SIGNAL-SAFETY(7)					     Linux Programmer's Manual						  SIGNAL-SAFETY(7)

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  incon-
       sistent 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 implementa-
       tions 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 execu-
       tion 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)

Linux								    2017-03-13							  SIGNAL-SAFETY(7)