1# Android bionic status 2 3This document details libc/libm/libdl additions and behavior changes. 4 5See also 6[Android linker changes for NDK developers](../android-changes-for-ndk-developers.md) 7for changes related to native code loading in various Android releases. 8 9## Bionic function availability 10 11### POSIX 12 13You can see the current status with respect to POSIX in the form of tests: 14https://android.googlesource.com/platform/bionic/+/main/tests/headers/posix/ 15 16Some POSIX functionality is not supported by the Linux kernel, and 17is guarded with tests for `__linux__`. Other functionality is not 18supported by bionic or glibc, and guarded with tests for `__BIONIC__` 19and `__GLIBC__`. In other cases historical accidents mean 32-bit 20bionic diverged but 64-bit bionic matches POSIX; these are guarded with 21`__LP64__`. 22 23Most bionic-only diversions should be accompanied by an explanatory comment. 24 25Missing functions are either obsolete or explicitly disallowed by SELinux: 26 * `a64l`/`l64a` 27 * `confstr` 28 * `crypt`/`encrypt`/`setkey` 29 * `gethostid` 30 * `shm_open`/`shm_unlink` 31 * `sockatmark` 32 * `ualarm` 33 34Missing functionality: 35 * `<aio.h>`. No particular reason not to have this other than that no-one's 36 needed it yet, and it's relatively complex. If/when llvm-libc adds this, 37 maybe we'll just reuse that. 38 * `<monetary.h>`. See 39 [discussion](https://github.com/android/ndk/issues/1182). 40 * `<wordexp.h>`. Unsafe because it passes user input to the shell (!), 41 and often should just be a call to glob() anyway. See also 42 [OpenBSD's discussion about adding wordexp()](https://www.mail-archive.com/tech@openbsd.org/msg02325.html). 43 * Locales. Although bionic contains the various `_l()` functions, the only 44 locale supported is a UTF-8 C/POSIX locale. Most of the POSIX APIs are 45 insufficient to support the wide range of languages used by Android users, 46 and apps should use icu4c (or do their i18n work in Java) instead. 47 * Robust mutexes. See 48 [discussion](https://github.com/android/ndk/issues/1181). 49 * Thread cancellation (`pthread_cancel`). Unlikely to ever be implemented 50 because of the difficulty and cost of implementing it, and the difficulty 51 of using it correctly. See 52 [This is why we can't have safe cancellation points](https://lwn.net/Articles/683118/) 53 for more about thread cancellation. 54 55Run `./libc/tools/check-symbols-glibc.py` in bionic/ for the current 56list of POSIX functions implemented by glibc but not by bionic. 57 58### libc 59 60Current libc symbols: https://android.googlesource.com/platform/bionic/+/main/libc/libc.map.txt 61 62New libc functions in API level 37: 63 * New system call wrappers: `sched_getattr()`/`sched_setattr()` (`<sched.h>`). 64 65New libc functions in API level 36: 66 * `qsort_r`, `sig2str`/`str2sig` (POSIX Issue 8 additions). 67 * GNU/BSD extension `lchmod`. 68 * GNU extensions `pthread_getaffinity_np`/`pthread_setaffinity_np`. 69 * New system call wrapper: `mseal` (`<sys/mman.h>`). 70 71New libc functions in V (API level 35): 72 * New `android_crash_detail_register`, `android_crash_detail_unregister`, 73 `android_crash_detail_replace_name`, and `android_crash_detail_replace_data` 74 functionality for adding arbitrary data to tombstones 75 (see `<android/crash_detail.h>` for full documentation). 76 * `tcgetwinsize`, `tcsetwinsize`, `_Fork` (POSIX Issue 8 additions). 77 * `timespec_getres` (C23 addition). 78 * `localtime_rz`, `mktime_z`, `tzalloc`, and `tzfree` (NetBSD 79 extensions implemented in tzcode, and the "least non-standard" 80 functions for avoiding $TZ if you need to use multiple timezones in 81 multi-threaded C). 