sigprocmask(2)



SIGPROCMASK(2)             Linux Programmer's Manual            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 allo-
              cated 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 ker-
       nel_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.  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  spe-
       cific 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),  sigsus-
       pend(2), pthread_sigmask(3), sigqueue(3), sigsetops(3), signal(7)

COLOPHON
       This  page  is  part of release 4.11 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                             2017-05-03                    SIGPROCMASK(2)

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