NAME

       sigprocmask, rt_sigprocmask - examine and change blocked signals

SYNOPSIS

       #include <signal.h>

       /* Prototype for the glibc wrapper function */
       int sigprocmask(int how, const sigset_t *set, sigset_t *oldset);

       /* Prototype for the underlying system call */
       int rt_sigprocmask(int how, const kernel_sigset_t *set,
                          kernel_sigset_t *oldset, size_t sigsetsize);

       /* Prototype for the legacy system call (deprecated) */
       int sigprocmask(int how, const old_kernel_sigset_t *set,
                       old_kernel_sigset_t *oldset);

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

       sigprocmask(): _POSIX_C_SOURCE

DESCRIPTION

       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.

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

       SIG_UNBLOCK
              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.

       SIG_SETMASK
              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).

       A set of functions for modifying and inspecting variables of type sigset_t ("signal sets")
       is described in sigsetops(3).

       The  use   of   sigprocmask()   is   unspecified   in   a   multithreaded   process;   see
       pthread_sigmask(3).

RETURN VALUE

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

ERRORS

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

       EINVAL Either the value specified in how was invalid or the kernel does  not  support  the
              size passed in sigsetsize.

CONFORMING TO

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

NOTES

       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.

       Note  that  it is permissible (although not very useful) to specify both set and oldset as
       NULL.

   C library/kernel differences
       The kernel's definition of sigset_t differs in size from that used by the C  library.   In
       this  manual  page, the former is referred to as kernel_sigset_t (it is nevertheless named
       sigset_t in the kernel sources).

       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  (referred  to  as
       old_kernel_sigset_t in this manual page) 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 (referred to as kernel_sigset_t in this manual page).   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 a fixed
       architecture specific value (equal to sizeof(kernel_sigset_t)).

       The  glibc  sigprocmask()  wrapper  function  hides  these  details from us, transparently
       calling rt_sigprocmask() when the kernel provides it.

SEE ALSO

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

COLOPHON

       This  page  is  part of release 4.15 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/.