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