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CleanSpec.mk A D04-Dec-20172.6 KiB

README.md A D04-Dec-201710.2 KiB

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 at least basic tests. Even a test that deliberately supplies
157     an invalid argument helps check that we're generating the right symbol
158     and have the right declaration in the header file. (And strace(1) can
159     confirm that the correct system call is being made.)
160
161
162Updating kernel header files
163----------------------------
164
165As mentioned above, this is currently a two-step process:
166
167  1. Use generate_uapi_headers.sh to go from a Linux source tree to appropriate
168     contents for external/kernel-headers/.
169  2. Run update_all.py to scrub those headers and import them into bionic.
170
171
172Updating tzdata
173---------------
174
175This is fully automated (and these days handled by the libcore team, because
176they own icu, and that needs to be updated in sync with bionic):
177
178  1. Run update-tzdata.py in external/icu/tools/.
179
180
181Verifying changes
182-----------------
183
184If you make a change that is likely to have a wide effect on the tree (such as a
185libc header change), you should run `make checkbuild`. A regular `make` will
186_not_ build the entire tree; just the minimum number of projects that are
187required for the device. Tests, additional developer tools, and various other
188modules will not be built. Note that `make checkbuild` will not be complete
189either, as `make tests` covers a few additional modules, but generally speaking
190`make checkbuild` is enough.
191
192
193Running the tests
194-----------------
195
196The tests are all built from the tests/ directory.
197
198### Device tests
199
200    $ mma
201    $ adb remount
202    $ adb sync
203    $ adb shell /data/nativetest/bionic-unit-tests/bionic-unit-tests32
204    $ adb shell \
205        /data/nativetest/bionic-unit-tests-static/bionic-unit-tests-static32
206    # Only for 64-bit targets
207    $ adb shell /data/nativetest64/bionic-unit-tests/bionic-unit-tests64
208    $ adb shell \
209        /data/nativetest64/bionic-unit-tests-static/bionic-unit-tests-static64
210
211### Host tests
212
213The host tests require that you have `lunch`ed either an x86 or x86_64 target.
214
215    $ mma
216    $ mm bionic-unit-tests-run-on-host32
217    $ mm bionic-unit-tests-run-on-host64  # For 64-bit *targets* only.
218
219### Against glibc
220
221As a way to check that our tests do in fact test the correct behavior (and not
222just the behavior we think is correct), it is possible to run the tests against
223the host's glibc. The executables are already in your path.
224
225    $ mma
226    $ bionic-unit-tests-glibc32
227    $ bionic-unit-tests-glibc64
228
229
230Gathering test coverage
231-----------------------
232
233For either host or target coverage, you must first:
234
235 * `$ export NATIVE_COVERAGE=true`
236     * Note that the build system is ignorant to this flag being toggled, i.e. if
237       you change this flag, you will have to manually rebuild bionic.
238 * Set `bionic_coverage=true` in `libc/Android.mk` and `libm/Android.mk`.
239
240### Coverage from device tests
241
242    $ mma
243    $ adb sync
244    $ adb shell \
245        GCOV_PREFIX=/data/local/tmp/gcov \
246        GCOV_PREFIX_STRIP=`echo $ANDROID_BUILD_TOP | grep -o / | wc -l` \
247        /data/nativetest/bionic-unit-tests/bionic-unit-tests32
248    $ acov
249
250`acov` will pull all coverage information from the device, push it to the right
251directories, run `lcov`, and open the coverage report in your browser.
252
253### Coverage from host tests
254
255First, build and run the host tests as usual (see above).
256
257    $ croot
258    $ lcov -c -d $ANDROID_PRODUCT_OUT -o coverage.info
259    $ genhtml -o covreport coverage.info # or lcov --list coverage.info
260
261The coverage report is now available at `covreport/index.html`.
262
263
264Attaching GDB to the tests
265--------------------------
266
267Bionic's test runner will run each test in its own process by default to prevent
268tests failures from impacting other tests. This also has the added benefit of
269running them in parallel, so they are much faster.
270
271However, this also makes it difficult to run the tests under GDB. To prevent
272each test from being forked, run the tests with the flag `--no-isolate`.
273
274
27532-bit ABI bugs
276---------------
277
278This probably belongs in the NDK documentation rather than here, but these
279are the known ABI bugs in the 32-bit ABI:
280
281 * `time_t` is 32-bit. <http://b/5819737>. In the 64-bit ABI, time_t is
282   64-bit.
283
284 * `off_t` is 32-bit. There is `off64_t`, and in newer releases there is
285   almost-complete support for `_FILE_OFFSET_BITS`. Unfortunately our stdio
286   implementation uses 32-bit offsets and -- worse -- function pointers to
287   functions that use 32-bit offsets, so there's no good way to implement
288   the last few pieces <http://b/24807045>. In the 64-bit ABI, off_t is
289   off64_t.
290
291 * `sigset_t` is too small on ARM and x86 (but correct on MIPS), so support
292   for real-time signals is broken. <http://b/5828899> In the 64-bit ABI,
293   `sigset_t` is the correct size for every architecture.
294