82 * `mbsrtowcs_l` and `wcsrtombs_l` aliases for `mbsrtowcs` and `wcsrtombs`. 83 * GNU extensions `strerrordesc_np` and `strerrorname_np`. 84 * New system call wrappers: `__riscv_flush_icache` (`<sys/cachectl.h>`), 85 `__riscv_hwprobe` (`<sys/hwprobe.h>`), `epoll_pwait2`/`epoll_pwait2_64` (`<sys/epoll.h>`). 86 87New libc behavior in V (API level 35): 88 * Added `LD_SHOW_AUXV` to the dynamic linker to dump the ELF auxiliary 89 vector if the environment variable is set. 90 * The printf family now supports `%#m` to print the name of the errno 91 constant (rather than the description printed by `%m`). 92 93New libc functions in U (API level 34): 94 * `close_range` and `copy_file_range` (Linux-specific GNU extensions). 95 * `memset_explicit` in <string.h> (C23 addition). 96 * `__freadahead` in <stdio_ext.h> (in musl but not glibc). 97 * `posix_spawn_file_actions_addchdir_np` and 98 `posix_spawn_file_actions_addfchdir_np` in <spawn.h> (in musl/glibc 99 and macOS, but not iOS). 100 101New libc behavior in U (API level 34): 102 * Support for `%b` and `%B` in the printf/wprintf family, `%b` in the 103 scanf/wscanf family, and `0b` prefixes with base 0 in the strtol/wcstol 104 family. 105 * Support for `wN` length modifiers in the printf/wprintf family. 106 * tmpfile() now respects $TMPDIR. 107 108New libc functions in T (API level 33): 109 * `backtrace`, `backtrace_symbols`, `backtrace_symbols_fd` (`<execinfo.h>`). 110 * New system call wrappers: `preadv2`, `preadv64v2`, `pwritev2`, 111 `pwritev64v2`. 112 113New libc functions in S (API level 31): 114 * New hooks for sanitizers for TLS access: `__libc_get_static_tls_bounds`, 115 `__libc_register_thread_exit_callback`, `__libc_iterate_dynamic_tls`, 116 `__libc_register_dynamic_tls_listeners`. 117 * New helper to allow the zygote to give each zygote child its own stack 118 cookie (currently unused): `android_reset_stack_guards`. 119 * Non-inline symbols for `ffsl`, `ffsll`. 120 * New system call wrappers: `pidfd_getfd`, `pidfd_open`, `pidfd_send_signal`, 121 `process_madvise`. 122 123New libc functions in R (API level 30): 124 * Full C11 `<threads.h>` (available as inlines for older API levels). 125 * `memfd_create` and `mlock2` (Linux-specific GNU extensions). 126 * `renameat2` and `statx` (Linux-specific GNU extensions). 127 * `pthread_cond_clockwait`/`pthread_mutex_clocklock`/`pthread_rwlock_clockrdlock`/`pthread_rwlock_clockwrlock`/`sem_clockwait` 128 129New libc behavior in R (API level 30): 130 * [fdsan](fdsan.md) now aborts when it detects common file descriptor errors, 131 rather than just logging. 132 133New libc functions in Q (API level 29): 134 * `timespec_get` (C11 `<time.h>` addition) 135 * `reallocarray` (BSD/GNU extension in `<malloc.h>` and `<stdlib.h>`) 136 * `res_randomid` (in `<resolv.h>`) 137 * `pthread_sigqueue` (GNU extension) 138 * `getloadavg` (BSD/GNU extension in <stdlib.h>) 139 140New libc behavior in Q (API level 29): 141 * Support for [ELF TLS](elf-tls.md). 142 * Whole printf family now supports the GNU `%m` extension, rather than a 143 special-case hack in `syslog`. 144 * `popen` now always uses `O_CLOEXEC`, not just with the `e` extension. 145 * Bug fixes to handling of UTF-8 U+fffe/U+ffff and code points above U+10ffff. 146 * `aligned_alloc` correctly verifies that `size` is a multiple of `alignment`. 147 * Using `%n` with the printf family is now reported as a FORTIFY failure. 148 Previous versions of Android would ignore the `%n` but not consume the 149 corresponding pointer argument, leading to obscure errors. The scanf family 150 is unchanged. 151 * Support in strptime for `%F`, `%G`, `%g`, `%P`, `%u`, `%V`, and `%v`. 