400 lines
16 KiB
Text
400 lines
16 KiB
Text
Bionic C Library Overview:
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==========================
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Introduction:
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Core Philosophy:
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The core idea behind Bionic's design is: KEEP IT REALLY SIMPLE.
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This implies that the C library should only provide lightweight wrappers
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around kernel facilities and not try to be too smart to deal with edge cases.
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The name "Bionic" comes from the fact that it is part-BSD and part-Linux:
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its source code consists in a mix of BSD C library pieces with custom
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Linux-specific bits used to deal with threads, processes, signals and a few
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others things.
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All original BSD pieces carry the BSD copyright disclaimer. Bionic-specific
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bits carry the Android Open Source Project copyright disclaimer. And
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everything is released under the BSD license.
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Architectures:
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Bionic currently supports the ARM and x86 instruction sets. In theory, it
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should be possible to support more, but this may require a little work (e.g.
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adding system call IDs to SYSCALLS.TXT, described below, or modifying the
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dynamic linker).
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The ARM-specific code is under arch-arm/ and the x86-specific one is under
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arch-x86/
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Note that the x86 version is only meant to run on an x86 Android device. We
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make absolutely no claim that you could build and use Bionic on a stock x86
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Linux distribution (though that would be cool, so patches are welcomed :-))
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Syscall stubs:
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Each system call function is implemented by a tiny assembler source fragment
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(called a "syscall stub"), which is generated automatically by
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tools/gensyscalls.py which reads the SYSCALLS.TXT file for input.
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SYSCALLS.TXT contains the list of all syscall stubs to generate, along with
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the corresponding syscall numeric identifier (which may differ between ARM
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and x86), and its signature
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If you modify this file, you may want to use tools/checksyscalls.py which
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checks its content against official Linux kernel header files, and will
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report errors when invalid syscall ids are used.
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Sometimes, the C library function is really a wrapper that calls the
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corresponding syscall with another name. For example, the exit() function
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is provided by the C library and calls the _exit() syscall stub.
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See SYSCALLS.TXT for documentation and details.
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time_t:
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time_t is 32-bit as defined by the kernel on 32-bit CPUs. A 64-bit version
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would be preferrable to avoid the Y2038 bug, but the kernel maintainers
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consider that this is not needed at the moment.
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Instead, Bionic provides a <time64.h> header that defines a time64_t type,
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and related functions like mktime64(), localtime64(), etc...
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strftime() uses time64_t internally, so the '%s' format (seconds since the
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epoch) is supported for dates >= 2038.
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strftime_tz():
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Bionic also provides the non-standard strftime_tz() function, a variant
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of strftime() which also accepts a time locale descriptor as defined
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by "struct strftime_locale" in <time.h>.
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This function is used by the low-level framework code in Android.
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Timezone management:
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The name of the current timezone is taken from the TZ environment variable,
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if defined. Otherwise, the system property named 'persist.sys.timezone' is
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checked instead.
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The zoneinfo timezone database and index files are located under directory
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/system/usr/share/zoneinfo, instead of the more Posix-compliant path of
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/usr/share/zoneinfo
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off_t:
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For similar reasons, off_t is 32-bit. We define loff_t as the 64-bit variant
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due to BSD inheritance, but off64_t should be available as a typedef to ease
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porting of current Linux-specific code.
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Linux kernel headers:
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Bionic comes with its own set of "clean" Linux kernel headers to allow
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user-space code to use kernel-specific declarations (e.g. IOCTLs, structure
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declarations, constants, etc...). They are located in:
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./kernel/common,
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./kernel/arch-arm
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./kernel/arch-x86
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These headers have been generated by a tool (kernel/tools/update-all.py) to
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only include the public definitions from the original Linux kernel headers.
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If you want to know why and how this is done, read kernel/README.TXT to get
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all the (gory) details.
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PThread implementation:
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Bionic's C library comes with its own pthread implementation bundled in.
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This is different from other historical C libraries which:
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- place it in an external library (-lpthread)
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- play linker tricks with weak symbols at dynamic link time
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The support for real-time features (a.k.a. -lrt) is also bundled in the
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C library.
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The implementation is based on futexes and strives to provide *very* short
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code paths for common operations. Notable features are the following:
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- pthread_mutex_t, pthread_cond_t are only 4 bytes each.
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- Normal, recursive and error-check mutexes are supported, and the code
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path is heavily optimized for the normal case, which is used most of
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the time.
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- Process-shared mutexes and condition variables are not supported.
