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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 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

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

COLOPHON

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