1 Working on bionic 2 ================= 3 4 What are the big pieces of bionic? 5 ---------------------------------- 6 7 libc/ --- libc.so, libc.a 8 The C library. Stuff like fopen(3) and kill(2). 9 libm/ --- 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. 13 libdl/ --- 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. 17 libstdc++/ --- 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 23 linker/ --- /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 31 tests/ --- unit tests 32 The tests/ directory contains unit tests. Roughly arranged as one file per 33 publicly-exported header file. 34 benchmarks/ --- benchmarks 35 The benchmarks/ directory contains benchmarks. 36 37 38 What's in libc/? 39 ---------------- 40 41 libc/ 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. 124 125 126 Adding system calls 127 ------------------- 128 129 Adding 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.) 144 145 146 Updating kernel header files 147 ---------------------------- 148 149 As 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. 154 155 156 Updating tzdata 157 --------------- 158 159 This is fully automated: 160 161 1. Run update-tzdata.py. 162 163
