Bionic C Library Overview
http://www.newsmth.net/nForum/#!article/MobileDev/3804?p=1
Bionic C Library Overview:
==========================
Introduction:
Core Philosophy:
The core idea behind Bionic's design is: KEEP IT REALLY SIMPLE.
This implies that the C library should only provide lightweight wrappers around kernel
facilities and not try to be too smart to deal with edge cases.
The name "Bionic" comes from the fact that it is part-BSD and part-Linux: its source
code consists in a mix of BSD C library pieces with custom Linux-specific bits used
to deal with threads, processes, signals and a few others things.
All original BSD pieces carry the BSD copyright disclaimer. Bionic-specific bits
carry the Android Open Source Project copyright disclaimer. And everything is released
under the BSD license.
Architectures:
Bionic currently supports the ARM and x86 instruction sets. In theory, it should be
possible to support more, but this may require a little work (e.g. adding system
call IDs to SYSCALLS.TXT, described below, or modifying the dynamic linker).
The ARM-specific code is under arch-arm/ and the x86-specific one is under arch-x86/
Note that the x86 version is only meant to run on an x86 Android device. We make
absolutely no claim that you could build and use Bionic on a stock x86 Linux
distribution (though that would be cool, so patches are welcomed :-))
Syscall stubs:
Each system call function is implemented by a tiny assembler source fragment
(called a "syscall stub"), which is generated automatically by tools/gensyscalls.py
which reads the SYSCALLS.TXT file for input.
SYSCALLS.TXT contains the list of all syscall stubs to generate, along with
the corresponding syscall numeric identifier (which may differ between ARM and x86),
and its signature
If you modify this file, you may want to use tools/checksyscalls.py which checks
its content against official Linux kernel header files, and will report errors when
invalid syscall ids are used.
Sometimes, the C library function is really a wrapper that calls the corresponding
syscall with another name. For example, the exit() function is provided by the C
library and calls the _exit() syscall stub.
See SYSCALLS.TXT for documentation and details.
time_t:
time_t is 32-bit as defined by the kernel on 32-bit CPUs. A 64-bit version would
be preferrable to avoid the Y2038 bug, but the kernel maintainers consider that
this is not needed at the moment.
off_t:
For similar reasons, off_t is 32-bit. We define loff_t as the 64-bit variant due
to BSD inheritance, but off64_t should be available as a typedef to ease porting of
current Linux-specific code.
Linux kernel headers:
Bionic comes with its own set of "clean" Linux kernel headers to allow user-space
code to use kernel-specific declarations (e.g. IOCTLs, structure declarations,
constants, etc...). They are located in:
./kernel/common,
./kernel/arch-arm
./kernel/arch-x86
These headers have been generated by a tool (kernel/tools/update-all.py) to only
include the public definitions from the original Linux kernel headers.
If you want to know why and how this is done, read kernel/README.TXT to get
all the (gory) details.
PThread implementation:
Bionic's C library comes with its own pthread implementation bundled in. This is
different from other historical C libraries which:
- place it in an external library (-lpthread)
- play linker tricks with weak symbols at dynamic link time
The support for real-time features (a.k.a. -lrt) is also bundled in the C library.
The implementation is based on futexes and strives to provide *very* short code paths
for common operations. Notable features are the following:
- pthread_mutex_t, pthread_cond_t are only 4 bytes each.
- Normal, recursive and error-check mutexes are supported, and the code path
is heavily optimized for the normal case, which is used most of the time.
- Process-shared mutexes and condition variables are not supported.
Their implementation requires far more complexity and was absolutely
not needed for Android (which uses other inter-process synchronization
capabilities).
Note that they could be added in the future without breaking the ABI
by specifying more sophisticated code paths (which may make the common
paths slightly slower though).
- There is currently no support for read/write locks, priority-ceiling in
mutexes and other more advanced features. Again, the main idea being that
this was not needed for Android at all but could be added in the future.
pthread_cancel():
pthread_cancel() will *not* be supported in Bionic, because doing this would
involve making the C library significantly bigger for very little benefit.
Consider that:
- A proper implementation must insert pthread cancellation checks in a lot
of different places of the C library. And conformance is very difficult to
test properly.
- A proper implementation must also clean up resources, like releasing memory,
or unlocking mutexes, properly if the cancellation happens in a complex
function (e.g. inside gethostbyname() or fprintf() + complex formatting
rules). This tends to slow down the path of many functions.
- pthread cancellation cannot stop all threads: e.g. it can't do anything
against an infinite loop
- pthread cancellation itself has short-comings and isn't very portable
(see http://advogato.org/person/slamb/diary.html?start=49 for example).
All of this is contrary to the Bionic design goals. If your code depends on
thread cancellation, please consider alternatives.
Note however that Bionic does implement pthread_cleanup_push() and pthread_cleanup_pop(),
which can be used to handle cleanups that happen when a thread voluntarily exits
through pthread_exit() or returning from its main function.
pthread_once():
Do not call fork() within a callback provided to pthread_once(). Doing this
may result in a deadlock in the child process the next time it calls pthread_once().
Also, you can't throw a C++ Exception from the callback (see C++ Exception Support
below).
The current implementation of pthread_once() lacks the necessary support of
multi-core-safe double-checked-locking (read and write barriers).
Thread-specific data
The thread-specific storage only provides for a bit less than 64 pthread_key_t
objects to each process. The implementation provides 64 real slots but also
uses about 5 of them (exact number may depend on implementation) for its
own use (e.g. two slots are pre-allocated by the C library to speed-up the
Android OpenGL sub-system).
Note that Posix mandates a minimum of 128 slots, but we do not claim to be
Posix-compliant.
