Skip to content

Latest commit

 

History

History
111 lines (88 loc) · 5.43 KB

32-bit-abi.md

File metadata and controls

111 lines (88 loc) · 5.43 KB
Raw

32-bit ABI bugs

32-bit off_t and _FILE_OFFSET_BITS=64

On 32-bit Android, off_t is a signed 32-bit integer. This limits functions that use off_t to working on files no larger than 2GiB.

Android does not require the _LARGEFILE_SOURCE macro to be used to make fseeko and ftello available. Instead they're always available from API level 24 where they were introduced, and never available before then.

Android also does not require the _LARGEFILE64_SOURCE macro to be used to make off64_t and corresponding functions such as ftruncate64 available. Instead, whatever subset of those functions was available at your target API level will be visible.

There are a couple of exceptions to note. Firstly, off64_t and the single function lseek64 were available right from the beginning in API 3. Secondly, Android has always silently inserted O_LARGEFILE into any open call, so if all you need are functions like read that don't take/return off_t, large files have always worked.

Android support for _FILE_OFFSET_BITS=64 (which turns off_t into off64_t and replaces each off_t function with its off64_t counterpart, such as lseek in the source becoming lseek64 at runtime) was added late. Even when it became available for the platform, it wasn't available from the NDK until r15. Before NDK r15, _FILE_OFFSET_BITS=64 silently did nothing: all code compiled with that was actually using a 32-bit off_t. With a new enough NDK, the situation becomes complicated. If you're targeting an API before 21, almost all functions that take an off_t become unavailable. You've asked for their 64-bit equivalents, and none of them (except lseek/lseek64) exist. As you increase your target API level, you'll have more and more of the functions available. API 12 adds some of the <unistd.h> functions, API 21 adds mmap, and by API 24 you have everything including <stdio.h>. See the linker map for full details. Note also that in NDK r16 and later, if you're using Clang we'll inline an mmap64 implementation in the headers when you target an API before 21 because it's an easy special case that's often needed. This means that code using _FILE_OFFSET_BITS=64 and mmap (but no other functions that are unavailable at your target API level) will always compile.

If your code stops compiling when you move to NDK r15 or later, removing every definition of _FILE_OFFSET_BITS=64 will restore the behavior you used to have: you'll have a 32-bit off_t and use the 32-bit functions. Make sure you grep thoroughly in both your source and your build system: many people aren't aware that _FILE_OFFSET_BITS is set. You might also have to remove references to __USE_FILE_OFFSET64 --- this is the internal flag that should never be set by user code but sometimes is (by zlib, for example). If you think you have removed these but your code still doesn't compile, you can insert this just before the line that's failing to double check:

#if _FILE_OFFSET_BITS == 64
#error "oops, file _FILE_OFFSET_BITS == 64"
#elif defined(__USE_FILE_OFFSET64)
#error "oops, __USE_FILE_OFFSET64 is defined"
#endif

In the 64-bit ABI, off_t is always 64-bit.

For source compatibility, the names containing 64 are also available in the 64-bit ABI even though they're identical to the non-64 names.

sigset_t is too small for real-time signals

On 32-bit Android, sigset_t is too small for ARM and x86. This means that there is no support for real-time signals in 32-bit code. Android P (API level 28) adds sigset64_t and a corresponding function for every function that takes a sigset_t (so sigprocmask64 takes a sigset64_t where sigprocmask takes a sigset_t).

On 32-bit Android, struct sigaction is also too small because it contains a sigset_t. We also offer a struct sigaction64 and sigaction64 function to work around this.

In the 64-bit ABI, sigset_t is the correct size for every architecture.

For source compatibility, the names containing 64 are also available in the 64-bit ABI even though they're identical to the non-64 names.

time_t is 32-bit on LP32 (y2038)

On 32-bit Android, time_t is 32-bit, which will overflow in 2038.

In the 64-bit ABI, time_t is 64-bit, which will not overflow until long after the death of the star around which we currently circle.

The header <time64.h> and type time64_t exist as a workaround, but the kernel interfaces exposed on 32-bit Android all use the 32-bit time_t and struct timespec/struct timeval. Linux 5.x kernels do offer extra interfaces so that 32-bit processes can pass 64-bit times to/from the kernel, but we do not plan on adding support for these to the C library. Convenient use of the new calls would require an equivalent to _FILE_OFFSET_BITS=64, which we wouldn't be able to globally flip for reasons similar to _FILE_OFFSET_BITS, mentioned above. All apps are already required to offer 64-bit variants, and we expect 64-bit-only devices within the next few years.

pthread_mutex_t is too small for large pids

This doesn't generally affect Android devices, because on devices /proc/sys/kernel/pid_max is usually too small to hit our 16-bit limit, but 32-bit bionic's pthread_mutex is a total of 32 bits, leaving just 16 bits for the owner thread id. This means bionic isn't able to support mutexes for tids that don't fit in 16 bits. This typically manifests as a hang in pthread_mutex_lock if the libc startup code doesn't detect this condition and abort.