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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