# 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. ## `sigset_t` is too small for real-time signals On 32-bit Android, `sigset_t` is too small for ARM and x86 (but correct for MIPS). This means that there is no support for real-time signals in 32-bit code. In the 64-bit ABI, `sigset_t` is the correct size for every architecture. ## `time_t` is 32-bit On 32-bit Android, `time_t` is 32-bit. 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`. In the 64-bit ABI, `time_t` is 64-bit. ## `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 the 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.