152 (strftime already supported them all.) 153 * [fdsan](fdsan.md) detects and logs common file descriptor errors at runtime. 154 155New libc functions in P (API level 28): 156 * `aligned_alloc` 157 * `__freading`/`__fwriting` (completing <stdio_ext.h>) 158 * `endhostent`/`endnetent`/`endprotoent`/`getnetent`/`getprotoent`/`sethostent`/`setnetent`/`setprotoent` (completing <netdb.h>) 159 * `fexecve` 160 * `fflush_unlocked`/`fgetc_unlocked`/`fgets_unlocked`/`fputc_unlocked`/`fputs_unlocked`/`fread_unlocked`/`fwrite_unlocked` 161 * `getentropy`/`getrandom` (adding <sys/random.h>) 162 * `getlogin_r` 163 * `glob`/`globfree` (adding <glob.h>) 164 * `hcreate`/`hcreate_r`/`hdestroy`/`hdestroy_r`/`hsearch`/`hsearch_r` (completing <search.h>) 165 * `iconv`/`iconv_close`/`iconv_open` (adding <iconv.h>) 166 * `pthread_attr_getinheritsched`/`pthread_attr_setinheritsched`/`pthread_setschedprio` 167 * `pthread_mutexattr_getprotocol`/`pthread_mutexattr_setprotocol` (mutex priority inheritance) 168 * <signal.h> support for `sigaction64_t` and `sigset64_t` allowing LP32 access to real-time signals 169 * <spawn.h> 170 * `swab` 171 * `syncfs` 172 173New libc behavior in P (API level 28): 174 * `%C` and `%S` support in the printf family (previously only the wprintf family supported these). 175 * `%mc`/`%ms`/`%m[` support in the scanf family. 176 * `%s` support in strptime (strftime already supported it). 177 * Using a `pthread_mutex_t` after it's been destroyed will be detected at 178 runtime and reported as a FORTIFY failure. 179 * Passing a null `FILE*` or `DIR*` to libc is now detected at runtime and 180 reported as a FORTIFY failure. 181 182New libc functions in O (API level 26): 183 * `sendto` FORTIFY support 184 * `__system_property_read_callback`/`__system_property_wait` 185 * legacy `bsd_signal` 186 * `catclose`/`catgets`/`catopen` (adding <nl_types.h>) 187 * `ctermid` 188 * all 6 <grp.h>/<pwd.h> (get|set|end)(gr|pw)ent functions 189 * `futimes`/`futimesat`/`lutimes` 190 * `getdomainname`/`setdomainname` 191 * `getsubopt` 192 * `hasmntopt` 193 * `mallopt` 194 * `mblen` 195 * 4 <sys/msg.h> `msg*` functions 196 * <langinfo.h> `nl_langinfo`/`nl_langinfo_l` 197 * `pthread_getname_np` 198 * 2 new Linux system calls `quotactl` and `sync_file_range` 199 * 4 <sys/sem.h> `sem*` functions 200 * 4 <sys/shm.h> `shm*` functions 201 * 5 legacy <signal.h> functions: `sighold`/`sigignore`/`sigpause`/`sigrelse`/`sigset` 202 * `strtod_l`/`strtof_l`/`strtol_l`/`strtoul_l` 203 * <wctype.h> `towctrans`/`towctrans_l`/`wctrans`/`wctrans_l` 204 205New libc behavior in O (API level 26): 206 * Passing an invalid `pthread_t` to libc is now detected at runtime and 207 reported as a FORTIFY failure. Most commonly this is a result of confusing 208 `pthread_t` and `pid_t`. 209 210New libc functions in N (API level 24): 211 * more FORTIFY support functions (`fread`/`fwrite`/`getcwd`/`pwrite`/`write`) 212 * all remaining `_FILE_OFFSET_BITS=64` functions, completing `_FILE_OFFSET_BITS=64` support in bionic (8) 213 * all 7 `pthread_barrier*` functions 214 * all 5 `pthread_spin*` functions 215 * `lockf`/`preadv`/`pwritev`/`scandirat` and `off64_t` variants 216 * `adjtimex`/`clock_adjtime` 217 * <ifaddrs.h> `getifaddrs`/`freeifaddrs`/`if_freenameindex`/`if_nameindex` 218 * `getgrgid_r`/`getgrnam_r` 219 * GNU extensions `fileno_unlocked`/`strchrnul` 220 * 32-bit `prlimit` 221 222New libc behavior in N (API level 24): 223 * `sem_wait` now returns EINTR when interrupted by a signal. 224 225New libc functions in M (API level 23): 226 * <dirent.h> `telldir`, `seekdir`. 227 * <malloc.h> `malloc_info`. 228 * <netdb.h> `gethostbyaddr_r`, `gethostbyname2_r`. 