0bfcbaf4d0
Also start breaking up the monolithic top level README.md, pulling the 32-bit ABI stuff out into its own file, and moving the remaining benchmark documentation in with the rest of the benchmark documentation. Bug: N/A Test: N/A Change-Id: Ic1b9995e27b5044199ed34883cc0b8faa894df0e
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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.