signal-safety

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 under-
       stood  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  main-
       tain  a statically allocated data buffer along with associated counters
       and indexes (or pointers) that record the amount of data and  the  cur-
       rent  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 inconsis-
       tent 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 complex-
       ity, 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 gen-
       eral, 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  func-
          tions.

   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) inter-
          nally  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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       https://www.kernel.org/doc/man-pages/.

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