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README.md

1Working on bionic
2=================
3
4What are the big pieces of bionic?
5----------------------------------
6
7#### libc/ --- libc.so, libc.a
8
9The C library. Stuff like `fopen(3)` and `kill(2)`.
10
11#### libm/ --- libm.so, libm.a
12
13The math library. Traditionally Unix systems kept stuff like `sin(3)` and
14`cos(3)` in a separate library to save space in the days before shared
15libraries.
16
17#### libdl/ --- libdl.so
18
19The dynamic linker interface library. This is actually just a bunch of stubs
20that the dynamic linker replaces with pointers to its own implementation at
21runtime. This is where stuff like `dlopen(3)` lives.
22
23#### libstdc++/ --- libstdc++.so
24
25The C++ ABI support functions. The C++ compiler doesn't know how to implement
26thread-safe static initialization and the like, so it just calls functions that
27are supplied by the system. Stuff like `__cxa_guard_acquire` and
28`__cxa_pure_virtual` live here.
29
30#### linker/ --- /system/bin/linker and /system/bin/linker64
31
32The dynamic linker. When you run a dynamically-linked executable, its ELF file
33has a `DT_INTERP` entry that says "use the following program to start me".  On
34Android, that's either `linker` or `linker64` (depending on whether it's a
3532-bit or 64-bit executable). It's responsible for loading the ELF executable
36into memory and resolving references to symbols (so that when your code tries to
37jump to `fopen(3)`, say, it lands in the right place).
38
39#### tests/ --- unit tests
40
41The `tests/` directory contains unit tests. Roughly arranged as one file per
42publicly-exported header file.
43
44#### benchmarks/ --- benchmarks
45
46The `benchmarks/` directory contains benchmarks.
47
48
49What's in libc/?
50----------------
51
52<pre>
53libc/
54  arch-arm/
55  arch-arm64/
56  arch-common/
57  arch-mips/
58  arch-mips64/
59  arch-x86/
60  arch-x86_64/
61    # Each architecture has its own subdirectory for stuff that isn't shared
62    # because it's architecture-specific. There will be a .mk file in here that
63    # drags in all the architecture-specific files.
64    bionic/
65      # Every architecture needs a handful of machine-specific assembler files.
66      # They live here.
67    include/
68      machine/
69        # The majority of header files are actually in libc/include/, but many
70        # of them pull in a <machine/something.h> for things like limits,
71        # endianness, and how floating point numbers are represented. Those
72        # headers 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</pre>
140
141
142Adding system calls
143-------------------
144
145Adding a system call usually involves:
146
147  1. Add entries to SYSCALLS.TXT.
148     See SYSCALLS.TXT itself for documentation on the format.
149  2. Run the gensyscalls.py script.
150  3. Add constants (and perhaps types) to the appropriate header file.
151     Note that you should check to see whether the constants are already in
152     kernel uapi header files, in which case you just need to make sure that
153     the appropriate POSIX header file in libc/include/ includes the
154     relevant file or files.
155  4. Add function declarations to the appropriate header file.
156  5. Add the function name to the correct section in libc/libc.map.txt and
157     run `./libc/tools/genversion-scripts.py`.
158  6. Add at least basic tests. Even a test that deliberately supplies
159     an invalid argument helps check that we're generating the right symbol
160     and have the right declaration in the header file, and that you correctly
161     updated the maps in step 5. (You can use strace(1) to confirm that the
162     correct system call is being made.)
163
164
165Updating kernel header files
166----------------------------
167
168As mentioned above, this is currently a two-step process:
169
170  1. Use generate_uapi_headers.sh to go from a Linux source tree to appropriate
171     contents for external/kernel-headers/.
172  2. Run update_all.py to scrub those headers and import them into bionic.
173
174
175Updating tzdata
176---------------
177
178This is fully automated (and these days handled by the libcore team, because
179they own icu, and that needs to be updated in sync with bionic):
180
181  1. Run update-tzdata.py in external/icu/tools/.
182
183
184Verifying changes
185-----------------
186
187If you make a change that is likely to have a wide effect on the tree (such as a
188libc header change), you should run `make checkbuild`. A regular `make` will
189_not_ build the entire tree; just the minimum number of projects that are
190required for the device. Tests, additional developer tools, and various other
191modules will not be built. Note that `make checkbuild` will not be complete
192either, as `make tests` covers a few additional modules, but generally speaking
193`make checkbuild` is enough.
194
195
196Running the tests
197-----------------
198
199The tests are all built from the tests/ directory.
200
201### Device tests
202
203    $ mma # In $ANDROID_ROOT/bionic.
204    $ adb root && adb remount && adb sync
205    $ adb shell /data/nativetest/bionic-unit-tests/bionic-unit-tests32
206    $ adb shell \
207        /data/nativetest/bionic-unit-tests-static/bionic-unit-tests-static32
208    # Only for 64-bit targets
209    $ adb shell /data/nativetest64/bionic-unit-tests/bionic-unit-tests64
210    $ adb shell \
211        /data/nativetest64/bionic-unit-tests-static/bionic-unit-tests-static64
212
213Note that we use our own custom gtest runner that offers a superset of the
214options documented at
215<https://github.com/google/googletest/blob/master/googletest/docs/AdvancedGuide.md#running-test-programs-advanced-options>,
216in particular for test isolation and parallelism (both on by default).
