safety(7) - Linux manual page (original) (raw)
signal-safety(7) Miscellaneous Information Manual signal-safety(7)
NAME top
signal-safety - async-signal-safe functionsDESCRIPTION top
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)](../man3/printf.3.html) 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)](../man3/printf.3.html), then the second
call to [printf(3)](../man3/printf.3.html) will operate on inconsistent data, with
unpredictable results.
To avoid problems with unsafe functions, there are two possible
choices:
(a) Ensure that (1) the signal handler calls only async-signal-
safe functions, and (2) the signal handler itself is
reentrant with respect to global variables in the main
program.
(b) 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)](../man3/abort.3.html) Added in POSIX.1-2001 TC1
[accept(2)](../man2/accept.2.html)
[access(2)](../man2/access.2.html)
[aio_error(3)](../man3/aio%5Ferror.3.html)
[aio_return(3)](../man3/aio%5Freturn.3.html)
[aio_suspend(3)](../man3/aio%5Fsuspend.3.html) See notes below
[alarm(2)](../man2/alarm.2.html)
[bind(2)](../man2/bind.2.html)
[cfgetispeed(3)](../man3/cfgetispeed.3.html)
[cfgetospeed(3)](../man3/cfgetospeed.3.html)
[cfsetispeed(3)](../man3/cfsetispeed.3.html)
[cfsetospeed(3)](../man3/cfsetospeed.3.html)
[chdir(2)](../man2/chdir.2.html)
[chmod(2)](../man2/chmod.2.html)
[chown(2)](../man2/chown.2.html)
[clock_gettime(2)](../man2/clock%5Fgettime.2.html)
[close(2)](../man2/close.2.html)
[connect(2)](../man2/connect.2.html)
[creat(2)](../man2/creat.2.html)
[dup(2)](../man2/dup.2.html)
[dup2(2)](../man2/dup2.2.html)
[execl(3)](../man3/execl.3.html) Added in POSIX.1-2008;
see notes below
[execle(3)](../man3/execle.3.html) See notes below
[execv(3)](../man3/execv.3.html) Added in POSIX.1-2008
[execve(2)](../man2/execve.2.html)
[_exit(2)](../man2/%5Fexit.2.html)
[_Exit(2)](../man2/%5FExit.2.html)
[faccessat(2)](../man2/faccessat.2.html) Added in POSIX.1-2008
[fchdir(2)](../man2/fchdir.2.html) Added in POSIX.1-2008 TC1
[fchmod(2)](../man2/fchmod.2.html)
[fchmodat(2)](../man2/fchmodat.2.html) Added in POSIX.1-2008
[fchown(2)](../man2/fchown.2.html)
[fchownat(2)](../man2/fchownat.2.html) Added in POSIX.1-2008
[fcntl(2)](../man2/fcntl.2.html)
[fdatasync(2)](../man2/fdatasync.2.html)
[fexecve(3)](../man3/fexecve.3.html) Added in POSIX.1-2008
[ffs(3)](../man3/ffs.3.html) Added in POSIX.1-2008 TC2
[fork(2)](../man2/fork.2.html) See notes below
[fstat(2)](../man2/fstat.2.html)
[fstatat(2)](../man2/fstatat.2.html) Added in POSIX.1-2008
[fsync(2)](../man2/fsync.2.html)
[ftruncate(2)](../man2/ftruncate.2.html)
[futimens(3)](../man3/futimens.3.html) Added in POSIX.1-2008
[getegid(2)](../man2/getegid.2.html)
[geteuid(2)](../man2/geteuid.2.html)
[getgid(2)](../man2/getgid.2.html)
[getgroups(2)](../man2/getgroups.2.html)
[getpeername(2)](../man2/getpeername.2.html)
[getpgrp(2)](../man2/getpgrp.2.html)
[getpid(2)](../man2/getpid.2.html)
