You can see the current status with respect to POSIX in the form of tests: https://android.googlesource.com/platform/bionic/+/master/tests/headers/posix/
Some POSIX functionality is not supported by the Linux kernel, and is guarded with tests for
__linux__. Other functionality is not supported by bionic or glibc, and guarded with tests for
__GLIBC__. In other cases historical accidents mean 32-bit bionic diverged but 64-bit bionic matches POSIX; these are guarded with
Most bionic-only diversions should be accompanied by an explanatory comment.
Missing functions are either obsolete or explicitly disallowed by SELinux:
./libc/tools/check-symbols-glibc.py in bionic/ for the current list of POSIX functions implemented by glibc but not by bionic.
Current libc symbols: https://android.googlesource.com/platform/bionic/+/master/libc/libc.map.txt
New libc functions in P:
pthread_mutexattr_setprotocol(mutex priority inheritance)
sigset64_tallowing LP32 access to real-time signals
New libc behavior in P:
%Ssupport in the printf family (previously only the wprintf family supported these)
%m[support in the scanf family
%ssupport in strptime (strftime already supported it)
New libc functions in O:
New libc functions in N:
_FILE_OFFSET_BITS=64support in bionic (8)
libc function count over time: G 803, H 825, I 826, J 846, J-MR1 873, J-MR2 881, K 896, L 1116, M 1181, N 1226, O 1278
ndk-r17$ for i in `ls -1v platforms/android-*/arch-arm/usr/lib/libc.so` ; do \ echo $i; nm $i | grep -vw [AbdNnt] | grep -vw B | wc -l ; done
Current libm symbols: https://android.googlesource.com/platform/bionic/+/master/libm/libm.map.txt
0 remaining missing POSIX libm functions.
19 new libm functions in O: complex trig/exp/log functions.
libm function count over time: G 158, J-MR2 164, L 220, M 265, O 284
Most bionic bug fixes and improvements have been made without checks for the app's
targetSdkVersion. As of O there were exactly two exceptions, but there are likely to be more in future because of Project Treble.
As part of a long-term goal to remove the global thread list, and in an attempt to flush out racy code, we changed how an invalid
pthread_t is handled. For
pthread_kill, instead of returning ESRCH when passed an invalid
pthread_t, if you‘re targeting O or above, they’ll abort with the message “attempt to use invalid pthread_t”.
Note that this doesn't change behavior as much as you might think: the old lookup only held the global thread list lock for the duration of the lookup, so there was still a race between that and the dereference in the caller, given that callers actually need the tid to pass to some syscall or other, and sometimes update fields in the
pthread_internal_t struct too.
We can‘t check a thread’s tid against 0 to see whether a
pthread_t is still valid because a dead thread gets its thread struct unmapped along with its stack, so the dereference isn't safe.
To fix your code, taking the affected functions one by one:
pthread_setschedparam should be fine. Unsafe calls to those seem highly unlikely.
pthread_detach callers probably want to switch to
pthread_attr_setdetachstate instead, or use
pthread_detach(pthread_self()); from the new thread's start routine rather than calling detach in the parent.
pthread_join calls should be safe anyway, because a joinable thread won‘t actually exit and unmap until it’s joined. If you‘re joining an unjoinable thread, the fix is to stop marking it detached. If you’re joining an already-joined thread, you need to rethink your design!
pthread_kill calls aren‘t portably fixable. (And are obviously inherently non-portable as-is.) The best alternative on Android is to use
pthread_gettid_np at some point that you know the thread to be alive, and then call
tgkill with signal 0 (which checks whether a process exists rather than actually sending a signal). That’s still not completely safe because if you're too late the tid may have been reused, but your code is inherently unsafe without a redesign anyway.
POSIX says that
sem_wait can be interrupted by delivery of a signal. This wasn't historically true in Android, and when we fixed this bug we found that existing code relied on the old behavior. To preserve compatibility,
sem_wait can only return EINTR on Android if the app targets N or later.