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Updated discussion for POSIX.1-2001 and SIGCHLD and sa_flags; formatting fixes
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Michael Kerrisk
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man2/sigaction.2
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man2/sigaction.2
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@ -32,10 +32,14 @@
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.\" Modified 2004-11-11 by Michael Kerrisk <mtk-manpages@gmx.net>
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.\" Added mention of SIGCONT under SA_NOCLDSTOP
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.\" Added SA_NOCLDWAIT
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.\" Modified 2004-11-17 by Michael Kerrisk <mtk-manpages@gmx.net>
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.\" Updated discussion for POSIX.1-2001 and SIGCHLD and sa_flags.
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.\" Formatting fixes
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.\"
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.TH SIGACTION 2 2004-11-11 "Linux 2.6.9" "Linux Programmer's Manual"
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.SH NAME
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sigaction, sigprocmask, sigpending, sigsuspend \- POSIX signal handling functions
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sigaction, sigprocmask, sigpending, sigsuspend \-
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POSIX signal handling API
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.SH SYNOPSIS
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.B #include <signal.h>
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.sp 2
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@ -50,7 +54,7 @@ sigaction, sigprocmask, sigpending, sigsuspend \- POSIX signal handling function
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.BI "int sigsuspend(const sigset_t *" mask );
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.SH DESCRIPTION
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The
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.B sigaction
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.BR sigaction ()
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system call is used to change the action taken by a process on
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receipt of a specific signal.
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.PP
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@ -72,7 +76,7 @@ is non\-null, the previous action is saved in
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.IR oldact .
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.PP
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The
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.B sigaction
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.I sigaction
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structure is defined as something like
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.sp
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.RS
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@ -124,9 +128,7 @@ gives a mask of signals which should be blocked during execution of
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the signal handler. In addition, the signal which triggered the handler
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will be blocked, unless the
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.B SA_NODEFER
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or
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.B SA_NOMASK
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flags are used.
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flag is used.
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.PP
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.I sa_flags
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specifies a set of flags which modify the behaviour of the signal handling
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@ -157,7 +159,7 @@ is
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.BR SIGCHLD ,
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do not transform children into zombies when they terminate.
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See also
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.BR wait (2).
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.BR waitpid (2).
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.TP
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.B SA_RESETHAND
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Restore the signal action to the default state once the signal handler
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@ -185,7 +187,9 @@ The signal handler takes 3 arguments, not one. In this case,
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.I sa_sigaction
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should be set instead of
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.IR sa_handler .
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(The sa_sigaction field was added in Linux 2.1.86.)
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(The
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.I sa_sigaction
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field was added in Linux 2.1.86.)
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.RE
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.PP
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The
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@ -220,21 +224,29 @@ siginfo_t {
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are defined for all signals.
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The rest of the struct may be a union, so that one should only
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read the fields that are meaningful for the given signal.
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.BR kill (2),
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POSIX.1b signals and SIGCHLD fill in
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POSIX.1b signals and
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.B SIGCHLD
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fill in
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.IR si_pid " and " si_uid .
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.BR
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SIGCHLD also fills in
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.B SIGCHLD
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also fills in
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.IR si_status ", " si_utime " and " si_stime .
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.IR si_int " and " si_ptr
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are specified by the sender of the POSIX.1b signal.
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.\" See
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.\" .BR sigqueue (2)
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.\" for more details.
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SIGILL, SIGFPE, SIGSEGV and SIGBUS fill in
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.BR SIGILL ,
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.BR SIGFPE ,
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.BR SIGSEGV ,
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and
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.B SIGBUS
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fill in
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.I si_addr
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with the address of the fault.
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SIGPOLL fills in
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.B SIGPOLL
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fills in
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.IR si_band " and " si_fd .
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.I si_code
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@ -341,7 +353,7 @@ POLL_HUP:device disconnected
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.PP
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The
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.B sigprocmask
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.BR sigprocmask ()
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call is used to change the list of currently blocked signals. The
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behaviour of the call is dependent on the value of
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.IR how ,
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@ -370,28 +382,26 @@ is non\-null, the previous value of the signal mask is stored in
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.IR oldset .
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.PP
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The
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.B sigpending
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.BR sigpending ()
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call allows the examination of pending signals (ones which have been
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raised while blocked). The signal mask of pending signals is stored
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in
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.IR set .
