README.md
1 Working on bionic
2 =================
3
4 What are the big pieces of bionic?
5 ----------------------------------
6
7 #### libc/ --- libc.so, libc.a
8
9 The C library. Stuff like `fopen(3)` and `kill(2)`.
10
11 #### libm/ --- libm.so, libm.a
12
13 The 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
15 libraries.
16
17 #### libdl/ --- libdl.so
18
19 The dynamic linker interface library. This is actually just a bunch of stubs
20 that the dynamic linker replaces with pointers to its own implementation at
21 runtime. This is where stuff like `dlopen(3)` lives.
22
23 #### libstdc++/ --- libstdc++.so
24
25 The C++ ABI support functions. The C++ compiler doesn't know how to implement
26 thread-safe static initialization and the like, so it just calls functions that
27 are 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
32 The dynamic linker. When you run a dynamically-linked executable, its ELF file
33 has a `DT_INTERP` entry that says "use the following program to start me". On
34 Android, that's either `linker` or `linker64` (depending on whether it's a
35 32-bit or 64-bit executable). It's responsible for loading the ELF executable
36 into memory and resolving references to symbols (so that when your code tries to
37 jump to `fopen(3)`, say, it lands in the right place).
38
39 #### tests/ --- unit tests
40
41 The `tests/` directory contains unit tests. Roughly arranged as one file per
42 publicly-exported header file.
43
44 #### benchmarks/ --- benchmarks
45
46 The `benchmarks/` directory contains benchmarks.
47
48
49 What's in libc/?
50 ----------------
51
52 <pre>
53 libc/
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
142 Adding system calls
143 -------------------
144
145 Adding 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
162 Updating kernel header files
163 ----------------------------
164
165 As 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
172 Updating tzdata
173 ---------------
174
175 This is fully automated (and these days handled by the libcore team, because
176 they 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
181 Verifying changes
182 -----------------
183
184 If you make a change that is likely to have a wide effect on the tree (such as a
185 libc 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
187 required for the device. Tests, additional developer tools, and various other
188 modules will not be built. Note that `make checkbuild` will not be complete
189 either, as `make tests` covers a few additional modules, but generally speaking
190 `make checkbuild` is enough.
191
192
193 Running the tests
194 -----------------
195
196 The 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
213 The 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
221 As a way to check that our tests do in fact test the correct behavior (and not
222 just the behavior we think is correct), it is possible to run the tests against
223 the 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
230 Gathering test coverage
231 -----------------------
232
233 For 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
251 directories, run `lcov`, and open the coverage report in your browser.
252
253 ### Coverage from host tests
254
255 First, 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
261 The coverage report is now available at `covreport/index.html`.
262
263
264 Attaching GDB to the tests
265 --------------------------
266
267 Bionic's test runner will run each test in its own process by default to prevent
268 tests failures from impacting other tests. This also has the added benefit of
269 running them in parallel, so they are much faster.
270
271 However, this also makes it difficult to run the tests under GDB. To prevent
272 each test from being forked, run the tests with the flag `--no-isolate`.
273
274
275 32-bit ABI bugs
276 ---------------
277
278 This probably belongs in the NDK documentation rather than here, but these
279 are 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