sigprocmask(2)

1SIGPROCMASK(2)             Linux Programmer's Manual            SIGPROCMASK(2)
2
3
4

NAME

6       sigprocmask, rt_sigprocmask - examine and change blocked signals
7

SYNOPSIS

9       #include <signal.h>
10
11       /* Prototype for the glibc wrapper function */
12       int sigprocmask(int how, const sigset_t *restrict set,
13                       sigset_t *restrict oldset);
14
15       #include <signal.h>           /* Definition of SIG_* constants */
16       #include <sys/syscall.h>      /* Definition of SYS_* constants */
17       #include <unistd.h>
18
19       /* Prototype for the underlying system call */
20       int syscall(SYS_rt_sigprocmask, int how, const kernel_sigset_t *set,
21                       kernel_sigset_t *oldset, size_t sigsetsize);
22
23       /* Prototype for the legacy system call (deprecated) */
24       int syscall(SYS_sigprocmask, int how, const old_kernel_sigset_t *set,
25                       old_kernel_sigset_t *oldset);
26
27   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
28
29       sigprocmask():
30           _POSIX_C_SOURCE
31

DESCRIPTION

33       sigprocmask()  is  used  to  fetch and/or change the signal mask of the
34       calling thread.  The signal mask is the set of signals  whose  delivery
35       is  currently  blocked  for the caller (see also signal(7) for more de‐
36       tails).
37
38       The behavior of the call is dependent on the value of how, as follows.
39
40       SIG_BLOCK
41              The set of blocked signals is the union of the current  set  and
42              the set argument.
43
44       SIG_UNBLOCK
45              The  signals  in set are removed from the current set of blocked
46              signals.  It is permissible to attempt to unblock a signal which
47              is not blocked.
48
49       SIG_SETMASK
50              The set of blocked signals is set to the argument set.
51
52       If  oldset is non-NULL, the previous value of the signal mask is stored
53       in oldset.
54
55       If set is NULL, then the signal mask is unchanged  (i.e.,  how  is  ig‐
56       nored),  but  the  current value of the signal mask is nevertheless re‐
57       turned in oldset (if it is not NULL).
58
59       A set of functions for  modifying  and  inspecting  variables  of  type
60       sigset_t ("signal sets") is described in sigsetops(3).
61
62       The use of sigprocmask() is unspecified in a multithreaded process; see
63       pthread_sigmask(3).
64

RETURN VALUE

66       sigprocmask() returns 0 on success.  On failure, -1 is returned and er‐
67       rno is set to indicate the error.
68

ERRORS

70       EFAULT The  set  or  oldset argument points outside the process's allo‐
71              cated address space.
72
73       EINVAL Either the value specified in how was invalid or the kernel does
74              not support the size passed in sigsetsize.
75

CONFORMING TO

77       POSIX.1-2001, POSIX.1-2008.
78

NOTES

80       It  is not possible to block SIGKILL or SIGSTOP.  Attempts to do so are
81       silently ignored.
82
83       Each of the threads in a process has its own signal mask.
84
85       A child created via fork(2) inherits a  copy  of  its  parent's  signal
86       mask; the signal mask is preserved across execve(2).
87
88       If  SIGBUS,  SIGFPE,  SIGILL,  or  SIGSEGV are generated while they are
89       blocked, the result is undefined, unless the signal  was  generated  by
90       kill(2), sigqueue(3), or raise(3).
91
92       See sigsetops(3) for details on manipulating signal sets.
93
94       Note  that it is permissible (although not very useful) to specify both
95       set and oldset as NULL.
96
97   C library/kernel differences
98       The kernel's definition of sigset_t differs in size from that  used  by
99       the  C library.  In this manual page, the former is referred to as ker‐
100       nel_sigset_t (it is nevertheless named sigset_t in the kernel sources).
101
102       The glibc wrapper function for sigprocmask() silently ignores  attempts
103       to block the two real-time signals that are used internally by the NPTL
104       threading implementation.  See nptl(7) for details.
105
106       The original Linux system call was named sigprocmask().  However,  with
107       the  addition of real-time signals in Linux 2.2, the fixed-size, 32-bit
108       sigset_t (referred to as old_kernel_sigset_t in this manual page)  type
109       supported  by  that  system call was no longer fit for purpose.  Conse‐
110       quently, a new system call, rt_sigprocmask(), was added to  support  an
111       enlarged  sigset_t  type (referred to as kernel_sigset_t in this manual
112       page).  The new system call takes a fourth argument, size_t sigsetsize,
113       which specifies the size in bytes of the signal sets in set and oldset.
114       This argument is currently required to have a fixed  architecture  spe‐
115       cific value (equal to sizeof(kernel_sigset_t)).
116
117       The  glibc  sigprocmask() wrapper function hides these details from us,
118       transparently calling rt_sigprocmask() when the kernel provides it.
119

COLOPHON

125       This  page  is  part of release 5.13 of the Linux man-pages project.  A
126       description of the project, information about reporting bugs,  and  the
127       latest     version     of     this    page,    can    be    found    at
128       https://www.kernel.org/doc/man-pages/.
129
130
131
132Linux                             2021-03-22                    SIGPROCMASK(2)