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 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.

   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 the value 8  (
   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 4.09 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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