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 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.) 163 164 165 Updating kernel header files 166 ---------------------------- 167 168 As 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. 173 174 175 Updating tzdata 176 --------------- 177 178 This is fully automated (and these days handled by the libcore team, because 179 they own icu, and that needs to be updated in sync with bionic): 180 181 1. Run update-tzdata.py in external/icu/tools/. 182 183 184 Verifying changes 185 ----------------- 186 187 If you make a change that is likely to have a wide effect on the tree (such as a 188 libc 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 190 required for the device. Tests, additional developer tools, and various other 191 modules will not be built. Note that `make checkbuild` will not be complete 192 either, as `make tests` covers a few additional modules, but generally speaking 193 `make checkbuild` is enough. 194 195 196 Running the tests 197 ----------------- 198 199 The 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 212 213 Note that we use our own custom gtest runner that offers a superset of the 214 options documented at 215 <https://github.com/google/googletest/blob/master/googletest/docs/AdvancedGuide.md#running-test-programs-advanced-options>, 216 in particular for test isolation and parallelism (both on by default). 217 218 ### Device tests via CTS 219 220 Most of the unit tests are executed by CTS. By default, CTS runs as 221 a non-root user, so the unit tests must also pass when not run as root. 222 Some tests cannot do any useful work unless run as root. In this case, 223 the test should check `getuid() == 0` and do nothing otherwise (typically 224 we log in this case to prevent accidents!). Obviously, if the test can be 225 rewritten to not require root, that's an even better solution. 226 227 Currently, the list of bionic CTS tests is generated at build time by 228 running a host version of the test executable and dumping the list of 229 all tests. In order for this to continue to work, all architectures must 230 have the same number of tests, and the host version of the executable 231 must also have the same number of tests. 232 233 Running the gtests directly is orders of magnitude faster than using CTS, 234 but 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 243 244 The host tests require that you have `lunch`ed either an x86 or x86_64 target. 245 Note that due to ABI limitations (specifically, the size of pthread_mutex_t), 246 32-bit bionic requires PIDs less than 65536. To enforce this, set /proc/sys/kernel/pid_max 247 to 65536. 248 249 $ ./tests/run-on-host.sh 32 250 $ ./tests/run-on-host.sh 64 # For x86_64-bit *targets* only. 251 252 You can supply gtest flags as extra arguments to this script. 253 254 ### Against glibc 255 256 As a way to check that our tests do in fact test the correct behavior (and not 257 just the behavior we think is correct), it is possible to run the tests against 258 the host's glibc. 259 260 $ ./tests/run-on-host.sh glibc 261 262 263 Gathering test coverage 264 ----------------------- 265 266 For 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 284 directories, run `lcov`, and open the coverage report in your browser. 285 286 ### Coverage from host tests 287 288 First, 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 293 294 The coverage report is now available at `covreport/index.html`. 295 296 297 Running 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 307 308 You can use `--benchmark_filter=getpid` to just run benchmarks with "getpid" 309 in their name. 310 311 ### Host benchmarks 312 313 See the "Host tests" section of "Running the tests" above. 314 315 316 Attaching GDB to the tests 317 -------------------------- 318 319 Bionic's test runner will run each test in its own process by default to prevent 320 tests failures from impacting other tests. This also has the added benefit of 321 running them in parallel, so they are much faster. 322 323 However, this also makes it difficult to run the tests under GDB. To prevent 324 each test from being forked, run the tests with the flag `--no-isolate`. 325 326 327 32-bit ABI bugs 328 --------------- 329 330 ### `off_t` is 32-bit. 331 332 On 32-bit Android, `off_t` is a signed 32-bit integer. This limits functions 333 that use `off_t` to working on files no larger than 2GiB. 334 335 Android does not require the `_LARGEFILE_SOURCE` macro to be used to make 336 `fseeko` and `ftello` available. Instead they're always available from API 337 level 24 where they were introduced, and never available before then. 338 339 Android also does not require the `_LARGEFILE64_SOURCE` macro to be used 340 to make `off64_t` and corresponding functions such as `ftruncate64` available. 341 Instead, whatever subset of those functions was available at your target API 342 level will be visible. 343 344 There are a couple of exceptions to note. Firstly, `off64_t` and the single 345 function `lseek64` were available right from the beginning in API 3. Secondly, 346 Android has always silently inserted `O_LARGEFILE` into any open call, so if 347 all you need are functions like `read` that don't take/return `off_t`, large 348 files have always worked. 349 350 Android support for `_FILE_OFFSET_BITS=64` (which turns `off_t` into `off64_t` 351 and replaces each `off_t` function with its `off64_t` counterpart, such as 352 `lseek` in the source becoming `lseek64` at runtime) was added late. Even when 353 it became available for the platform, it wasn't available from the NDK until 354 r15. Before NDK r15, `_FILE_OFFSET_BITS=64` silently did nothing: all code 355 compiled with that was actually using a 32-bit `off_t`. With a new enough NDK, 356 the situation becomes complicated. If you're targeting an API before 21, almost 357 all functions that take an `off_t` become unavailable. You've asked for their 358 64-bit equivalents, and none of them (except `lseek`/`lseek64`) exist. As you 359 increase your target API level, you'll have more and more of the functions 360 available. API 12 adds some of the `<unistd.h>` functions, API 21 adds `mmap`, 361 and by API 24 you have everything including `<stdio.h>`. See the 362 [linker map](libc/libc.map.txt) for full details. 363 364 In the 64-bit ABI, `off_t` is always 64-bit. 365 366 ### `sigset_t` is too small for real-time signals. 367 368 On 32-bit Android, `sigset_t` is too small for ARM and x86 (but correct for 369 MIPS). This means that there is no support for real-time signals in 32-bit 370 code. 371 372 In the 64-bit ABI, `sigset_t` is the correct size for every architecture. 373 374 ### `time_t` is 32-bit. 375 376 On 32-bit Android, `time_t` is 32-bit. The header `<time64.h>` and type 377 `time64_t` exist as a workaround, but the kernel interfaces exposed on 32-bit 378 Android all use the 32-bit `time_t`. 379 380 In the 64-bit ABI, `time_t` is 64-bit. 381