[getppid(2)](../man2/getppid.2.html)
[getsockname(2)](../man2/getsockname.2.html)
[getsockopt(2)](../man2/getsockopt.2.html)
[getuid(2)](../man2/getuid.2.html)
[htonl(3)](../man3/htonl.3.html) Added in POSIX.1-2008 TC2
[htons(3)](../man3/htons.3.html) Added in POSIX.1-2008 TC2
[kill(2)](../man2/kill.2.html)
[link(2)](../man2/link.2.html)
[linkat(2)](../man2/linkat.2.html) Added in POSIX.1-2008
[listen(2)](../man2/listen.2.html)
[longjmp(3)](../man3/longjmp.3.html) Added in POSIX.1-2008
TC2; see notes below
[lseek(2)](../man2/lseek.2.html)
[lstat(2)](../man2/lstat.2.html)
[memccpy(3)](../man3/memccpy.3.html) Added in POSIX.1-2008 TC2
[memchr(3)](../man3/memchr.3.html) Added in POSIX.1-2008 TC2
[memcmp(3)](../man3/memcmp.3.html) Added in POSIX.1-2008 TC2
[memcpy(3)](../man3/memcpy.3.html) Added in POSIX.1-2008 TC2
[memmove(3)](../man3/memmove.3.html) Added in POSIX.1-2008 TC2
[memset(3)](../man3/memset.3.html) Added in POSIX.1-2008 TC2
[mkdir(2)](../man2/mkdir.2.html)
[mkdirat(2)](../man2/mkdirat.2.html) Added in POSIX.1-2008
[mkfifo(3)](../man3/mkfifo.3.html)
[mkfifoat(3)](../man3/mkfifoat.3.html) Added in POSIX.1-2008
[mknod(2)](../man2/mknod.2.html) Added in POSIX.1-2008
[mknodat(2)](../man2/mknodat.2.html) Added in POSIX.1-2008
[ntohl(3)](../man3/ntohl.3.html) Added in POSIX.1-2008 TC2
[ntohs(3)](../man3/ntohs.3.html) Added in POSIX.1-2008 TC2
[open(2)](../man2/open.2.html)
[openat(2)](../man2/openat.2.html) Added in POSIX.1-2008
[pause(2)](../man2/pause.2.html)
[pipe(2)](../man2/pipe.2.html)
[poll(2)](../man2/poll.2.html)
**posix_trace_event**(3)
[pselect(2)](../man2/pselect.2.html)
[pthread_kill(3)](../man3/pthread%5Fkill.3.html) Added in POSIX.1-2008 TC1
[pthread_self(3)](../man3/pthread%5Fself.3.html) Added in POSIX.1-2008 TC1
[pthread_sigmask(3)](../man3/pthread%5Fsigmask.3.html) Added in POSIX.1-2008 TC1
[raise(3)](../man3/raise.3.html)
[read(2)](../man2/read.2.html)
[readlink(2)](../man2/readlink.2.html)
[readlinkat(2)](../man2/readlinkat.2.html) Added in POSIX.1-2008
[recv(2)](../man2/recv.2.html)
[recvfrom(2)](../man2/recvfrom.2.html)
[recvmsg(2)](../man2/recvmsg.2.html)
[rename(2)](../man2/rename.2.html)
[renameat(2)](../man2/renameat.2.html) Added in POSIX.1-2008
[rmdir(2)](../man2/rmdir.2.html)
[select(2)](../man2/select.2.html)
[sem_post(3)](../man3/sem%5Fpost.3.html)
[send(2)](../man2/send.2.html)
[sendmsg(2)](../man2/sendmsg.2.html)
[sendto(2)](../man2/sendto.2.html)
[setgid(2)](../man2/setgid.2.html)
[setpgid(2)](../man2/setpgid.2.html)
[setsid(2)](../man2/setsid.2.html)
[setsockopt(2)](../man2/setsockopt.2.html)
[setuid(2)](../man2/setuid.2.html)
[shutdown(2)](../man2/shutdown.2.html)
[sigaction(2)](../man2/sigaction.2.html)
[sigaddset(3)](../man3/sigaddset.3.html)
[sigdelset(3)](../man3/sigdelset.3.html)
[sigemptyset(3)](../man3/sigemptyset.3.html)
[sigfillset(3)](../man3/sigfillset.3.html)
[sigismember(3)](../man3/sigismember.3.html)