Except for the main thread, the TLS area is stored at the top of the stack. See
comments in bionic/libc/bionic/pthread.c for details.
At the moment, thread-local storage defined through the __thread compiler keyword
is not supported by the Bionic C library and dynamic linker.
Multi-core support
At the moment, Bionic does not provide or use read/write memory barriers.
This means that using it on certain multi-core systems might not be supported,
depending on its exact CPU architecture.
Android-specific features:
Bionic provides a small number of Android-specific features to its clients:
- access to system properties:
Android provides a simple shared value/key space to all processes on the
system. It stores a liberal number of 'properties', each of them being a
simple size-limited string that can be associated to a size-limited string
value.
The header
and also defines the maximum size of keys and values.
- Android-specific user/group management:
There is no /etc/passwd or /etc/groups in Android. By design, it is meant to
be used by a single handset user. On the other hand, Android uses the Linux
user/group management features extensively to secure process permissions,
like access to various filesystem directories.
In the Android scheme, each installed application gets its own uid_t/gid_t
starting from 10000; lower numerical ids are reserved for system daemons.
getpwnam() recognizes some hard-coded subsystems names (e.g. "radio") and
will translate them to their low-user-id values. It also recognizes "app_1234"
as the synthetic name of the application that was installed with uid 10000 + 1234,
which is 11234. getgrnam() works similarly
getgrouplist() will always return a single group for any user name, which is
the one passed as an input parameter.
getgrgid() will similarly only return a structure that contains a single-element
members list, corresponding to the user with the same numerical value than the
group.
See bionic/libc/bionic/stubs.c for more details.
- getservent()
There is no /etc/services on Android. Instead the C library embeds a constant
list of services in its executable, which is parsed on demand by the various
functions that depend on it. See bionic/libc/netbsd/net/getservent.c and
bionic/libc/netbsd/net/services.h
The list of services defined internally might change liberally in the future.
This feature is mostly historically and is very rarely used.
The getservent() returns thread-local data. getservbyport() and getservbyname()
are also implemented in a similar fashion.
- getprotoent()
There is no /etc/protocol on Android. Bionic does not currently implement
getprotoent() and related functions. If we add it, it will likely be done
in a way similar to getservent()
DNS resolver:
Bionic uses a NetBSD-derived resolver library which has been modified in the following
ways:
- don't implement the name-server-switch feature (a.k.a.
- read /system/etc/resolv.conf instead of /etc/resolv.conf
- read the list of servers from system properties. the code looks for
'net.dns1', 'net.dns2', etc.. Each property should contain the IP address
of a DNS server.
these properties are set/modified by other parts of the Android system
(e.g. the dhcpd daemon).
the implementation also supports per-process DNS server list, using the
properties 'net.dns1.
for the numerical ID of the current process.
- when performing a query, use a properly randomized Query ID (instead of
a incremented one), for increased security.
- when performing a query, bind the local client socket to a random port
for increased security.
- get rid of *many* unfortunate thread-safety issues in the original code
Bionic does *not* expose implementation details of its DNS resolver; the content
of
change completely in the future.
PThread Real-Time Timers:
timer_create(), timer_gettime(), timer_settime() and timer_getoverrun() are
supported.
Bionic also now supports SIGEV_THREAD real-time timers (see timer_create()).
The implementation simply uses a single thread per timer, unlike GLibc which
uses complex heuristics to try to use the less threads possible when several
timers with compatible properties are used.
This means that if your code uses a lot of SIGEV_THREAD timers, your program
may consume a lot of memory. However, if your program needs many of these timers,
it'd better handle timeout events directly instead.
Other timers (e.g. SIGEV_SIGNAL) are handled by the kernel and use much less
system resources.
Binary Compatibility:
Bionic is *not* in any way binary-compatible with the GNU C Library, ucLibc or any
known Linux C library. This means several things:
- You cannot expect to build something against the GNU C Library headers and have
it dynamically link properly to Bionic later.
- You should *really* use the Android toolchain to build your program against Bionic.
The toolchain deals with many important details that are crucial to get something
working properly.
Failure to do so will usually result in the inability to run or link your program,
or even runtime crashes. Several random web pages on the Internet describe how you
can succesfully write a "hello-world" program with the ARM GNU toolchain. These
examples usually work by chance, if anything else, and you should not follow these
instructions unless you want to waste a lot of your time in the process.
Note however that you *can* generate a binary that is built against the GNU C Library
headers and then statically linked to it. The corresponding executable should be able
to run (if it doesn't use dlopen()/dlsym())
Dynamic Linker:
Bionic comes with its own dynamic linker (just like ld.so on Linux really comes from
GLibc). This linker does not support all the relocations generated by other GCC ARM
toolchains.
C++ Exceptions Support:
At the moment, Bionic doesn't support C++ exceptions, what this really means is the
following:
- If pthread_once() is called with a C++ callback that throws an exception,
then the C library will keep the corresponding pthread_once_t mutex locked.
Any further call to pthread_once() will result in a deadlock.
A proper implementation should be able to register a C++ exception cleanup
handler before the callback to properly unlock the pthread_once_t. Unfortunately
this requires tricky assembly code that is highly dependent on the compiler.
This feature is not planned to be supported anytime soon.
- The same problem may arise if you throw an exception within a callback called
from the C library. Fortunately, these cases are very rare in the real-world,
but any callback you provide to the C library should *not* throw an exception.
- Bionic lacks a few support functions to have exception support work properly.
Include Paths:
The Android build system should automatically provide the necessary include paths
required to build against the C library headers. However, if you want to do that
yourself, you will need to add:
libc/arch-$ARCH/include
libc/include
libc/kernel/common
libc/kernel/arch-$ARCH
to your C include path.