures from impacting other tests. This also has the added benefit of
312running them in parallel, so they are much faster.
314However, this also makes it difficult to run the tests under GDB. To prevent
315each test from being forked, run the tests with the flag `--no-isolate`.
318## 32-bit ABI bugs
320See [32-bit ABI bugs](docs/32-bit-abi.md).