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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:

  • <aio.h>
  • <wordexp.h>
  • 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 P:

  • __freading/__fwriting (completing <stdio_ext.h>)
  • endhostent/endnetent/endprotoent/getnetent/getprotoent/sethostent/setnetent/setprotoent (completing <netdb.h>)
  • fexecve
  • fflush_unlocked/fgetc_unlocked/fgets_unlocked/fputc_unlocked/fputs_unlocked/fread_unlocked/fwrite_unlocked
  • getentropy/getrandom (adding <sys/random.h>)
  • getlogin_r
  • glob/globfree (adding <glob.h>)
  • hcreate/hcreate_r/hdestroy/hdestroy_r/hsearch/hsearch_r (completing <search.h>)
  • iconv/iconv_close/iconv_open (adding <iconv.h>)
  • pthread_attr_getinheritsched/pthread_attr_setinheritsched/pthread_setschedprio
  • pthread_mutexattr_getprotocol/pthread_mutexattr_setprotocol (mutex priority inheritance)
  • <signal.h> support for sigaction64_t and sigset64_t allowing LP32 access to real-time signals
  • <spawn.h>
  • 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 <nl_types.h>)
  • ctermid
  • all 6 <grp.h>/<pwd.h> (get|set|end)(gr|pw)ent functions
  • futimes/futimesat/lutimes
  • getdomainname/setdomainname
  • getsubopt
  • hasmntopt
  • mallopt
  • mblen
  • 4 <sys/msg.h> msg* functions
  • <langinfo.h> nl_langinfo/nl_langinfo_l
  • pthread_getname_np
  • 2 new Linux system calls quotactl and sync_file_range
  • 4 <sys/sem.h> sem* functions
  • 4 <sys/shm.h> shm* functions
  • 5 legacy <signal.h> functions: sighold/sigignore/sigpause/sigrelse/sigset
  • strtod_l/strtof_l/strtol_l/strtoul_l
  • <wctype.h> 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.