.\" Copyright (c) 2005 Michael Kerrisk
.\" based on earlier work by faith@cs.unc.edu and
.\" Mike Battersby <mib@deakin.edu.au>
.\"
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.\" 2005-09-15, mtk, Created new page by splitting off from sigaction.2
.\"
.TH SIGPROCMASK 2 2016-03-15 "Linux" "Linux Programmer's Manual"
.SH NAME
sigprocmask, rt_sigprocmask \- examine and change blocked signals
.SH SYNOPSIS
.B #include <signal.h>
.sp
.BI "int sigprocmask(int " how ", const sigset_t *" set ,
.BI "sigset_t *" oldset );
.sp
.in -4n
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.in
.sp
.ad l
.BR sigprocmask ():
_POSIX_C_SOURCE
.ad b
.SH DESCRIPTION
.BR 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
.BR signal (7)
for more details).

The behavior of the call is dependent on the value of
.IR how ,
as follows.
.TP
.B SIG_BLOCK
The set of blocked signals is the union of the current set and the
.I set
argument.
.TP
.B SIG_UNBLOCK
The signals in
.I set
are removed from the current set of blocked signals.
It is permissible to attempt to unblock a signal which is not blocked.
.TP
.B SIG_SETMASK
The set of blocked signals is set to the argument
.IR set .
.PP
If
.I oldset
is non-NULL, the previous value of the signal mask is stored in
.IR oldset .

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

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

The use of
.BR sigprocmask ()
is unspecified in a multithreaded process; see
.BR pthread_sigmask (3).
.SH RETURN VALUE
.BR sigprocmask ()
returns 0 on success and \-1 on error.
In the event of an error,
.I errno
is set to indicate the cause.
.SH ERRORS
.TP
.B EFAULT
The
.I set
or
.I oldset
argument points outside the process's allocated address space.
.TP
.B EINVAL
The value specified in
.I how
was invalid.
.SH CONFORMING TO
POSIX.1-2001, POSIX.1-2008.
.SH NOTES
It is not possible to block
.BR 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
.BR fork (2)
inherits a copy of its parent's signal mask;
the signal mask is preserved across
.BR execve (2).

If
.BR SIGBUS ,
.BR SIGFPE ,
.BR SIGILL ,
or
.B SIGSEGV
are generated
while they are blocked, the result is undefined,
unless the signal was generated by
.BR kill (2),
.BR sigqueue (3),
or
.BR raise (3).
.PP
See
.BR sigsetops (3)
for details on manipulating signal sets.
.\"
.SS C library/kernel differences
The glibc wrapper function for
.BR sigprocmask ()
silently ignores attempts to block the two real-time signals that
are used internally by the NPTL threading implementation.
See
.BR nptl (7)
for details.

The original Linux system call was named
.BR sigprocmask ().
However, with the addition of real-time signals in Linux 2.2,
the fixed-size, 32-bit
.IR sigset_t
type supported by that system call was no longer fit for purpose.
Consequently, a new system call,
.BR rt_sigprocmask (),
was added to support an enlarged
.IR sigset_t
type.
The new system call takes a fourth argument,
.IR "size_t sigsetsize" ,
which specifies the size in bytes of the signal sets in
.IR set
and
.IR oldset .
This argument is currently required to have the value
.IR sizeof(sigset_t)
(or the error
.B EINVAL
results).
The glibc
.BR sigprocmask ()
wrapper function hides these details from us, transparently calling
.BR rt_sigprocmask ()
when the kernel provides it.
.\"
.SH SEE ALSO
.BR kill (2),
.BR pause (2),
.BR sigaction (2),
.BR signal (2),
.BR sigpending (2),
.BR sigsuspend (2),
.BR pthread_sigmask (3),
.BR sigqueue (3),
.BR sigsetops (3),
.BR signal (7)