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.PP
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The
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.B sigsuspend
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.BR sigsuspend ()
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call temporarily replaces the signal mask for the process with that
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given by
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.I mask
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and then suspends the process until a signal is received.
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.SH "RETURN VALUE"
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The functions
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.BR sigaction ,
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.BR sigprocmask ,
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.BR sigaction (),
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.BR sigprocmask (),
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and
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.B sigpending
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.BR sigpending ()
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return 0 on success and \-1 on error.
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The function
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.B sigsuspend
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.BR sigsuspend ()
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always returns \-1, normally with the error
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.BR EINTR .
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@ -417,22 +427,55 @@ It is not possible to block
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with the sigprocmask call. Attempts to do so will be silently ignored.
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.PP
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According to POSIX, the behaviour of a process is undefined after it
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ignores a SIGFPE, SIGILL, or SIGSEGV signal that was not generated
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by the \fIkill()\fP or the \fIraise()\fP functions.
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ignores a
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.BR SIGFPE ,
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.BR SIGILL ,
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or
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.BR SIGSEGV
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signal that was not generated by
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.BR kill ()
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or
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.BR raise ().
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Integer division by zero has undefined result.
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On some architectures it will generate a SIGFPE signal.
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On some architectures it will generate a
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.B SIGFPE
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signal.
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(Also dividing the most negative integer by \-1 may generate SIGFPE.)
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Ignoring this signal might lead to an endless loop.
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.PP
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POSIX (B.3.3.1.3) disallows setting the action for SIGCHLD to SIG_IGN.
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The BSD and SYSV behaviours differ, causing BSD software
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that sets the action for SIGCHLD to SIG_IGN to fail on Linux.
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POSIX.1-1990 disallowed setting the action for
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.B SIGCHLD
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to
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.BR SIG_IGN .
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POSIX.1-2001 allows this possibility, so that ignoring
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.B SIGCHLD
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can be used to prevent the creation of zombies (see
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.BR wait (2)).
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Nevertheless, the historical BSD and System V behaviours for ignoring
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.B SIGCHLD
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differ, so that the only completely portable method of ensuring that
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terminated children do not become zombies is to catch the
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.B SIGCHLD
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signal and perform a
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.BR wait (2)
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or similar.
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.PP
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The POSIX spec only defines
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POSIX.1-1990 only specified
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.BR SA_NOCLDSTOP .
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Use of other
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POSIX.1-2001 added
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.BR SA_NOCLDWAIT ,
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.BR SA_RESETHAND ,
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.BR SA_NODEFER ,
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and
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.BR SA_SIGINFO .
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Use of these latter values in
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.I sa_flags
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is non\-portable.
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may be less portable in applications intended for older
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Unix implementations.
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.PP
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Support for
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.B SA_SIGINFO
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was added in Linux 2.2.
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.PP
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The
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.B SA_RESETHAND
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@ -447,16 +490,13 @@ allowed the receipt of any signal, not just the one we are installing
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.I sa_mask
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settings).
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.PP
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.\" FIXME What "library" is being referred to in the following sentence?
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The
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.BR SA_RESETHAND " and " SA_NODEFER
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names for SVr4 compatibility are present only in library versions 3.0.9
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and greater.
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.PP
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The
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.B SA_SIGINFO
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flag is specified by POSIX.1b. Support for it was added in Linux 2.2.
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.PP
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.B sigaction
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.BR sigaction ()
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can be called with a null second argument to query the current signal
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handler. It can also be used to check whether a given signal is valid for
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the current machine by calling it with null second and third arguments.
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@ -471,7 +511,9 @@ POSIX, SVr4. SVr4 does not document the EINTR condition.
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Before the introduction of
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.B SA_SIGINFO
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it was also possible to get some additional information,
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namely by using a sa_handler with second argument of type
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namely by using a
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.I sa_handler
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with second argument of type
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.IR "struct sigcontext".
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See the relevant kernel sources for details.
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This use is obsolete now.
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@ -484,6 +526,7 @@ This use is obsolete now.
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.BR signal (2),
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.BR sigqueue (2),
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.BR sigvec (2),
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.BR wait (2),
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.BR killpg (3),
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.BR raise (3),
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.BR siginterrupt (3),
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