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README.md

1# bionic
2
3[bionic](https://en.wikipedia.org/wiki/Bionic_(software)) is Android's
4C library, math library, and dynamic linker.
5
6# Using bionic as an app developer
7
8See the [user documentation](docs/).
9
10# Working on bionic itself
11
12This documentation is about making changes to bionic itself.
13
14## What are the big pieces of bionic?
15
16#### libc/ --- libc.so, libc.a
17
18The C library. Stuff like `fopen(3)` and `kill(2)`.
19
20#### libm/ --- libm.so, libm.a
21
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
24libraries.
25
26#### libdl/ --- libdl.so
27
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.
31
32#### libstdc++/ --- libstdc++.so
33
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.
38
39#### linker/ --- /system/bin/linker and /system/bin/linker64
40
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).
47
48#### tests/ --- unit tests
49
50The `tests/` directory contains unit tests. Roughly arranged as one file per
51publicly-exported header file.
52
53#### benchmarks/ --- benchmarks
54
55The `benchmarks/` directory contains benchmarks, with its own [documentation](benchmarks/README.md).
56
57
58## What's in libc/?
59
60```
61libc/
62  arch-arm/
63  arch-arm64/
64  arch-common/
65  arch-x86/
66  arch-x86_64/
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.
70    bionic/
71      # Every architecture needs a handful of machine-specific assembler files.
72      # They live here.
73    string/
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.
77    syscalls/
78      # The syscalls directories contain script-generated assembler files.
79      # See 'Adding system calls' later.
80
81  include/
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.
84
85  kernel/
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/.
92
93  private/
94    # These are private header files meant for use within bionic itself.
95
96  dns/
97    # Contains the DNS resolver (originates from NetBSD code).
98
99  upstream-freebsd/
100  upstream-netbsd/
101  upstream-openbsd/
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...
106    android/
107      include/
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.
112
113  bionic/
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.
120
121  malloc_debug/
122    # The code that implements the functionality to enable debugging of
123    # native allocation problems.
124
125  stdio/
126    # These are legacy files of dubious provenance. We're working to clean
127    # this mess up, and this directory should disappear.
128
129  tools/
130    # Various tools used to maintain bionic.
131
132  tzcode/
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.
136  zoneinfo/
137    # Android-format time zone data.
138    # See 'Updating tzdata' later.
139```
140
141
142## Adding libc wrappers for system calls
143
144The first question you should ask is "should I add a libc wrapper for
145this system call?". The answer is usually "no".
146
147The answer is "yes" if the system call is part of the POSIX standard.
148
149The answer is probably "yes" if the system call has a wrapper in at
150least one other C library (typically glibc/musl or Apple's libc).
151
152The answer may be "yes" if the system call has three/four distinct
153users in different projects, and there isn't a more specific higher-level
154library that would make more sense as the place to add the wrapper.
155
156In all other cases, you should use
157[syscall(3)](http://man7.org/linux/man-pages/man2/syscall.2.html) instead.
158
159Adding a system call usually involves:
160
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()`. If you need to create a new
170     header file, libc/include/sys/sysinfo.h is a good short example to copy and
171     paste from.
172  4. Add basic documentation to the header file. libc/include/sys/sysinfo.h is a
173     good short example that shows the expected style. Most of the detail
174     should actually be left to the man7.org page, with only a brief
175     one-sentence explanation in our documentation. Alway include the return
176     value/error reporting details. Explicitly say which version of Android the
177     function was added to. Explicitly call out any Android-specific
178     changes/additions/limitations because they won't be on the man7.org page.
179  5. Add the function name to the correct section in libc/libc.map.txt.
180  6. Add a basic test. Don't try to test everything; concentrate on just testing
181     the code that's actually in *bionic*, not all the functionality that's
182     implemented in the kernel. For simple syscalls, that's just the
183     auto-generated argument and return value marshalling.
184
185     A trivial test that deliberately supplies an invalid argument helps check
186     that we're generating the right symbol and have the right declaration in
187     the header file, and that the change to libc.map.txt from step 5 is
188     correct. (You can use strace(1) manually to confirm that the correct
189     system call is being made.)
190
191     For testing the *kernel* side of things, we should prefer to rely on
192     https://github.com/linux-test-project/ltp for kernel testing, but you'll
193     want to check that external/ltp does contain tests for the syscall you're
194     adding. Also check that external/ltp is using the libc wrapper for the
195     syscall rather than calling it "directly" via syscall(3)!
196
197Some system calls are harder than others. The most common problem is a 64-bit
198argument such as `off64_t` (a *pointer* to a 64-bit argument is fine, since
199pointers are always the "natural" size for the architecture regardless of the
200size of the thing they point to). Whenever you have a function that takes
201`off_t` or `off64_t`, you'll need to consider whether you actually need a foo()
202and a foo64(), and whether they will use the same underlying system call or are
203implemented as two different system calls. It's usually easiest to find a
204similar system call and copy and paste from that. You'll definitely need to test
205both on 32-bit and 64-bit. (These special cases warrant more testing than the
206easy cases, even if only manual testing with strace. Sadly it isn't always
207feasible to write a working test for the interesting cases -- offsets larger
208than 2GiB, say -- so you may end up just writing a "meaningless" program whose
209only purpose is to give you patterns to look for when run under strace(1).)