[siglongjmp(3)](../man3/siglongjmp.3.html) Added in POSIX.1-2008
TC2; see notes below
[signal(2)](../man2/signal.2.html)
[sigpause(3)](../man3/sigpause.3.html)
[sigpending(2)](../man2/sigpending.2.html)
[sigprocmask(2)](../man2/sigprocmask.2.html)
**sigqueue**(2)
[sigset(3)](../man3/sigset.3.html)
[sigsuspend(2)](../man2/sigsuspend.2.html)
[sleep(3)](../man3/sleep.3.html)
[sockatmark(3)](../man3/sockatmark.3.html) Added in POSIX.1-2001 TC2
[socket(2)](../man2/socket.2.html)
[socketpair(2)](../man2/socketpair.2.html)
[stat(2)](../man2/stat.2.html)
[stpcpy(3)](../man3/stpcpy.3.html) Added in POSIX.1-2008 TC2
[stpncpy(3)](../man3/stpncpy.3.html) Added in POSIX.1-2008 TC2
[strcat(3)](../man3/strcat.3.html) Added in POSIX.1-2008 TC2
[strchr(3)](../man3/strchr.3.html) Added in POSIX.1-2008 TC2
[strcmp(3)](../man3/strcmp.3.html) Added in POSIX.1-2008 TC2
[strcpy(3)](../man3/strcpy.3.html) Added in POSIX.1-2008 TC2
[strcspn(3)](../man3/strcspn.3.html) Added in POSIX.1-2008 TC2
[strlen(3)](../man3/strlen.3.html) Added in POSIX.1-2008 TC2
[strncat(3)](../man3/strncat.3.html) Added in POSIX.1-2008 TC2
[strncmp(3)](../man3/strncmp.3.html) Added in POSIX.1-2008 TC2
[strncpy(3)](../man3/strncpy.3.html) Added in POSIX.1-2008 TC2
[strnlen(3)](../man3/strnlen.3.html) Added in POSIX.1-2008 TC2
[strpbrk(3)](../man3/strpbrk.3.html) Added in POSIX.1-2008 TC2
[strrchr(3)](../man3/strrchr.3.html) Added in POSIX.1-2008 TC2
[strspn(3)](../man3/strspn.3.html) Added in POSIX.1-2008 TC2
[strstr(3)](../man3/strstr.3.html) Added in POSIX.1-2008 TC2
[strtok_r(3)](../man3/strtok%5Fr.3.html) Added in POSIX.1-2008 TC2
[symlink(2)](../man2/symlink.2.html)
[symlinkat(2)](../man2/symlinkat.2.html) Added in POSIX.1-2008
[tcdrain(3)](../man3/tcdrain.3.html)
[tcflow(3)](../man3/tcflow.3.html)
[tcflush(3)](../man3/tcflush.3.html)
[tcgetattr(3)](../man3/tcgetattr.3.html)
[tcgetpgrp(3)](../man3/tcgetpgrp.3.html)
[tcsendbreak(3)](../man3/tcsendbreak.3.html)
[tcsetattr(3)](../man3/tcsetattr.3.html)
[tcsetpgrp(3)](../man3/tcsetpgrp.3.html)
[time(2)](../man2/time.2.html)
[timer_getoverrun(2)](../man2/timer%5Fgetoverrun.2.html)
[timer_gettime(2)](../man2/timer%5Fgettime.2.html)
[timer_settime(2)](../man2/timer%5Fsettime.2.html)
[times(2)](../man2/times.2.html)
[umask(2)](../man2/umask.2.html)
[uname(2)](../man2/uname.2.html)
[unlink(2)](../man2/unlink.2.html)
[unlinkat(2)](../man2/unlinkat.2.html) Added in POSIX.1-2008
[utime(2)](../man2/utime.2.html)
[utimensat(2)](../man2/utimensat.2.html) Added in POSIX.1-2008
[utimes(2)](../man2/utimes.2.html) Added in POSIX.1-2008
[wait(2)](../man2/wait.2.html)
[waitpid(2)](../man2/waitpid.2.html)
[wcpcpy(3)](../man3/wcpcpy.3.html) Added in POSIX.1-2008 TC2
[wcpncpy(3)](../man3/wcpncpy.3.html) Added in POSIX.1-2008 TC2
[wcscat(3)](../man3/wcscat.3.html) Added in POSIX.1-2008 TC2
[wcschr(3)](../man3/wcschr.3.html) Added in POSIX.1-2008 TC2
[wcscmp(3)](../man3/wcscmp.3.html) Added in POSIX.1-2008 TC2
