From 560cee6bffeb7ba5079d82bea75d24c420488a01 Mon Sep 17 00:00:00 2001 From: Elliott Hughes Date: Tue, 18 Feb 2014 22:08:56 -0800 Subject: [PATCH] Top-level documentation about working on bionic. Change-Id: If0a531865fa9c0d57486b98d4b78e3efcbf4e1d1 --- HACKING.txt | 163 ++++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 163 insertions(+) create mode 100644 HACKING.txt diff --git a/HACKING.txt b/HACKING.txt new file mode 100644 index 000000000..5785b3072 --- /dev/null +++ b/HACKING.txt @@ -0,0 +1,163 @@ +Working on bionic +================= + +What are the big pieces of bionic? +---------------------------------- + +libc/ --- libc.so, libc.a + The C library. Stuff like fopen(3) and kill(2). +libm/ --- libm.so, libm.a + The math library. Traditionally Unix systems kept stuff like sin(3) and + cos(3) in a separate library to save space in the days before shared + libraries. +libdl/ --- libdl.so + The dynamic linker interface library. This is actually just a bunch of + stubs that the dynamic linker replaces with pointers to its own + implementation at runtime. This is where stuff like dlopen(3) lives. +libstdc++/ --- libstdc++.so + The C++ ABI support functions. The C++ compiler doesn't know how to + implement thread-safe static initialization and the like, so it just calls + functions that are supplied by the system. Stuff like __cxa_guard_acquire + and __cxa_pure_virtual live here. + +linker/ --- /system/bin/linker and /system/bin/linker64 + The dynamic linker. When you run a dynamically-linked executable, its ELF + file has a DT_INTERP entry that says "use the following program to start me". + On Android, that's either linker or linker64 (depending on whether it's a + 32-bit or 64-bit executable). It's responsible for loading the ELF executable + into memory and resolving references to symbols (so that when your code tries + to jump to fopen(3), say, it lands in the right place). + +tests/ --- unit tests + The tests/ directory contains unit tests. Roughly arranged as one file per + publicly-exported header file. +benchmarks/ --- benchmarks + The benchmarks/ directory contains benchmarks. + + +What's in libc/? +---------------- + +libc/ + arch-arm/ + arch-arm64/ + arch-common/ + arch-mips/ + arch-mips64/ + arch-x86/ + arch-x86_64/ + # Each architecture has its own subdirectory for stuff that isn't shared + # because it's architecture-specific. There will be a .mk file in here that + # drags in all the architecture-specific files. + bionic/ + # Every architecture needs a handful of machine-specific assembler files. + # They live here. + include/ + machine/ + # The majority of header files are actually in libc/include/, but many + # of them pull in a for things like limits, + # endianness, and how floating point numbers are represented. Those + # headers live here. + string/ + # Most architectures have a handful of optional assembler files + # implementing optimized versions of various routines. The + # functions are particular favorites. + syscalls/ + # The syscalls directories contain script-generated assembler files. + # See 'Adding system calls' later. + + include/ + # The public header files on everyone's include path. These are a mixture of + # files written by us and files taken from BSD. + + kernel/ + # The kernel uapi header files. These are scrubbed copies of the originals + # in external/kernel-headers/. These files must not be edited directly. The + # generate_uapi_headers.sh script should be used to go from a kernel tree to + # external/kernel-headers/ --- this takes care of the architecture-specific + # details. The update_all.py script should be used to regenerate bionic's + # scrubbed headers from external/kernel-headers/. + + private/ + # These are private header files meant for use within bionic itself. + + netbsd/ + stdio/ + stdlib/ + string/ + unistd/ + wchar/ + # These are legacy files of unknown provenance. In the past, bionic was a + # mess of random versions of random files from all three of FreeBSD, NetBSD, + # and OpenBSD! We've been working to clean that up, but these directories + # are basically where all the stuff we haven't got to yet lives. + # The 'netbsd' directory misleadingly contains the DNS resolver (which will + # probably be forked sometime soon, and that directory simply renamed). + # The other directories contain stuff that still needs to be sorted. + + upstream-dlmalloc/ + upstream-freebsd/ + upstream-netbsd/ + upstream-openbsd/ + # These directories contain unmolested upstream source. Any time we can + # just use a BSD implementation of something unmodified, we should. + # See files like netbsd-compat.h for various ways in which we manage to + # build BSD source in bionic. + + bionic/ + # This is the biggest mess. The C++ files are files we own, typically + # because the Linux kernel interface is sufficiently different that we + # can't use any of the BSD implementations. The C files are usually + # legacy mess that needs to be sorted out, either by replacing it with + # current upstream source in one of the upstream directories or by + # switching the file to C++ and cleaning it up. + + tools/ + # Various tools used to maintain bionic. + + tzcode/ + # A modified superset of the IANA tzcode. Most of the modifications relate + # to Android's use of a single file (with corresponding index) to contain + # time zone data. + zoneinfo/ + # Android-format time zone data. + # See 'Updating tzdata' later. + + +Adding system calls +------------------- + +Adding a system call usually involves: + + 1. Add entries to SYSCALLS.TXT. + See SYSCALLS.TXT itself for documentation on the format. + 2. Run the gensyscalls.py script. + 3. Add constants (and perhaps types) to the appropriate header file. + Note that you should check to see whether the constants are already in + kernel uapi header files, in which case you just need to make sure that + that appropriate POSIX header file in libc/include/ includes the + relevant file or files. + 4. Add function declarations to the appropriate header file. + 5. Add at least basic tests. Even a test that deliberately supplies + an invalid argument helps check that we're generating the right symbol + and have the right declaration in the header file. (And strace(1) can + confirm that the correct system call is being made.) + + +Updating kernel header files +---------------------------- + +As mentioned above, this is currently a two-step process: + + 1. Use generate_uapi_headers.sh to go from a Linux source tree to appropriate + contents for external/kernel-headers/. + 2. Run update_all.py to scrub those headers and import them into bionic. + + +Updating tzdata +--------------- + +This is fully automated: + + 1. Run update-tzdata.py. +