# 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 `` functions, API 21 adds `mmap`, and by API 24 you have everything including ``. See the [linker map](libc/libc.map.txt) 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 `` 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. ## `getuid()` and friends wrongly set errno for very large results This doesn't generally affect Android devices, because we don't have any uids/gids/pids large enough, but 32-bit Android doesn't take into account that functions like getuid() potentially have return values that cover the entire 32-bit, and can't fail. This means that the usual "if the result is between -1 and -4096, set errno and return -1" code is inappropriate for these functions. Since LP32 is unlikely to be still supported long before those limits could ever matter, although -- unlike the others in this document -- this defect is actually fixable, it doesn't seem worth fixing.