[wcscpy(3)](../man3/wcscpy.3.html) Added in POSIX.1-2008 TC2
[wcscspn(3)](../man3/wcscspn.3.html) Added in POSIX.1-2008 TC2
[wcslen(3)](../man3/wcslen.3.html) Added in POSIX.1-2008 TC2
[wcsncat(3)](../man3/wcsncat.3.html) Added in POSIX.1-2008 TC2
[wcsncmp(3)](../man3/wcsncmp.3.html) Added in POSIX.1-2008 TC2
[wcsncpy(3)](../man3/wcsncpy.3.html) Added in POSIX.1-2008 TC2
[wcsnlen(3)](../man3/wcsnlen.3.html) Added in POSIX.1-2008 TC2
[wcspbrk(3)](../man3/wcspbrk.3.html) Added in POSIX.1-2008 TC2
[wcsrchr(3)](../man3/wcsrchr.3.html) Added in POSIX.1-2008 TC2
[wcsspn(3)](../man3/wcsspn.3.html) Added in POSIX.1-2008 TC2
[wcsstr(3)](../man3/wcsstr.3.html) Added in POSIX.1-2008 TC2
[wcstok(3)](../man3/wcstok.3.html) Added in POSIX.1-2008 TC2
[wmemchr(3)](../man3/wmemchr.3.html) Added in POSIX.1-2008 TC2
[wmemcmp(3)](../man3/wmemcmp.3.html) Added in POSIX.1-2008 TC2
[wmemcpy(3)](../man3/wmemcpy.3.html) Added in POSIX.1-2008 TC2
[wmemmove(3)](../man3/wmemmove.3.html) Added in POSIX.1-2008 TC2
[wmemset(3)](../man3/wmemset.3.html) Added in POSIX.1-2008 TC2
[write(2)](../man2/write.2.html)
Notes:
• POSIX.1-2001 and POSIX.1-2001 TC2 required the functions
[fpathconf(3)](../man3/fpathconf.3.html), [pathconf(3)](../man3/pathconf.3.html), and [sysconf(3)](../man3/sysconf.3.html) 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)](../man3/longjmp.3.html)
or [siglongjmp(3)](../man3/siglongjmp.3.html) 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)](../man2/fork.2.html)
from a signal handler and any of the fork handlers registered
by [pthread_atfork(3)](../man3/pthread%5Fatfork.3.html) 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)](../man2/fork.2.html) from the list of async-
signal-safe functions.
• Asynchronous signal handlers that call functions which are
cancelation points and nest over regions of deferred
cancelation may trigger cancelation whose behavior is as if
asynchronous cancelation 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)](../man3/execl.3.html) and [execle(3)](../man3/execle.3.html) employed [realloc(3)](../man3/realloc.3.html)
internally and were consequently not async-signal-safe. This
was fixed in glibc 2.24.
• The glibc implementation of [aio_suspend(3)](../man3/aio%5Fsuspend.3.html) is not async-signal-
safe because it uses [pthread_mutex_lock(3)](../man3/pthread%5Fmutex%5Flock.3.html) internally.SEE ALSO top
[sigaction(2)](../man2/sigaction.2.html), [signal(7)](../man7/signal.7.html), [standards(7)](../man7/standards.7.html)COLOPHON top
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man-pages@man7.orgLinux man-pages 6.10 2024-08-31 signal-safety(7)
Pages that refer to this page:fork(2), sigaction(2), signal(2), pthread_atfork(3), sd_journal_get_fd(3), sd_journal_print(3), sd_journal_stream_fd(3), seccomp_load(3), seccomp_precompute(3), sem_post(3), setjmp(3), strerror(3), attributes(7), signal(7)