libc/ --- libc.so, libc.a
The C library. Stuff like
libm/ --- libm.so, libm.a
The math library. Traditionally Unix systems kept stuff like
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
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_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
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
tests/ directory contains unit tests. Roughly arranged as one file per publicly-exported header file.
benchmarks/ --- benchmarks
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.