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NAME
signal-safety - async-signal-safe functions
DESCRIPTION
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.1-2003
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.1-2013
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.1-2016
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.1-2016
htons(3) Added in POSIX.1-2016
kill(2)
link(2)
linkat(2) Added in POSIX.1-2008
listen(2)
longjmp(3) Added in POSIX.1-2016; see notes below
lseek(2)
lstat(2)
memccpy(3) Added in POSIX.1-2016
memchr(3) Added in POSIX.1-2016
memcmp(3) Added in POSIX.1-2016
memcpy(3) Added in POSIX.1-2016
memmove(3) Added in POSIX.1-2016
memset(3) Added in POSIX.1-2016
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.1-2016
ntohs(3) Added in POSIX.1-2016
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.1-2013
pthread_self(3) Added in POSIX.1-2013
pthread_sigmask(3) Added in POSIX.1-2013
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.1-2016; see notes below
signal(2)
sigpause(3)
sigpending(2)
sigprocmask(2)
sigqueue(2)
sigset(3)
sigsuspend(2)
sleep(3)
sockatmark(3) Added in POSIX.1-2004
socket(2)
socketpair(2)
stat(2)
stpcpy(3) Added in POSIX.1-2016
stpncpy(3) Added in POSIX.1-2016
strcat(3) Added in POSIX.1-2016
strchr(3) Added in POSIX.1-2016
strcmp(3) Added in POSIX.1-2016
strcpy(3) Added in POSIX.1-2016
strcspn(3) Added in POSIX.1-2016
strlen(3) Added in POSIX.1-2016
strncat(3) Added in POSIX.1-2016
strncmp(3) Added in POSIX.1-2016
strncpy(3) Added in POSIX.1-2016
strnlen(3) Added in POSIX.1-2016
strpbrk(3) Added in POSIX.1-2016
strrchr(3) Added in POSIX.1-2016
strspn(3) Added in POSIX.1-2016
strstr(3) Added in POSIX.1-2016
strtok_r(3) Added in POSIX.1-2016
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.1-2016
wcpncpy(3) Added in POSIX.1-2016
wcscat(3) Added in POSIX.1-2016
wcschr(3) Added in POSIX.1-2016
wcscmp(3) Added in POSIX.1-2016
wcscpy(3) Added in POSIX.1-2016
wcscspn(3) Added in POSIX.1-2016
wcslen(3) Added in POSIX.1-2016
wcsncat(3) Added in POSIX.1-2016
wcsncmp(3) Added in POSIX.1-2016
wcsncpy(3) Added in POSIX.1-2016
wcsnlen(3) Added in POSIX.1-2016
wcspbrk(3) Added in POSIX.1-2016
wcsrchr(3) Added in POSIX.1-2016
wcsspn(3) Added in POSIX.1-2016
wcsstr(3) Added in POSIX.1-2016
wcstok(3) Added in POSIX.1-2016
wmemchr(3) Added in POSIX.1-2016
wmemcmp(3) Added in POSIX.1-2016
wmemcpy(3) Added in POSIX.1-2016
wmemmove(3) Added in POSIX.1-2016
wmemset(3) Added in POSIX.1-2016
write(2)
Notes:
* POSIX.1-2001 and POSIX.1-2004 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.1-2003 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.
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.
SEE ALSO
sigaction(2), signal(7), standards(7)
COLOPHON
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