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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | STANDARDS | HISTORY | NOTES | SEE ALSO | COLOPHON |
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sigprocmask(2) System Calls Manual sigprocmask(2)
sigprocmask, rt_sigprocmask - examine and change blocked signals
Standard C library (libc, -lc)
#include <signal.h>
/* Prototype for the glibc wrapper function */
int sigprocmask(int how, const sigset_t *_Nullable restrict set,
sigset_t *_Nullable restrict oldset);
#include <signal.h> /* Definition of SIG_* constants */
#include <sys/syscall.h> /* Definition of SYS_* constants */
#include <unistd.h>
/* Prototype for the underlying system call */
int syscall(SYS_rt_sigprocmask, int how,
const kernel_sigset_t *_Nullable set,
kernel_sigset_t *_Nullable oldset,
size_t sigsetsize);
/* Prototype for the legacy system call */
[[deprecated]] int syscall(SYS_sigprocmask, int how,
const old_kernel_sigset_t *_Nullable set,
old_kernel_sigset_t *_Nullable oldset);
Feature Test Macro Requirements for glibc (see
feature_test_macros(7)):
sigprocmask():
_POSIX_C_SOURCE
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).
sigprocmask() returns 0 on success. On failure, -1 is returned
and errno is set to indicate the error.
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.
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 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.
POSIX.1-2008.
POSIX.1-2001.
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.
kill(2), pause(2), sigaction(2), signal(2), sigpending(2),
sigsuspend(2), pthread_sigmask(3), sigqueue(3), sigsetops(3),
signal(7)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
man-pages@man7.org
Linux man-pages 6.15 2025-05-17 sigprocmask(2)
Pages that refer to this page: env(1), clone(2), io_uring_enter2(2), io_uring_enter(2), poll(2), ptrace(2), rt_sigqueueinfo(2), seccomp(2), select(2), select_tut(2), sgetmask(2), sigaction(2), signal(2), signalfd(2), sigpending(2), sigsuspend(2), sigwaitinfo(2), syscalls(2), getcontext(3), makecontext(3), posix_spawn(3), pthread_attr_setsigmask_np(3), pthread_sigmask(3), sd_event_add_child(3), sd_event_add_signal(3), sigpause(3), sigset(3), sigsetops(3), sigvec(3), sigwait(3), system(3), systemd.exec(5), nptl(7), signal(7), signal-safety(7), system_data_types(7)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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