217
218### Device tests via CTS
219
220Most of the unit tests are executed by CTS. By default, CTS runs as
221a non-root user, so the unit tests must also pass when not run as root.
222Some tests cannot do any useful work unless run as root. In this case,
223the test should check `getuid() == 0` and do nothing otherwise (typically
224we log in this case to prevent accidents!). Obviously, if the test can be
225rewritten to not require root, that's an even better solution.
226
227Currently, the list of bionic CTS tests is generated at build time by
228running a host version of the test executable and dumping the list of
229all tests. In order for this to continue to work, all architectures must
230have the same number of tests, and the host version of the executable
231must also have the same number of tests.
232
233Running the gtests directly is orders of magnitude faster than using CTS,
234but in cases where you really have to run CTS:
235
236    $ make cts # In $ANDROID_ROOT.
237    $ adb unroot # Because real CTS doesn't run as root.
238    # This will sync any *test* changes, but not *code* changes:
239    $ cts-tradefed \
240        run singleCommand cts --skip-preconditions -m CtsBionicTestCases
241
242### Host tests
243
244The host tests require that you have `lunch`ed either an x86 or x86_64 target.
245Note that due to ABI limitations (specifically, the size of pthread_mutex_t),
24632-bit bionic requires PIDs less than 65536. To enforce this, set /proc/sys/kernel/pid_max
247to 65536.
248
249    $ ./tests/run-on-host.sh 32
250    $ ./tests/run-on-host.sh 64   # For x86_64-bit *targets* only.
251
252You can supply gtest flags as extra arguments to this script.
253
254### Against glibc
255
256As a way to check that our tests do in fact test the correct behavior (and not
257just the behavior we think is correct), it is possible to run the tests against
258the host's glibc.
259
260    $ ./tests/run-on-host.sh glibc
261
262
263Gathering test coverage
264-----------------------
265
266For either host or target coverage, you must first:
267
268 * `$ export NATIVE_COVERAGE=true`
269     * Note that the build system is ignorant to this flag being toggled, i.e. if
270       you change this flag, you will have to manually rebuild bionic.
271 * Set `bionic_coverage=true` in `libc/Android.mk` and `libm/Android.mk`.
272
273### Coverage from device tests
274
275    $ mma
276    $ adb sync
277    $ adb shell \
278        GCOV_PREFIX=/data/local/tmp/gcov \
279        GCOV_PREFIX_STRIP=`echo $ANDROID_BUILD_TOP | grep -o / | wc -l` \
280        /data/nativetest/bionic-unit-tests/bionic-unit-tests32
281    $ acov
282
283`acov` will pull all coverage information from the device, push it to the right
284directories, run `lcov`, and open the coverage report in your browser.
285
286### Coverage from host tests
287
288First, build and run the host tests as usual (see above).
289
290    $ croot
291    $ lcov -c -d $ANDROID_PRODUCT_OUT -o coverage.info
292    $ genhtml -o covreport coverage.info # or lcov --list coverage.info
293
294The coverage report is now available at `covreport/index.html`.
295
296
297Running the benchmarks
298----------------------
299
300### Device benchmarks
301
302    $ mma
303    $ adb remount
304    $ adb sync
305    $ adb shell /data/nativetest/bionic-benchmarks/bionic-benchmarks
306    $ adb shell /data/nativetest64/bionic-benchmarks/bionic-benchmarks
307
308You can use `--benchmark_filter=getpid` to just run benchmarks with "getpid"
309in their name.
310
311### Host benchmarks
312
313See the "Host tests" section of "Running the tests" above.
314
315
316Attaching GDB to the tests
317--------------------------
318
319Bionic's test runner will run each test in its own process by default to prevent
320tests failures from impacting other tests. This also has the added benefit of
321running them in parallel, so they are much faster.
322
323However, this also makes it difficult to run the tests under GDB. To prevent
324each test from being forked, run the tests with the flag `--no-isolate`.
325
326
32732-bit ABI bugs
328---------------
329
330This probably belongs in the NDK documentation rather than here, but these
331are the known ABI bugs in the 32-bit ABI:
332
333 * `time_t` is 32-bit. <http://b/5819737>. In the 64-bit ABI, time_t is
334   64-bit.
335
336 * `off_t` is 32-bit. There is `off64_t`, and in newer releases there is
337   almost-complete support for `_FILE_OFFSET_BITS`. Unfortunately our stdio
338   implementation uses 32-bit offsets and -- worse -- function pointers to
339   functions that use 32-bit offsets, so there's no good way to implement
340   the last few pieces <http://b/24807045>. In the 64-bit ABI, off_t is
341   off64_t.
342
343 * `sigset_t` is too small on ARM and x86 (but correct on MIPS), so support
344   for real-time signals is broken. <http://b/5828899> In the 64-bit ABI,
345   `sigset_t` is the correct size for every architecture.
346