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Their implementation requires far more complexity and was absolutely
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not needed for Android (which uses other inter-process synchronization
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capabilities).
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Note that they could be added in the future without breaking the ABI
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by specifying more sophisticated code paths (which may make the common
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paths slightly slower though).
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- There is currently no support for read/write locks, priority-ceiling in
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mutexes and other more advanced features. Again, the main idea being
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that this was not needed for Android at all but could be added in the
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future.
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pthread_cancel():
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pthread_cancel() will *not* be supported in Bionic, because doing this would
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involve making the C library significantly bigger for very little benefit.
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Consider that:
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- A proper implementation must insert pthread cancellation checks in a lot
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of different places of the C library. And conformance is very difficult
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to test properly.
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- A proper implementation must also clean up resources, like releasing
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memory, or unlocking mutexes, properly if the cancellation happens in a
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complex function (e.g. inside gethostbyname() or fprintf() + complex
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formatting rules). This tends to slow down the path of many functions.
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- pthread cancellation cannot stop all threads: e.g. it can't do anything
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against an infinite loop
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- pthread cancellation itself has short-comings and isn't very portable
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(see http://advogato.org/person/slamb/diary.html?start=49 for example).
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All of this is contrary to the Bionic design goals. If your code depends on
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thread cancellation, please consider alternatives.
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Note however that Bionic does implement pthread_cleanup_push() and
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pthread_cleanup_pop(), which can be used to handle cleanups that happen when
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a thread voluntarily exits through pthread_exit() or returning from its
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main function.
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pthread_once():
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Do not call fork() within a callback provided to pthread_once(). Doing this
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may result in a deadlock in the child process the next time it calls
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pthread_once().
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Also, you can't throw a C++ Exception from the callback (see C++ Exception
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Support below).
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The current implementation of pthread_once() lacks the necessary support of
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multi-core-safe double-checked-locking (read and write barriers).
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Thread-specific data
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The thread-specific storage only provides for a bit less than 64
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pthread_key_t objects to each process. The implementation provides 64 real
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slots but also uses about 5 of them (exact number may depend on
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implementation) for its own use (e.g. two slots are pre-allocated by the C
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library to speed-up the Android OpenGL sub-system).
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Note that Posix mandates a minimum of 128 slots, but we do not claim to be
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Posix-compliant.
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Except for the main thread, the TLS area is stored at the top of the stack.
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See comments in bionic/libc/bionic/pthread.c for details.
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At the moment, thread-local storage defined through the __thread compiler
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keyword is not supported by the Bionic C library and dynamic linker.
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Multi-core support
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At the moment, Bionic does not provide or use read/write memory barriers.
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This means that using it on certain multi-core systems might not be
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supported, depending on its exact CPU architecture.
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Android-specific features:
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Bionic provides a small number of Android-specific features to its clients:
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- access to system properties:
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Android provides a simple shared value/key space to all processes on the
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system. It stores a liberal number of 'properties', each of them being a
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simple size-limited string that can be associated to a size-limited
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string value.
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The header <sys/system_properties.h> can be used to read system
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properties and also defines the maximum size of keys and values.
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- Android-specific user/group management:
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There is no /etc/passwd or /etc/groups in Android. By design, it is
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meant to be used by a single handset user. On the other hand, Android
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uses the Linux user/group management features extensively to secure
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process permissions, like access to various filesystem directories.
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In the Android scheme, each installed application gets its own
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uid_t/gid_t starting from 10000; lower numerical ids are reserved for
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system daemons.
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getpwnam() recognizes some hard-coded subsystems names (e.g. "radio")
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and will translate them to their low-user-id values. It also recognizes
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"app_1234" as the synthetic name of the application that was installed
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with uid 10000 + 1234, which is 11234. getgrnam() works similarly
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getgrouplist() will always return a single group for any user name,
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which is the one passed as an input parameter.
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getgrgid() will similarly only return a structure that contains a
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single-element members list, corresponding to the user with the same
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numerical value than the group.
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See bionic/libc/bionic/stubs.c for more details.
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- getservent()
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There is no /etc/services on Android. Instead the C library embeds a
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constant list of services in its executable, which is parsed on demand
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by the various functions that depend on it. See
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bionic/libc/netbsd/net/getservent.c and
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bionic/libc/netbsd/net/services.h
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The list of services defined internally might change liberally in the
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future. This feature is mostly historically and is very rarely used.