229 * <pthread.h> `pthread_rwlockattr_getkind_np`/`pthread_rwlockattr_setkind_np`. 230 * <pty.h> `forkpty`, `openpty`. 231 * <signal.h> `sigqueue`, `sigtimedwait`, `sigwaitinfo`. 232 * <stdio.h> `fmemopen`, `open_memstream`, `feof_unlocked`, `ferror_unlocked`, `clearerr_unlocked`. 233 * <stdio_ext.h> `__flbf`, `__freadable`, `__fsetlocking`, `__fwritable`, `__fbufsize`, `__fpending`, `_flushlbf`, `__fpurge`. 234 * <stdlib.h> `mkostemp`/`mkostemps`, `lcong48`. 235 * <string.h> `basename`, `strerror_l`, `strerror_r`, `mempcpy`. 236 * <sys/sysinfo.h> `get_nprocs_conf`/`get_nprocs`, `get_phys_pages`, `get_avphys_pages`. 237 * <sys/uio.h> `process_vm_readv`/`process_vm_writev`. 238 * `clock_getcpuclockid`, `login_tty`, `mkfifoat`, `posix_madvise`, `sethostname`, `strcasecmp_l`/`strncasecmp_l`. 239 * <wchar.h> `open_wmemstream`, `wcscasecmp_l`/`wcsncasecmp_l`, `wmempcpy`. 240 * all of <error.h>. 241 * re-introduced various <resolv.h> functions: `ns_format_ttl`, `ns_get16`, `ns_get32`, `ns_initparse`, `ns_makecanon`, `ns_msg_getflag`, `ns_name_compress`, `ns_name_ntol`, `ns_name_ntop`, `ns_name_pack`, `ns_name_pton`, `ns_name_rollback`, `ns_name_skip`, `ns_name_uncompress`, `ns_name_unpack`, `ns_parserr`, `ns_put16`, `ns_put32`, `ns_samename`, `ns_skiprr`, `ns_sprintrr`, and `ns_sprintrrf`. 242 243New libc functions in L (API level 21): 244 * <android/dlext.h>. 245 * <android/set_abort_message.h>. 246 * <arpa/inet.h> `inet_lnaof`, `inet_netof`, `inet_network`, `inet_makeaddr`. 247 * <wctype.h> `iswblank`. 248 * <ctype.h> `isalnum_l`, `isalpha_l`, `isblank_l`, `icntrl_l`, `isdigit_l`, `isgraph_l`, `islower_l`, `isprint_l`, `ispunct_l`, `isspace_l`, `isupper_l`, `isxdigit_l`, `_tolower`, `tolower_l`, `_toupper`, `toupper_l`. 249 * <fcntl.h> `fallocate`, `posix_fadvise`, `posix_fallocate`, `splice`, `tee`, `vmsplice`. 250 * <inttypes.h> `wcstoimax`, `wcstoumax`. 251 * <link.h> `dl_iterate_phdr`. 252 * <mntent.h> `setmntent`, `endmntent`, `getmntent_r`. 253 * <poll.h> `ppoll`. 254 * <pthread.h> `pthread_condattr_getclock`, `pthread_condattr_setclock`, `pthread_mutex_timedlock`, `pthread_gettid_np`. 255 * <sched.h> `setns`. 256 * <search.h> `insque`, `remque`, `lfind`, `lsearch`, `twalk`. 257 * <stdio.h> `dprintf`, `vdprintf`. 258 * <stdlib.h> `initstate`, `setstate`, `getprogname`/`setprogname`, `atof`/`strtof`, `at_quick_exit`/`_Exit`/`quick_exit`, `grantpt`, `mbtowc`/`wctomb`, `posix_openpt`, `rand_r`/`rand`/`random`/`srand`/`srandom`, `strtold_l`/`strtoll_l`/`strtoull_l`. 259 * <string.h> `strcoll_l`/`strxfrm_l`, `stpcpy`/`stpncpy`. 260 * <sys/resource.h> `prlimit`. 261 * <sys/socket.h> `accept4`, `sendmmsg`. 262 * <sys/stat.h> `mkfifo`/`mknodat`. 263 * <time.h> `strftime_l`. 264 * <unistd.h> `dup3`, `execvpe`, `getpagesize`, `linkat`/`symlinkat`/`readlinkat`, `truncate`. 265 * <wchar.h> `wcstof`, `vfwscanf`/`vswscanf`/`vwscanf`, `wcstold_l`/`wcstoll`/`wcstoll_l`/`wcstoull`/`wcstoull_l`, `mbsnrtowcs`/`wcsnrtombs`, `wcscoll_l`/`wcsxfrm_l`. 266 * <wctype.h> `iswalnum_l`/`iswalpha_l`/`iswblank_l`/`iswcntrl_l`/`iswctype_l`/`iswdigit_l`/`iswgraph_l`/`iswlower_l`/`iswprint_l`/`iswpunct_l`/`iswspace_l`/`iswupper_l`/`iswxdigit_l`, `wctype_l`, `towlower_l`/`towupper_l`. 267 * all of <fts.h>. 268 * all of <locale.h>. 269 * all of <sys/epoll.h>. 270 * all of <sys/fsuid.h>. 271 * all of <sys/inotify.h>. 272 * all of <uchar.h>. 273 274New libc functions in K (API level 19): 275 * <inttypes.h> `imaxabs`, `imaxdiv`. 276 * <stdlib.h> `abs`, `labs`, `llabs`. 277 * <sys/stat.h> `futimens`. 278 * all of <sys/statvfs.h>. 279 * all of <sys/swap.h>. 280 * all of <sys/timerfd.h>. 