# Android bionic status ## Bionic function availability ### POSIX 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 `__BIONIC__` and `__GLIBC__`. In other cases historical accidents mean 32-bit bionic diverged but 64-bit bionic matches POSIX; these are guarded with `__LP64__`. Most bionic-only diversions should be accompanied by an explanatory comment. Missing functions are either obsolete or explicitly disallowed by SELinux: * `a64l`/`l64a` * `confstr` * `crypt`/`encrypt`/`setkey` * `gethostid` * `shm_open`/`shm_unlink` * `sockatmark` Missing functionality: * `` * `` * Thread cancellation * Robust mutexes Run `./libc/tools/check-symbols-glibc.py` in bionic/ for the current list of POSIX functions implemented by glibc but not by bionic. ### libc Current libc symbols: https://android.googlesource.com/platform/bionic/+/master/libc/libc.map.txt New libc functions in Q: * `timespec_get` (C11 `` addition) * `res_randomid` (in ``) New libc functions in P: * `__freading`/`__fwriting` (completing ) * `endhostent`/`endnetent`/`endprotoent`/`getnetent`/`getprotoent`/`sethostent`/`setnetent`/`setprotoent` (completing ) * `fexecve` * `fflush_unlocked`/`fgetc_unlocked`/`fgets_unlocked`/`fputc_unlocked`/`fputs_unlocked`/`fread_unlocked`/`fwrite_unlocked` * `getentropy`/`getrandom` (adding ) * `getlogin_r` * `glob`/`globfree` (adding ) * `hcreate`/`hcreate_r`/`hdestroy`/`hdestroy_r`/`hsearch`/`hsearch_r` (completing ) * `iconv`/`iconv_close`/`iconv_open` (adding ) * `pthread_attr_getinheritsched`/`pthread_attr_setinheritsched`/`pthread_setschedprio` * `pthread_mutexattr_getprotocol`/`pthread_mutexattr_setprotocol` (mutex priority inheritance) * support for `sigaction64_t` and `sigset64_t` allowing LP32 access to real-time signals * * `swab` * `syncfs` New libc behavior in P: * `%C` and `%S` support in the printf family (previously only the wprintf family supported these) * `%mc`/`%ms`/`%m[` support in the scanf family * `%s` support in strptime (strftime already supported it) New libc functions in O: * `sendto` FORTIFY support * `__system_property_read_callback`/`__system_property_wait` * legacy `bsd_signal` * `catclose`/`catgets`/`catopen` (adding ) * `ctermid` * all 6 / (get|set|end)(gr|pw)ent functions * `futimes`/`futimesat`/`lutimes` * `getdomainname`/`setdomainname` * `getsubopt` * `hasmntopt` * `mallopt` * `mblen` * 4 `msg*` functions * `nl_langinfo`/`nl_langinfo_l` * `pthread_getname_np` * 2 new Linux system calls `quotactl` and `sync_file_range` * 4 `sem*` functions * 4 `shm*` functions * 5 legacy functions: `sighold`/`sigignore`/`sigpause`/`sigrelse`/`sigset` * `strtod_l`/`strtof_l`/`strtol_l`/`strtoul_l` * `towctrans`/`towctrans_l`/`wctrans`/`wctrans_l` New libc functions in N: * more FORTIFY support functions (`fread`/`fwrite`/`getcwd`/`pwrite`/`write`) * all remaining `_FILE_OFFSET_BITS=64` functions, completing `_FILE_OFFSET_BITS=64` support in bionic (8) * all 7 `pthread_barrier*` functions * all 5 `pthread_spin*` functions * `lockf`/`preadv`/`pwritev`/`scandirat` and `off64_t` variants * `adjtimex`/`clock_adjtime` * `getifaddrs`/`freeifaddrs`/`if_freenameindex`/`if_nameindex` * `getgrgid_r`/`getgrnam_r` * GNU extensions `fileno_unlocked`/`strchrnul` * 32-bit `prlimit` 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 ``` ### libm 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 ## Target API level behavioral differences 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. ### Invalid `pthread_t` handling (targetSdkVersion >= O) 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_detach`, `pthread_getcpuclockid`, `pthread_getschedparam`/`pthread_setschedparam`, `pthread_join`, and `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_getcpuclockid` and `pthread_getschedparam`/`pthread_setschedparam` should be fine. Unsafe calls to those seem highly unlikely. * Unsafe `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! * Unsafe `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 `kill`/`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. ### Interruptable `sem_wait` (targetSdkVersion >= N) 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. ## FORTIFY The `_FORTIFY_SOURCE` macro can be used to enable extra automatic bounds checking for common libc functions. If a buffer overrun is detected, the program is safely aborted as in this (example)[https://source.android.com/devices/tech/debug/native-crash#fortify]. Note that in recent releases Android's FORTIFY has been extended to cover other issues. It can now detect, for example, passing `O_CREAT` to open(2) without specifying a mode. It also performs some checking regardless of whether the caller was built with FORTIFY enabled. In P, for example, calling a `pthread_mutex_` function on a destroyed mutex, calling a `` function on a null pointer, using `%n` with the printf(3) family, or using the scanf(3) `m` modifier incorrectly will all result in FORTIFY failures even for code not built with FORTIFY. More background information is available in our (FORTIFY in Android)[https://android-developers.googleblog.com/2017/04/fortify-in-android.html] blog post. The Android platform is built with `-D_FORTIFY_SOURCE=2`, but NDK users need to manually enable FORTIFY by setting that themselves in whatever build system they're using. The exact subset of FORTIFY available to NDK users will depend on their target ABI level, because when a FORTIFY check can't be guaranteed at compile-time, a call to a run-time `_chk` function is added.