signal-safety(7)

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 mo-
       ment, 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  re-
          spect 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  general, a function is async-signal-safe either because it is reen-
       trant 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.1-2001 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  fpath-
          conf(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  be-
          havior 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 func-
          tions.

       *  Asynchronous signal handlers that call functions which are cancella-
          tion points and nest over regions of deferred cancellation may trig-
          ger cancellation whose behavior is as if  asynchronous  cancellation
          had occurred and may cause application state to become inconsistent.

   Deviations in the GNU C library
       The following known deviations from the standard occur in the GNU C li-
       brary:

       *  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
       This  page  is  part of release 5.07 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
       https://www.kernel.org/doc/man-pages/.

Linux                             2020-06-09                  SIGNAL-SAFETY(7)