SIGPROCMASK(2)            (2017-09-15)             SIGPROCMASK(2)

     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 sig-
          nals 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).

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          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().

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          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.   Conse-
          quently,  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 5.10  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/.

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