281 282New libc functions in J-MR2 (API level 18): 283 * <stdio.h> `getdelim` and `getline`. 284 * <sys/auxv.h> `getauxval`. 285 * <sys/signalfd.h> `signalfd`. 286 287New libc functions in J-MR1 (API level 17): 288 * <ftw.h>. 289 * <signal.h> `psiginfo` and `psignal`. 290 * `getsid`, `malloc_usable_size`, `mlockall`/`munlockall`, `posix_memalign`, `unshare`. 291 292New libc functions in J (API level 16): 293 * the <search.h> tree functions `tdelete`, `tdestroy`, `tfind`, and `tsearch`. 294 * `faccessat`, `readahead`, `tgkill`. 295 * all of <sys/xattr.h>. 296 297libc function count over time: 298 299| API level | Function count | 300|-----------|----------------| 301| 16 | 842 | 302| 17 | 870 | 303| 18 | 878 | 304| 19 | 893 | 305| 21 | 1016 | 306| 22 | 1038 | 307| 23 | 1103 | 308| 24 | 1147 | 309| 25 | 1147 | 310| 26 | 1199 | 311| 27 | 1199 | 312| 28 | 1298 | 313| 29 | 1312 | 314| 30 | 1368 | 315| 31 | 1379 | 316| 32 | 1379 | 317| 33 | 1386 | 318| 34 | 1392 | 319 320Data collected by: 321``` 322ndk-r26c$ for i in `ls -1v toolchains/llvm/prebuilt/linux-x86_64/sysroot/usr/lib/aarch64-linux-android/*/libc.so` ; \ 323 do echo $i; nm $i | grep -w T | wc -l ; done 324``` 325 326### libm 327 328Current libm symbols: https://android.googlesource.com/platform/bionic/+/main/libm/libm.map.txt 329 3300 remaining missing C11/POSIX libm functions. 331 332New libm functions in O (API level 26): 333 * <complex.h> `clog`/`clogf`, `cpow`/`cpowf` functions. 334 335New libm functions in M (API level 23): 336 * <complex.h> `cabs`, `carg`, `cimag`, `cacos`, `cacosh`, `casin`, `casinh`, `catan`, `catanh`, `ccos`, `ccosh`, `cexp`, `conj`, `cproj`, `csin`, `csinh`, `csqrt`, `ctan`, `ctanh`, `creal`, `cabsf`, `cargf`, `cimagf`, `cacosf`, `cacoshf`, `casinf`, `casinhf`, `catanf`, `catanhf`, `ccosf`, `ccoshf`, `cexpf`, `conjf`, `cprojf`, `csinf`, `csinhf`, `csqrtf`, `ctanf`, `ctanhf`, `crealf`, `cabsl`, `cprojl`, `csqrtl`. 337 * <math.h> `lgammal_r`. 338 339New libm functions in L (API level 21): 340 * <complex.h> `cabsl`, `cprojl`, `csqrtl`. 341 * <math.h> `isinf`, `significandl`. 342 343New libm functions in J-MR2 (API level 18): 344 * <math.h> `log2`, `log2f`. 345 346 347## Target API level behavioral differences 348 349Most bionic bug fixes and improvements have been made without checks for 350the app's `targetSdkVersion`. There are a handful of exceptions. (If in 351doubt, search the source for `android_get_application_target_sdk_version()`.) 352 353### Destroyed mutex checking (targetSdkVersion >= 28) 354 355If a destroyed `pthread_mutex_t` is passed to any of the mutex functions, apps 356targeting API level 28 or higher will see a 357"<function> called on a destroyed mutex" fortify failure. Apps targeting older 358API levels will just have the function fail with EBUSY (matching the likely 359behavior before we added the check). 360 361### Invalid `pthread_t` handling (targetSdkVersion >= 26) 362 363As part of a long-term goal to remove the global thread list, 364and in an attempt to flush out racy code, we changed how an invalid 365`pthread_t` is handled. For `pthread_detach`, `pthread_getcpuclockid`, 366`pthread_getschedparam`/`pthread_setschedparam`, `pthread_join`, and 367`pthread_kill`, instead of returning ESRCH when passed an invalid 368`pthread_t`, if you're targeting API level 26 or above, they'll abort with the 369message "attempt to use invalid pthread\_t". 