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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() 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 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 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.


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.


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.






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)

2023-03-30 Linux man-pages 6.05.01