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, with its own documentation.
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.
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.
dns/
# Contains the DNS resolver (originates from NetBSD code).
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.
# The structure under these directories mimics the upstream tree,
# but there's also...
android/
include/
# This is where we keep the hacks necessary to build BSD source
# in our world. The *-compat.h files are automatically included
# using -include, but we also provide equivalents for missing
# header/source files needed by the BSD implementation.
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.
malloc_debug/
# The code that implements the functionality to enable debugging of
# native allocation problems.
stdio/
# These are legacy files of dubious provenance. We're working to clean
# this mess up, and this directory should disappear.
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.