sigprocmask(2) — Linux manual page


sigprocmask(2)             System Calls Manual            sigprocmask(2)

NAME         top

       sigprocmask, rt_sigprocmask - examine and change blocked signals

LIBRARY         top

       Standard C library (libc, -lc)

SYNOPSIS         top

       #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


DESCRIPTION         top

       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

              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).

RETURN VALUE         top

       sigprocmask() returns 0 on success.  On failure, -1 is returned
       and errno is set to indicate the error.

ERRORS         top

       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.

VERSIONS         top

   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

       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.

STANDARDS         top


HISTORY         top


NOTES         top

       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.

SEE ALSO         top

       kill(2), pause(2), sigaction(2), signal(2), sigpending(2),
       sigsuspend(2), pthread_sigmask(3), sigqueue(3), sigsetops(3),

Linux man-pages (unreleased)     (date)                   sigprocmask(2)

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