SIGPROCMASK(2) Linux Programmer's Manual SIGPROCMASK(2)
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(7)):
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
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 set argument.
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.
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
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
sigprocmask() returns 0 on success and -1 on error. In the event of an
error, errno is set to indicate the cause.
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.
It is not possible to block SIGKILL or SIGSTOP. Attempts to do so are
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.
kill(2), pause(2), sigaction(2), signal(2), sigpending(2), sigsus‐
pend(2), pthread_sigmask(3), sigqueue(3), sigsetops(3), signal(7)
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Linux 2017-09-15 SIGPROCMASK(2)