32-bit ABI bugs

off_t is 32-bit

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.

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 <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.

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.