sigprocmask, rt_sigprocmask - examine and change blocked signals
/* 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,
Feature Test Macro Requirements for glibc (see feature_test_macros
() 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.
- The set of blocked signals is the union of the current set and the
- 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
- The set of blocked signals is set to the argument set.
is non-NULL, the previous value of the signal mask is stored in
is NULL, then the signal mask is unchanged (i.e., how
ignored), but the current value of the signal mask is nevertheless returned in
(if it is not NULL).
A set of functions for modifying and inspecting variables of type
("signal sets") is described in sigsetops
The use of sigprocmask
() is unspecified in a multithreaded process; see
() returns 0 on success and -1 on error. In the event of an
is set to indicate the cause.
- The set or oldset argument points outside the process's
allocated address space.
- Either the value specified in how was invalid or the kernel does
not support the size passed in sigsetsize.
It is not possible to block SIGKILL
. 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
, or SIGSEGV
while they are blocked, the result is undefined, unless the signal was
generated by kill
(3), or raise
(3) for details on manipulating signal sets.
Note that it is permissible (although not very useful) to specify both
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
(it is nevertheless named sigset_t
in the kernel
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
(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
this manual page). The new system call takes a fourth argument, size_t
, which specifies the size in bytes of the signal sets in
. 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.