370 371Note that this doesn't change behavior as much as you might think: the 372old lookup only held the global thread list lock for the duration of 373the lookup, so there was still a race between that and the dereference 374in the caller, given that callers actually need the tid to pass to some 375syscall or other, and sometimes update fields in the `pthread_internal_t` 376struct too. 377 378We can't check a thread's tid against 0 to see whether a `pthread_t` 379is still valid because a dead thread gets its thread struct unmapped 380along with its stack, so the dereference isn't safe. 381 382To fix your code, taking the affected functions one by one: 383 384 * `pthread_getcpuclockid` and `pthread_getschedparam`/`pthread_setschedparam` 385 should be fine. Unsafe calls to those seem highly unlikely. 386 387 * Unsafe `pthread_detach` callers probably want to switch to 388 `pthread_attr_setdetachstate` instead, or use 389 `pthread_detach(pthread_self());` from the new thread's start routine 390 rather than calling detach in the parent. 391 392 * `pthread_join` calls should be safe anyway, because a joinable thread 393 won't actually exit and unmap until it's joined. If you're joining an 394 unjoinable thread, the fix is to stop marking it detached. If you're 395 joining an already-joined thread, you need to rethink your design! 396 397 * Unsafe `pthread_kill` calls aren't portably fixable. (And are obviously 398 inherently non-portable as-is.) The best alternative on Android is to 399 use `pthread_gettid_np` at some point that you know the thread to be 400 alive, and then call `kill`/`tgkill` with signal 0 (which checks 401 whether a process exists rather than actually sending a 402 signal). That's still not completely safe because if you're too late 403 the tid may have been reused, but your code is inherently unsafe without 404 a redesign anyway. 405 406### Interruptable `sem_wait` (targetSdkVersion >= 24) 407 408POSIX says that `sem_wait` can be interrupted by delivery of a 409signal. This wasn't historically true in Android, and when we fixed this 410bug we found that existing code relied on the old behavior. To preserve 411compatibility, `sem_wait` can only return EINTR on Android if the app 412targets API level 24 or later. 413 414 415## FORTIFY 416 417The `_FORTIFY_SOURCE` macro can be used to enable extra 418automatic bounds checking for common libc functions. If a buffer 419overrun is detected, the program is safely aborted as in this 420[example](https://source.android.com/devices/tech/debug/native-crash#fortify). 421 422Note that Android's FORTIFY has been extended to cover other issues. It can 423detect, for example, passing `O_CREAT` to open(2) without specifying a mode. It 424also performs some checking regardless of whether the caller was built with 425FORTIFY enabled. From API level 28, for example, calling a `pthread_mutex_` 426function on a destroyed mutex, calling a `<dirent.h>` function on a null 427pointer, using `%n` with the printf(3) family, or using the scanf(3) `m` 428modifier incorrectly will all result in FORTIFY failures even for code not built 429with FORTIFY. 430 431More background information is available in our 432[FORTIFY in Android](https://android-developers.googleblog.com/2017/04/fortify-in-android.html) 433blog post, and there's more detail about the implementation in 434[The Anatomy of Clang FORTIFY](clang_fortify_anatomy.md). 435 436The Android platform is built with `-D_FORTIFY_SOURCE=2`. Users of ndk-build 437or the NDK's CMake toolchain file also get this by default with NDK r21 or 438newer. Users of other build systems 439need to manually enable FORTIFY by setting `_FORTIFY_SOURCE` themselves in 440whatever build system they're using. The exact subset of FORTIFY available to 441NDK users will depend on their target ABI level, because when a FORTIFY 442check can't be guaranteed at compile-time, a call to a run-time `_chk` 443function is added. 444