An async-signal-safe function is one that can be safely called
from within a signal handler. Many functions are not async-
signal-safe. In particular, nonreentrant functions are generally
unsafe to call from a signal handler.
The kinds of issues that render a function unsafe can be quickly
understood when one considers the implementation of the stdio
library, all of whose functions are not async-signal-safe.
When performing buffered I/O on a file, the stdio functions must
maintain a statically allocated data buffer along with associated
counters and indexes (or pointers) that record the amount of data
and the current position in the buffer. Suppose that the main
program is in the middle of a call to a stdio function such as
printf(3) where the buffer and associated variables have been
partially updated. If, at that moment, the program is
interrupted by a signal handler that also calls printf(3), then
the second call to printf(3) will operate on inconsistent data,
with unpredictable results.
To avoid problems with unsafe functions, there are two possible
choices:
1. Ensure that (a) the signal handler calls only async-signal-
safe functions, and (b) the signal handler itself is reentrant
with respect to global variables in the main program.
2. Block signal delivery in the main program when calling
functions that are unsafe or operating on global data that is
also accessed by the signal handler.
Generally, the second choice is difficult in programs of any
complexity, so the first choice is taken.
POSIX.1 specifies a set of functions that an implementation must
make async-signal-safe. (An implementation may provide safe
implementations of additional functions, but this is not required
by the standard and other implementations may not provide the
same guarantees.)
In general, a function is async-signal-safe either because it is
reentrant or because it is atomic with respect to signals (i.e.,
its execution can't be interrupted by a signal handler).
The set of functions required to be async-signal-safe by POSIX.1
is shown in the following table. The functions not otherwise
noted were required to be async-signal-safe in POSIX.1-2001; the
table details changes in the subsequent standards.
Function Notes
abort(3) Added in POSIX.1-2001 TC1
accept(2)
access(2)
aio_error(3)
aio_return(3)
aio_suspend(3) See notes below
alarm(2)
bind(2)
cfgetispeed(3)
cfgetospeed(3)
cfsetispeed(3)
cfsetospeed(3)
chdir(2)
chmod(2)
chown(2)
clock_gettime(2)
close(2)
connect(2)
creat(2)
dup(2)
dup2(2)
execl(3) Added in POSIX.1-2008;
see notes below
execle(3) See notes below
execv(3) Added in POSIX.1-2008
execve(2)
_exit(2)
_Exit(2)
faccessat(2) Added in POSIX.1-2008
fchdir(2) Added in POSIX.1-2008 TC1
fchmod(2)
fchmodat(2) Added in POSIX.1-2008
fchown(2)
fchownat(2) Added in POSIX.1-2008
fcntl(2)
fdatasync(2)
fexecve(3) Added in POSIX.1-2008
ffs(3) Added in POSIX.1-2008 TC2
fork(2) See notes below
fstat(2)
fstatat(2) Added in POSIX.1-2008
fsync(2)
ftruncate(2)
futimens(3) Added in POSIX.1-2008
getegid(2)
geteuid(2)
getgid(2)
getgroups(2)
getpeername(2)
getpgrp(2)
getpid(2)
getppid(2)
getsockname(2)
getsockopt(2)
getuid(2)
htonl(3) Added in POSIX.1-2008 TC2
htons(3) Added in POSIX.1-2008 TC2
kill(2)
link(2)
linkat(2) Added in POSIX.1-2008
listen(2)
longjmp(3) Added in POSIX.1-2008
TC2; see notes below
lseek(2)
lstat(2)
memccpy(3) Added in POSIX.1-2008 TC2
memchr(3) Added in POSIX.1-2008 TC2
memcmp(3) Added in POSIX.1-2008 TC2
memcpy(3) Added in POSIX.1-2008 TC2
memmove(3) Added in POSIX.1-2008 TC2
memset(3) Added in POSIX.1-2008 TC2
mkdir(2)
mkdirat(2) Added in POSIX.1-2008
mkfifo(3)
mkfifoat(3) Added in POSIX.1-2008
mknod(2) Added in POSIX.1-2008
mknodat(2) Added in POSIX.1-2008
ntohl(3) Added in POSIX.1-2008 TC2
ntohs(3) Added in POSIX.1-2008 TC2
open(2)
openat(2) Added in POSIX.1-2008
pause(2)
pipe(2)
poll(2)
posix_trace_event(3)
pselect(2)
pthread_kill(3) Added in POSIX.1-2008 TC1
pthread_self(3) Added in POSIX.1-2008 TC1
