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bionic is Android's C library, math library, and dynamic linker.

Using bionic as an app developer

See the user documentation.

Working on bionic itself

This documentation is about making changes to bionic itself.

What are the big pieces of bionic?

libc/ ---, libc.a

The C library. Stuff like


libm/ ---, libm.a

The math library. Traditionally Unix systems kept stuff like

in a separate library to save space in the days before shared libraries.

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


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

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

entry that says "use the following program to start me". On Android, that's either
(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
, say, it lands in the right place).

tests/ --- unit tests


directory contains unit tests. Roughly arranged as one file per publicly-exported header file.

benchmarks/ --- benchmarks


directory contains benchmarks, with its own documentation.

What's in libc/?

    # Each architecture has its own subdirectory for stuff that isn't shared
    # because it's architecture-specific. There will be a .mk file in here that
    # drags in all the architecture-specific files.
      # Every architecture needs a handful of machine-specific assembler files.
      # They live here.
      # Most architectures have a handful of optional assembler files
      # implementing optimized versions of various routines. The 
      # functions are particular favorites.
      # The syscalls directories contain script-generated assembler files.
      # See 'Adding system calls' later.

include/ # The public header files on everyone's include path. These are a mixture of # files written by us and files taken from BSD.

kernel/ # The kernel uapi header files. These are scrubbed copies of the originals # in external/kernel-headers/. These files must not be edited directly. The # script should be used to go from a kernel tree to # external/kernel-headers/ --- this takes care of the architecture-specific # details. The script should be used to regenerate bionic's # scrubbed headers from external/kernel-headers/.

private/ # These are private header files meant for use within bionic itself.

dns/ # Contains the DNS resolver (originates from NetBSD code).

upstream-freebsd/ upstream-netbsd/ upstream-openbsd/ # These directories contain unmolested upstream source. Any time we can # just use a BSD implementation of something unmodified, we should. # The structure under these directories mimics the upstream tree, # but there's also... android/ include/ # This is where we keep the hacks necessary to build BSD source # in our world. The *-compat.h files are automatically included # using -include, but we also provide equivalents for missing # header/source files needed by the BSD implementation.

bionic/ # This is the biggest mess. The C++ files are files we own, typically # because the Linux kernel interface is sufficiently different that we # can't use any of the BSD implementations. The C files are usually # legacy mess that needs to be sorted out, either by replacing it with # current upstream source in one of the upstream directories or by # switching the file to C++ and cleaning it up.

malloc_debug/ # The code that implements the functionality to enable debugging of # native allocation problems.

stdio/ # These are legacy files of dubious provenance. We're working to clean # this mess up, and this directory should disappear.

tools/ # Various tools used to maintain bionic.

tzcode/ # A modified superset of the IANA tzcode. Most of the modifications relate # to Android's use of a single file (with corresponding index) to contain # time zone data. zoneinfo/ # Android-format time zone data. # See 'Updating tzdata' later. </string.h>

Adding libc wrappers for system calls

The first question you should ask is "should I add a libc wrapper for this system call?". The answer is usually "no".

The answer is "yes" if the system call is part of the POSIX standard.

The answer is probably "yes" if the system call has a wrapper in at least one other C library.

The answer may be "yes" if the system call has three/four distinct users in different projects, and there isn't a more specific library that would make more sense as the place to add the wrapper.

In all other cases, you should use syscall(3) instead.

Adding a system call usually involves:

  1. Add entries to SYSCALLS.TXT. See SYSCALLS.TXT itself for documentation on the format.
  2. 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.
  3. Add function declarations to the appropriate header file. Don't forget to include the appropriate
  4. Add the function name to the correct section in libc/
  5. 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:

  1. Use to go from a Linux source tree to appropriate contents for external/kernel-headers/.
  2. Run to scrub those headers and import them into bionic.

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

and friends described in

Updating tzdata

This is handled by the libcore team, because they own icu, and that needs to be updated in sync with bionic). See system/timezone/

Verifying changes

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
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.

Device tests

$ mma # In $ANDROID_ROOT/bionic.
$ adb root && adb remount && adb sync
$ adb shell /data/nativetest/bionic-unit-tests/bionic-unit-tests
$ adb shell \
# Only for 64-bit targets
$ adb shell /data/nativetest64/bionic-unit-tests/bionic-unit-tests
$ adb shell \

Note that we use our own custom gtest runner that offers a superset of the options documented at, in particular for test isolation and parallelism (both on by default).

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:

$ 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

Host tests

The host tests require that you have

ed either an x86 or x8664 target. Note that due to ABI limitations (specifically, the size of pthreadmutext), 32-bit bionic requires PIDs less than 65536. To enforce this, set /proc/sys/kernel/pidmax to 65536.
$ ./tests/ 32
$ ./tests/ 64   # For x86_64-bit *targets* only.

You can supply gtest flags as extra arguments to this script.

Against glibc

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/ glibc

Gathering test coverage

To get test coverage for bionic, use

. Before running, follow the instructions at the top of the file to rebuild bionic with coverage instrumentation.

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 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


32-bit ABI bugs

See 32-bit ABI bugs.

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