signal-safety(7) — Linux manual page

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.1-2001 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.

SEE ALSO         top

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

COLOPHON         top

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Linux                          2020-12-21               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)sem_post(3)setjmp(3)attributes(7)signal(7)