pthread_sigmask(3) Added in POSIX.1-2008 TC1
raise(3)
read(2)
readlink(2)
readlinkat(2) Added in POSIX.1-2008
recv(2)
recvfrom(2)
recvmsg(2)
rename(2)
renameat(2) Added in POSIX.1-2008
rmdir(2)
select(2)
sem_post(3)
send(2)
sendmsg(2)
sendto(2)
setgid(2)
setpgid(2)
setsid(2)
setsockopt(2)
setuid(2)
shutdown(2)
sigaction(2)
sigaddset(3)
sigdelset(3)
sigemptyset(3)
sigfillset(3)
sigismember(3)
siglongjmp(3) Added in POSIX.1-2008
TC2; see notes below
signal(2)
sigpause(3)
sigpending(2)
sigprocmask(2)
sigqueue(2)
sigset(3)
sigsuspend(2)
sleep(3)
sockatmark(3) Added in POSIX.1-2001 TC2
socket(2)
socketpair(2)
stat(2)
stpcpy(3) Added in POSIX.1-2008 TC2
stpncpy(3) Added in POSIX.1-2008 TC2
strcat(3) Added in POSIX.1-2008 TC2
strchr(3) Added in POSIX.1-2008 TC2
strcmp(3) Added in POSIX.1-2008 TC2
strcpy(3) Added in POSIX.1-2008 TC2
strcspn(3) Added in POSIX.1-2008 TC2
strlen(3) Added in POSIX.1-2008 TC2
strncat(3) Added in POSIX.1-2008 TC2
strncmp(3) Added in POSIX.1-2008 TC2
strncpy(3) Added in POSIX.1-2008 TC2
strnlen(3) Added in POSIX.1-2008 TC2
strpbrk(3) Added in POSIX.1-2008 TC2
strrchr(3) Added in POSIX.1-2008 TC2
strspn(3) Added in POSIX.1-2008 TC2
strstr(3) Added in POSIX.1-2008 TC2
strtok_r(3) Added in POSIX.1-2008 TC2
symlink(2)
symlinkat(2) Added in POSIX.1-2008
tcdrain(3)
tcflow(3)
tcflush(3)
tcgetattr(3)
tcgetpgrp(3)
tcsendbreak(3)
tcsetattr(3)
tcsetpgrp(3)
time(2)
timer_getoverrun(2)
timer_gettime(2)
timer_settime(2)
times(2)
umask(2)
uname(2)
unlink(2)
unlinkat(2) Added in POSIX.1-2008
utime(2)
utimensat(2) Added in POSIX.1-2008
utimes(2) Added in POSIX.1-2008
wait(2)
waitpid(2)
wcpcpy(3) Added in POSIX.1-2008 TC2
wcpncpy(3) Added in POSIX.1-2008 TC2
wcscat(3) Added in POSIX.1-2008 TC2
wcschr(3) Added in POSIX.1-2008 TC2
wcscmp(3) Added in POSIX.1-2008 TC2
wcscpy(3) Added in POSIX.1-2008 TC2
wcscspn(3) Added in POSIX.1-2008 TC2
wcslen(3) Added in POSIX.1-2008 TC2
wcsncat(3) Added in POSIX.1-2008 TC2
wcsncmp(3) Added in POSIX.1-2008 TC2
wcsncpy(3) Added in POSIX.1-2008 TC2
wcsnlen(3) Added in POSIX.1-2008 TC2
wcspbrk(3) Added in POSIX.1-2008 TC2
wcsrchr(3) Added in POSIX.1-2008 TC2
wcsspn(3) Added in POSIX.1-2008 TC2
wcsstr(3) Added in POSIX.1-2008 TC2
wcstok(3) Added in POSIX.1-2008 TC2
wmemchr(3) Added in POSIX.1-2008 TC2
wmemcmp(3) Added in POSIX.1-2008 TC2
wmemcpy(3) Added in POSIX.1-2008 TC2
wmemmove(3) Added in POSIX.1-2008 TC2
wmemset(3) Added in POSIX.1-2008 TC2
write(2)
Notes:
* POSIX.1-2001 and POSIX.1-2001 TC2 required the functions
fpathconf(3), pathconf(3), and sysconf(3) to be async-signal-
safe, but this requirement was removed in POSIX.1-2008.
* If a signal handler interrupts the execution of an unsafe
function, and the handler terminates via a call to longjmp(3)
or siglongjmp(3) and the program subsequently calls an unsafe
function, then the behavior of the program is undefined.
* POSIX.1-2001 TC1 clarified that if an application calls
fork(2) from a signal handler and any of the fork handlers
registered by pthread_atfork(3) calls a function that is not
async-signal-safe, the behavior is undefined. A future
revision of the standard is likely to remove fork(2) from the
list of async-signal-safe functions.
* Asynchronous signal handlers that call functions which are
cancellation points and nest over regions of deferred
cancellation may trigger cancellation whose behavior is as if
asynchronous cancellation had occurred and may cause
application state to become inconsistent.
errno
Fetching and setting the value of errno is async-signal-safe
provided that the signal handler saves errno on entry and
restores its value before returning.
Deviations in the GNU C library
The following known deviations from the standard occur in the GNU
C library:
* Before glibc 2.24, execl(3) and execle(3) employed realloc(3)
internally and were consequently not async-signal-safe. This
was fixed in glibc 2.24.
* The glibc implementation of aio_suspend(3) is not async-
signal-safe because it uses pthread_mutex_lock(3) internally.