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The getservent() returns thread-local data. getservbyport() and
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getservbyname() are also implemented in a similar fashion.
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- getprotoent()
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There is no /etc/protocol on Android. Bionic does not currently
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implement getprotoent() and related functions. If added, it will
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likely be done in a way similar to getservent()
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DNS resolver:
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Bionic uses a NetBSD-derived resolver library which has been modified in
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the following ways:
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- don't implement the name-server-switch feature (a.k.a. <nsswitch.h>)
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- read /system/etc/resolv.conf instead of /etc/resolv.conf
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- read the list of servers from system properties. the code looks for
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'net.dns1', 'net.dns2', etc.. Each property should contain the IP
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address of a DNS server.
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these properties are set/modified by other parts of the Android system
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(e.g. the dhcpd daemon).
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the implementation also supports per-process DNS server list, using the
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properties 'net.dns1.<pid>', 'net.dns2.<pid>', etc... Where <pid> stands
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for the numerical ID of the current process.
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- when performing a query, use a properly randomized Query ID (instead of
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a incremented one), for increased security.
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- when performing a query, bind the local client socket to a random port
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for increased security.
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- get rid of *many* unfortunate thread-safety issues in the original code
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Bionic does *not* expose implementation details of its DNS resolver; the
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content of <arpa/nameser.h> is intentionally blank. The resolver
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implementation might change completely in the future.
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PThread Real-Time Timers:
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timer_create(), timer_gettime(), timer_settime() and timer_getoverrun() are
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supported.
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Bionic also now supports SIGEV_THREAD real-time timers (see timer_create()).
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The implementation simply uses a single thread per timer, unlike GLibc which
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uses complex heuristics to try to use the less threads possible when several
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timers with compatible properties are used.
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This means that if your code uses a lot of SIGEV_THREAD timers, your program
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may consume a lot of memory. However, if your program needs many of these
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timers, it'd better handle timeout events directly instead.
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Other timers (e.g. SIGEV_SIGNAL) are handled by the kernel and use much less
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system resources.
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Binary Compatibility:
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Bionic is *not* in any way binary-compatible with the GNU C Library, ucLibc
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or any known Linux C library. This means several things:
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- You cannot expect to build something against the GNU C Library headers and
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have it dynamically link properly to Bionic later.
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- You should *really* use the Android toolchain to build your program against
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Bionic. The toolchain deals with many important details that are crucial
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to get something working properly.
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Failure to do so will usually result in the inability to run or link your
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program, or even runtime crashes. Several random web pages on the Internet
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describe how you can succesfully write a "hello-world" program with the
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ARM GNU toolchain. These examples usually work by chance, if anything else,
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and you should not follow these instructions unless you want to waste a lot
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of your time in the process.
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Note however that you *can* generate a binary that is built against the
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GNU C Library headers and then statically linked to it. The corresponding
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executable should be able to run (if it doesn't use dlopen()/dlsym())
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Dynamic Linker:
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Bionic comes with its own dynamic linker (just like ld.so on Linux really
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comes from GLibc). This linker does not support all the relocations
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generated by other GCC ARM toolchains.
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C++ Exceptions Support:
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At the moment, Bionic doesn't support C++ exceptions, what this really means
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is the following:
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- If pthread_once() is called with a C++ callback that throws an exception,
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then the C library will keep the corresponding pthread_once_t mutex
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locked. Any further call to pthread_once() will result in a deadlock.
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A proper implementation should be able to register a C++ exception
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cleanup handler before the callback to properly unlock the
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pthread_once_t. Unfortunately this requires tricky assembly code that
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is highly dependent on the compiler.
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This feature is not planned to be supported anytime soon.
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- The same problem may arise if you throw an exception within a callback
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called from the C library. Fortunately, these cases are very rare in the
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real-world, but any callback you provide to the C library should *not*
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throw an exception.
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- Bionic lacks a few support functions to have exception support work
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properly.
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System V IPCs:
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Bionic intentionally does not provide support for System-V IPCs mechanisms,
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like the ones provided by semget(), shmget(), msgget(). The reason for this
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is to avoid denial-of-service. For a detailed rationale about this, please
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read the file docs/SYSV-IPCS.TXT.
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Include Paths:
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The Android build system should automatically provide the necessary include
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paths required to build against the C library headers. However, if you want
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to do that yourself, you will need to add:
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libc/arch-$ARCH/include
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libc/include
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libc/kernel/common
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libc/kernel/arch-$ARCH
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to your C include path.
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