NAME | DESCRIPTION | SEE ALSO | COLOPHON

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

NAME         top

       signal-safety - async-signal-safe functions

DESCRIPTION         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) 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         top

       sigaction(2), signal(7), standards(7)

COLOPHON         top

       This page is part of release 4.16 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at
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Linux                            2017-03-13                 SIGNAL-SAFETY(7)

Pages that refer to this page: fork(2)sigaction(2)signal(2)pthread_atfork(3)sem_post(3)setjmp(3)signal(7)