210
211## Updating kernel header files
212
213As mentioned above, this is currently a two-step process:
214
215  1. Use generate_uapi_headers.sh to go from a Linux source tree to appropriate
216     contents for external/kernel-headers/.
217  2. Run update_all.py to scrub those headers and import them into bionic.
218
219Note that if you're actually just trying to expose device-specific headers to
220build your device drivers, you shouldn't modify bionic. Instead use
221`TARGET_DEVICE_KERNEL_HEADERS` and friends described in [config.mk](https://android.googlesource.com/platform/build/+/master/core/config.mk#186).
222
223
224## Updating tzdata
225
226This is handled by the libcore team, because they own icu, and that needs to be
227updated in sync with bionic). See
228[system/timezone/README.android](https://android.googlesource.com/platform/system/timezone/+/master/README.android).
229
230
231## Verifying changes
232
233If you make a change that is likely to have a wide effect on the tree (such as a
234libc header change), you should run `make checkbuild`. A regular `make` will
235_not_ build the entire tree; just the minimum number of projects that are
236required for the device. Tests, additional developer tools, and various other
237modules will not be built. Note that `make checkbuild` will not be complete
238either, as `make tests` covers a few additional modules, but generally speaking
239`make checkbuild` is enough.
240
241
242## Running the tests
243
244The tests are all built from the tests/ directory.
245
246### Device tests
247
248    $ mma # In $ANDROID_ROOT/bionic.
249    $ adb root && adb remount && adb sync
250    $ adb shell /data/nativetest/bionic-unit-tests/bionic-unit-tests
251    $ adb shell \
252        /data/nativetest/bionic-unit-tests-static/bionic-unit-tests-static
253    # Only for 64-bit targets
254    $ adb shell /data/nativetest64/bionic-unit-tests/bionic-unit-tests
255    $ adb shell \
256        /data/nativetest64/bionic-unit-tests-static/bionic-unit-tests-static
257
258Note that we use our own custom gtest runner that offers a superset of the
259options documented at
260<https://github.com/google/googletest/blob/master/googletest/docs/AdvancedGuide.md#running-test-programs-advanced-options>,
261in particular for test isolation and parallelism (both on by default).
262
263### Device tests via CTS
264
265Most of the unit tests are executed by CTS. By default, CTS runs as
266a non-root user, so the unit tests must also pass when not run as root.
267Some tests cannot do any useful work unless run as root. In this case,
268the test should check `getuid() == 0` and do nothing otherwise (typically
269we log in this case to prevent accidents!). Obviously, if the test can be
270rewritten to not require root, that's an even better solution.
271
272Currently, the list of bionic CTS tests is generated at build time by
273running a host version of the test executable and dumping the list of
274all tests. In order for this to continue to work, all architectures must
275have the same number of tests, and the host version of the executable
276must also have the same number of tests.
277
278Running the gtests directly is orders of magnitude faster than using CTS,
279but in cases where you really have to run CTS:
280
281    $ make cts # In $ANDROID_ROOT.
282    $ adb unroot # Because real CTS doesn't run as root.
283    # This will sync any *test* changes, but not *code* changes:
284    $ cts-tradefed \
285        run singleCommand cts --skip-preconditions -m CtsBionicTestCases
286
287### Host tests
288
289The host tests require that you have `lunch`ed either an x86 or x86_64 target.
290Note that due to ABI limitations (specifically, the size of pthread_mutex_t),
29132-bit bionic requires PIDs less than 65536. To enforce this, set /proc/sys/kernel/pid_max
292to 65536.
293
294    $ ./tests/run-on-host.sh 32
295    $ ./tests/run-on-host.sh 64   # For x86_64-bit *targets* only.
296
297You can supply gtest flags as extra arguments to this script.
298
299### Against glibc
300
301As a way to check that our tests do in fact test the correct behavior (and not
302just the behavior we think is correct), it is possible to run the tests against
303the host's glibc.
304
305    $ ./tests/run-on-host.sh glibc
306
307## Gathering test coverage
308
309To get test coverage for bionic, use `//bionic/build/coverage.sh`. Before
310running, follow the instructions at the top of the file to rebuild bionic with
311coverage instrumentation.
312
313## Attaching GDB to the tests
314
315Bionic's test runner will run each test in its own process by default to prevent
316tests failures from impacting other tests. This also has the added benefit of
317running them in parallel, so they are much faster.
318
319However, this also makes it difficult to run the tests under GDB. To prevent
320each test from being forked, run the tests with the flag `--no-isolate`.
321
322
323## 32-bit ABI bugs
324
325See [32-bit ABI bugs](docs/32-bit-abi.md).
326