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       sigprocmask, rt_sigprocmask - examine and change blocked signals


       #include <signal.h>

       int sigprocmask(int how, const sigset_t *set, sigset_t *oldset);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       sigprocmask(): _POSIX_C_SOURCE >= 1 || _XOPEN_SOURCE || _POSIX_SOURCE


       sigprocmask()  is  used to fetch and/or change the signal mask of the calling thread.  The
       signal mask is the set of signals whose delivery is currently blocked for the caller  (see
       also signal(7) for more details).

       The behavior of the call is dependent on the value of how, as follows.

              The set of blocked signals is the union of the current set and the set argument.

              The  signals  in  set  are  removed from the current set of blocked signals.  It is
              permissible to attempt to unblock a signal which is not blocked.

              The set of blocked signals is set to the argument set.

       If oldset is non-NULL, the previous value of the signal mask is stored in oldset.

       If set is NULL, then the signal mask is unchanged (i.e., how is ignored), but the  current
       value of the signal mask is nevertheless returned in oldset (if it is not NULL).

       The   use   of   sigprocmask()   is   unspecified   in   a   multithreaded   process;  see


       sigprocmask() returns 0 on success and -1 on error.  In the event of an  error,  errno  is
       set to indicate the cause.


       EFAULT The set or oldset argument points outside the process's allocated address space.

       EINVAL The value specified in how was invalid.


       POSIX.1-2001, POSIX.1-2008.


       It is not possible to block SIGKILL or SIGSTOP.  Attempts to do so are silently ignored.

       Each of the threads in a process has its own signal mask.

       A  child  created via fork(2) inherits a copy of its parent's signal mask; the signal mask
       is preserved across execve(2).

       If SIGBUS, SIGFPE, SIGILL, or SIGSEGV are generated while they are blocked, the result  is
       undefined, unless the signal was generated by kill(2), sigqueue(3), or raise(3).

       See sigsetops(3) for details on manipulating signal sets.

   C library/kernel differences
       The  glibc  wrapper  function for sigprocmask() silently ignores attempts to block the two
       real-time signals that are used internally by  the  NPTL  threading  implementation.   See
       nptl(7) for details.

       The  original  Linux  system  call was named sigprocmask().  However, with the addition of
       real-time signals in Linux 2.2, the fixed-size, 32-bit sigset_t  type  supported  by  that
       system   call   was  no  longer  fit  for  purpose.   Consequently,  a  new  system  call,
       rt_sigprocmask(), was added to support an enlarged sigset_t type.   The  new  system  call
       takes  a  fourth  argument,  size_t  sigsetsize,  which specifies the size in bytes of the
       signal sets in set and oldset.  This argument is currently  required  to  have  the  value
       sizeof(sigset_t)  (or the error EINVAL results).  The glibc sigprocmask() wrapper function
       hides these details from  us,  transparently  calling  rt_sigprocmask()  when  the  kernel
       provides it.


       kill(2),     pause(2),     sigaction(2),    signal(2),    sigpending(2),    sigsuspend(2),
       pthread_sigmask(3), sigqueue(3), sigsetops(3), signal(7)


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