HACKING.txt revision b76613627d045acd3bdb7294f424f14c21584872 
1Working on bionic
2=================
3
4What are the big pieces of bionic?
5----------------------------------
6
7libc/ --- libc.so, libc.a
8  The C library. Stuff like fopen(3) and kill(2).
9libm/ --- libm.so, libm.a
10  The math library. Traditionally Unix systems kept stuff like sin(3) and
11  cos(3) in a separate library to save space in the days before shared
12  libraries.
13libdl/ --- libdl.so
14  The dynamic linker interface library. This is actually just a bunch of
15  stubs that the dynamic linker replaces with pointers to its own
16  implementation at runtime. This is where stuff like dlopen(3) lives.
17libstdc++/ --- libstdc++.so
18  The C++ ABI support functions. The C++ compiler doesn't know how to
19  implement thread-safe static initialization and the like, so it just calls
20  functions that are supplied by the system. Stuff like __cxa_guard_acquire
21  and __cxa_pure_virtual live here.
22
23linker/ --- /system/bin/linker and /system/bin/linker64
24  The dynamic linker. When you run a dynamically-linked executable, its ELF
25  file has a DT_INTERP entry that says "use the following program to start me".
26  On Android, that's either linker or linker64 (depending on whether it's a
27  32-bit or 64-bit executable). It's responsible for loading the ELF executable
28  into memory and resolving references to symbols (so that when your code tries
29  to jump to fopen(3), say, it lands in the right place).
30
31tests/ --- unit tests
32  The tests/ directory contains unit tests. Roughly arranged as one file per
33  publicly-exported header file.
34benchmarks/ --- benchmarks
35  The benchmarks/ directory contains benchmarks.
36
37
38What's in libc/?
39----------------
40
41libc/
42  arch-arm/
43  arch-arm64/
44  arch-common/
45  arch-mips/
46  arch-mips64/
47  arch-x86/
48  arch-x86_64/
49    # Each architecture has its own subdirectory for stuff that isn't shared
50    # because it's architecture-specific. There will be a .mk file in here that
51    # drags in all the architecture-specific files.
52    bionic/
53      # Every architecture needs a handful of machine-specific assembler files.
54      # They live here.
55    include/
56      machine/
57        # The majority of header files are actually in libc/include/, but many
58        # of them pull in a <machine/something.h> for things like limits,
59        # endianness, and how floating point numbers are represented. Those
60        # headers live here.
61    string/
62      # Most architectures have a handful of optional assembler files
63      # implementing optimized versions of various routines. The <string.h>
64      # functions are particular favorites.
65    syscalls/
66      # The syscalls directories contain script-generated assembler files.
67      # See 'Adding system calls' later.
68
69  include/
70    # The public header files on everyone's include path. These are a mixture of
71    # files written by us and files taken from BSD.
72
73  kernel/
74    # The kernel uapi header files. These are scrubbed copies of the originals
75    # in external/kernel-headers/. These files must not be edited directly. The
76    # generate_uapi_headers.sh script should be used to go from a kernel tree to
77    # external/kernel-headers/ --- this takes care of the architecture-specific
78    # details. The update_all.py script should be used to regenerate bionic's
79    # scrubbed headers from external/kernel-headers/.
80
81  private/
82    # These are private header files meant for use within bionic itself.
83
84  dns/
85    # Contains the DNS resolver (originates from NetBSD code).
86
87  upstream-dlmalloc/
88  upstream-freebsd/
89  upstream-netbsd/
90  upstream-openbsd/
91    # These directories contain unmolested upstream source. Any time we can
92    # just use a BSD implementation of something unmodified, we should.
93    # The structure under these directories mimics the upstream tree,
94    # but there's also...
95    android/
96      include/
97        # This is where we keep the hacks necessary to build BSD source
98        # in our world. The *-compat.h files are automatically included
99        # using -include, but we also provide equivalents for missing
100        # header/source files needed by the BSD implementation.
101
102  bionic/
103    # This is the biggest mess. The C++ files are files we own, typically
104    # because the Linux kernel interface is sufficiently different that we
105    # can't use any of the BSD implementations. The C files are usually
106    # legacy mess that needs to be sorted out, either by replacing it with
107    # current upstream source in one of the upstream directories or by
108    # switching the file to C++ and cleaning it up.
109
110  stdio/
111    # These are legacy files of dubious provenance. We're working to clean
112    # this mess up, and this directory should disappear.
113
114  tools/
115    # Various tools used to maintain bionic.
116
117  tzcode/
118    # A modified superset of the IANA tzcode. Most of the modifications relate
119    # to Android's use of a single file (with corresponding index) to contain
120    # time zone data.
121  zoneinfo/
122    # Android-format time zone data.
123    # See 'Updating tzdata' later.
124
125
126Adding system calls
127-------------------
128
129Adding a system call usually involves:
130
131  1. Add entries to SYSCALLS.TXT.
132     See SYSCALLS.TXT itself for documentation on the format.
133  2. Run the gensyscalls.py script.
134  3. Add constants (and perhaps types) to the appropriate header file.
135     Note that you should check to see whether the constants are already in
136     kernel uapi header files, in which case you just need to make sure that
137     the appropriate POSIX header file in libc/include/ includes the
138     relevant file or files.
139  4. Add function declarations to the appropriate header file.
140  5. Add at least basic tests. Even a test that deliberately supplies
141     an invalid argument helps check that we're generating the right symbol
142     and have the right declaration in the header file. (And strace(1) can
143     confirm that the correct system call is being made.)
144
145
146Updating kernel header files
147----------------------------
148
149As mentioned above, this is currently a two-step process:
150
151  1. Use generate_uapi_headers.sh to go from a Linux source tree to appropriate
152     contents for external/kernel-headers/.
153  2. Run update_all.py to scrub those headers and import them into bionic.
154
155
156Updating tzdata
157---------------
158
159This is fully automated:
160
161  1. Run update-tzdata.py.
162
163