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 <machine/something.h> 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 <string.h>
      # 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.

  stdio/
  stdlib/
  unistd/
    # 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.
  dns/
    # Contains the DNS resolver (originates from NetBSD code).

  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
     the 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.