man2/sigprocmask.2

   1 .\" Copyright (c) 2005 Michael Kerrisk
   2 .\" based on earlier work by faith@cs.unc.edu and
   3 .\" Mike Battersby <mib@deakin.edu.au>
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  27 .\" 2005-09-15, mtk, Created new page by splitting off from sigaction.2
  28 .\"
  29 .TH SIGPROCMASK 2 2017-09-15 "Linux" "Linux Programmer's Manual"
  30 .SH NAME
  31 sigprocmask, rt_sigprocmask \- examine and change blocked signals
  32 .SH SYNOPSIS
  33 .B #include <signal.h>
  34 .PP
  35 .nf
  36 /* Prototype for the glibc wrapper function */
  37 .BI "int sigprocmask(int " how ", const sigset_t *" set ", sigset_t *" oldset );
  38 .PP
  39 /* Prototype for the underlying system call */
  40 .BI "int rt_sigprocmask(int " how ", const kernel_sigset_t *" set ,
  41 .BI "                   kernel_sigset_t *" oldset ", size_t " sigsetsize );
  42 .PP
  43 /* Prototype for the legacy system call (deprecated) */
  44 .BI "int sigprocmask(int " how ", const old_kernel_sigset_t *" set ,
  45 .BI "                old_kernel_sigset_t *" oldset ); "
  46 .fi
  47 .PP
  48 .in -4n
  49 Feature Test Macro Requirements for glibc (see
  50 .BR feature_test_macros (7)):
  51 .in
  52 .PP
  53 .ad l
  54 .BR sigprocmask ():
  55 _POSIX_C_SOURCE
  56 .ad b
  57 .SH DESCRIPTION
  58 .BR sigprocmask ()
  59 is used to fetch and/or change the signal mask of the calling thread.
  60 The signal mask is the set of signals whose delivery is currently
  61 blocked for the caller
  62 (see also
  63 .BR signal (7)
  64 for more details).
  65 .PP
  66 The behavior of the call is dependent on the value of
  67 .IR how ,
  68 as follows.
  69 .TP
  70 .B SIG_BLOCK
  71 The set of blocked signals is the union of the current set and the
  72 .I set
  73 argument.
  74 .TP
  75 .B SIG_UNBLOCK
  76 The signals in
  77 .I set
  78 are removed from the current set of blocked signals.
  79 It is permissible to attempt to unblock a signal which is not blocked.
  80 .TP
  81 .B SIG_SETMASK
  82 The set of blocked signals is set to the argument
  83 .IR set .
  84 .PP
  85 If
  86 .I oldset
  87 is non-NULL, the previous value of the signal mask is stored in
  88 .IR oldset .
  89 .PP
  90 If
  91 .I set
  92 is NULL, then the signal mask is unchanged (i.e.,
  93 .I how
  94 is ignored),
  95 but the current value of the signal mask is nevertheless returned in
  96 .I oldset
  97 (if it is not NULL).
  98 .PP
  99 A set of functions for modifying and inspecting variables of type
 100 .I sigset_t
 101 ("signal sets") is described in
 102 .BR sigsetops (3).
 103 .PP
 104 The use of
 105 .BR sigprocmask ()
 106 is unspecified in a multithreaded process; see
 107 .BR pthread_sigmask (3).
 108 .SH RETURN VALUE
 109 .BR sigprocmask ()
 110 returns 0 on success and \-1 on error.
 111 In the event of an error,
 112 .I errno
 113 is set to indicate the cause.
 114 .SH ERRORS
 115 .TP
 116 .B EFAULT
 117 The
 118 .I set
 119 or
 120 .I oldset
 121 argument points outside the process's allocated address space.
 122 .TP
 123 .B EINVAL
 124 Either the value specified in
 125 .I how
 126 was invalid or the kernel does not support the size passed in
 127 .I sigsetsize.
 128 .SH CONFORMING TO
 129 POSIX.1-2001, POSIX.1-2008.
 130 .SH NOTES
 131 It is not possible to block
 132 .BR SIGKILL " or " SIGSTOP .
 133 Attempts to do so are silently ignored.
 134 .PP
 135 Each of the threads in a process has its own signal mask.
 136 .PP
 137 A child created via
 138 .BR fork (2)
 139 inherits a copy of its parent's signal mask;
 140 the signal mask is preserved across
 141 .BR execve (2).
 142 .PP
 143 If
 144 .BR SIGBUS ,
 145 .BR SIGFPE ,
 146 .BR SIGILL ,
 147 or
 148 .B SIGSEGV
 149 are generated
 150 while they are blocked, the result is undefined,
 151 unless the signal was generated by
 152 .BR kill (2),
 153 .BR sigqueue (3),
 154 or
 155 .BR raise (3).
 156 .PP
 157 See
 158 .BR sigsetops (3)
 159 for details on manipulating signal sets.
 160 .PP
 161 Note that it is permissible (although not very useful) to specify both
 162 .I set
 163 and
 164 .I oldset
 165 as NULL.
 166 .\"
 167 .SS C library/kernel differences
 168 .PP
 169 The kernel's definition of
 170 .IR sigset_t
 171 differs in size from that used
 172 by the C library.
 173 In this manual page, the former is referred to as
 174 .I kernel_sigset_t
 175 (it is nevertheless named
 176 .I sigset_t
 177 in the kernel sources).
 178 .PP
 179 The glibc wrapper function for
 180 .BR sigprocmask ()
 181 silently ignores attempts to block the two real-time signals that
 182 are used internally by the NPTL threading implementation.
 183 See
 184 .BR nptl (7)
 185 for details.
 186 .PP
 187 The original Linux system call was named
 188 .BR sigprocmask ().
 189 However, with the addition of real-time signals in Linux 2.2,
 190 the fixed-size, 32-bit
 191 .IR sigset_t
 192 (referred to as
 193 .IR old_kernel_sigset_t
 194 in this manual page)
 195 type supported by that system call was no longer fit for purpose.
 196 Consequently, a new system call,
 197 .BR rt_sigprocmask (),
 198 was added to support an enlarged
 199 .IR sigset_t
 200 type
 201 (referred to as
 202 .IR kernel_sigset_t
 203 in this manual page).
 204 The new system call takes a fourth argument,
 205 .IR "size_t sigsetsize" ,
 206 which specifies the size in bytes of the signal sets in
 207 .IR set
 208 and
 209 .IR oldset .
 210 This argument is currently required to have a fixed architecture specific value
 211 (equal to
 212 .IR sizeof(kernel_sigset_t) ).
 213 .\" sizeof(kernel_sigset_t) == _NSIG / 8,
 214 .\" which equals to 8 on most architectures, but e.g. on MIPS it's 16.
 215 .PP
 216 The glibc
 217 .BR sigprocmask ()
 218 wrapper function hides these details from us, transparently calling
 219 .BR rt_sigprocmask ()
 220 when the kernel provides it.
 221 .\"
 222 .SH SEE ALSO
 223 .BR kill (2),
 224 .BR pause (2),
 225 .BR sigaction (2),
 226 .BR signal (2),
 227 .BR sigpending (2),
 228 .BR sigsuspend (2),
 229 .BR pthread_sigmask (3),
 230 .BR sigqueue (3),
 231 .BR sigsetops (3),
 232 .BR signal (7)