man-pages/man2/sigaction.2

'\" t
.\" Copyright (c) 1994,1995 Mike Battersby <mib@deakin.edu.au>
.\"           and 2004, 2005 Michael Kerrisk <mtk-manpages@gmx.net>
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.\" Modified, aeb, 960424
.\" Modified Fri Jan 31 17:31:20 1997 by Eric S. Raymond <esr@thyrsus.com>
.\" Modified Thu Nov 26 02:12:45 1998 by aeb - add SIGCHLD stuff.
.\" Modified Sat May  8 17:40:19 1999 by Matthew Wilcox
.\"	add POSIX.1b signals
.\" Modified Sat Dec 29 01:44:52 2001 by Evan Jones <ejones@uwaterloo.ca>
.\"	SA_ONSTACK
.\" Modified 2004-11-11 by Michael Kerrisk <mtk-manpages@gmx.net>
.\"	Added mention of SIGCONT under SA_NOCLDSTOP
.\"	Added SA_NOCLDWAIT
.\" Modified 2004-11-17 by Michael Kerrisk <mtk-manpages@gmx.net>
.\"	Updated discussion for POSIX.1-2001 and SIGCHLD and sa_flags.
.\"	Formatting fixes
.\" 2004-12-09, mtk, added SI_TKILL + other minor changes
.\" 2005-09-15, mtk, split sigpending(), sigprocmask(), sigsuspend()
.\"	out of this page into separate pages.
.\"
.TH SIGACTION 2 2005-09-15 "Linux 2.6.14" "Linux Programmer's Manual"
.SH NAME
sigaction \- examine and change a signal action
.SH SYNOPSIS
.B #include <signal.h>
.sp 
.BI "int sigaction(int " signum ", const struct sigaction *" act ,
.BI "struct sigaction *" oldact );
.SH DESCRIPTION
The
.BR sigaction ()
system call is used to change the action taken by a process on
receipt of a specific signal.
.PP
.I signum
specifies the signal and can be any valid signal except
.B SIGKILL
and
.BR SIGSTOP .
.PP
If
.I act
is non\-null, the new action for signal
.I signum
is installed from
.IR act .
If
.I oldact
is non\-null, the previous action is saved in
.IR oldact .
.PP
The
.I sigaction
structure is defined as something like
.sp
.RS
.nf
struct sigaction {
    void (*sa_handler)(int);
    void (*sa_sigaction)(int, siginfo_t *, void *);
    sigset_t sa_mask;
    int sa_flags;
    void (*sa_restorer)(void);
}
.fi
.RE
.PP
On some architectures a union is involved: do not assign to both
.I sa_handler
and
.IR sa_sigaction .
.PP
The
.I sa_restorer
element is obsolete and should not be used.
POSIX does not specify a
.I sa_restorer
element.
.PP
.I sa_handler
specifies the action to be associated with
.I signum
and may be
.B SIG_DFL
for the default action, 
.B SIG_IGN
to ignore this signal, or a pointer to a signal handling function.
This function receives the signal number as its only argument.
.PP
If
.B SA_SIGINFO
is specified in
.IR sa_flags , 
then
.I sa_sigaction
(instead of
.IR sa_handler )
specifies the signal-handling function for
.IR signum .
This function receives the signal number as its first argument, a
pointer to a
.I siginfo_t
as its second argument and a pointer to a
.I ucontext_t
(cast to void *) as its third argument.
.PP
.I sa_mask
gives a mask of signals which should be blocked during execution of
the signal handler.  In addition, the signal which triggered the handler
will be blocked, unless the
.B SA_NODEFER
flag is used.
.PP
.I sa_flags
specifies a set of flags which modify the behaviour of the signal handling
process. It is formed by the bitwise OR of zero or more of the following:
.RS
.TP
.B SA_NOCLDSTOP
If
.I signum
is
.BR SIGCHLD ,
do not receive notification when child processes stop (i.e., when they
receive one of
.BR SIGSTOP ", " SIGTSTP ", " SIGTTIN
or
.BR SIGTTOU )
or resume (i.e., they receive
.BR SIGCONT )
(see
.BR wait (2)).
.TP
.B SA_NOCLDWAIT
(Linux 2.6 and later)
.\" To be precise: Linux 2.5.60 -- MTK
If
.I signum
is
.BR SIGCHLD ,
do not transform children into zombies when they terminate.
See also
.BR waitpid (2).
.TP
.B SA_RESETHAND
Restore the signal action to the default state once the signal handler
has been called.
.B SA_ONESHOT
is an obsolete, non-standard synonym for this flag.
.TP
.BR SA_ONSTACK
Call the signal handler on an alternate signal stack provided by 
.BR sigaltstack (2).
If an alternate stack is not available, the default stack will be used.
.TP
.B SA_RESTART
Provide behaviour compatible with BSD signal semantics by making certain
system calls restartable across signals.
.TP
.B SA_NODEFER
Do not prevent the signal from being received from within its own signal
handler.
.B SA_NOMASK
is an obsolete, non-standard synonym for this flag.
.TP
.B SA_SIGINFO
The signal handler takes 3 arguments, not one.  In this case,
.I sa_sigaction
should be set instead of
.IR sa_handler .
(The
.I sa_sigaction
field was added in Linux 2.1.86.)
.RE
.PP
The
.I siginfo_t
parameter to
.I sa_sigaction
is a struct with the following elements
.sp
.RS
.nf
.ta 4 13 24
siginfo_t {
	int	si_signo;	/* Signal number */
	int	si_errno;	/* An errno value */
	int	si_code;	/* Signal code */
	pid_t	si_pid;	/* Sending process ID */
	uid_t	si_uid;	/* Real user ID of sending process */
	int	si_status;	/* Exit value or signal */
	clock_t	si_utime;	/* User time consumed */
	clock_t	si_stime;	/* System time consumed */
	sigval_t	si_value;	/* Signal value */
	int	si_int;	/* POSIX.1b signal */
	void *	si_ptr;	/* POSIX.1b signal */
	void *	si_addr;	/* Memory location which caused fault */
	int	si_band;	/* Band event */
	int	si_fd;	/* File descriptor */
}
.fi
.RE

.IR si_signo ", " si_errno " and " si_code
are defined for all signals.
.RI ( si_signo
is unused on Linux.)
The rest of the struct may be a union, so that one should only
read the fields that are meaningful for the given signal.
POSIX.1b signals and
.B SIGCHLD
fill in
.IR si_pid " and " si_uid .
.BR 
.B SIGCHLD
also fills in
.IR si_status ", " si_utime " and " si_stime .
.IR si_int " and " si_ptr
are specified by the sender of the POSIX.1b signal.
.\" See
.\" .BR sigqueue (2)
.\" for more details.
.BR SIGILL , 
.BR SIGFPE ,
.BR SIGSEGV , 
and
.B SIGBUS
fill in
.I si_addr
with the address of the fault.
.B SIGPOLL
fills in
.IR si_band " and " si_fd .

.I si_code
indicates why this signal was sent.  It is a value, not a bitmask.  The
values which are possible for any signal are listed in this table:
.TS
tab(:) allbox;
c s
l l.
\fIsi_code\fR
Value:Signal origin
SI_USER:kill(), sigsend(), or raise()
SI_KERNEL:The kernel
SI_QUEUE:sigqueue()
SI_TIMER:POSIX timer expired
SI_MESGQ:POSIX message queue state changed (since Linux 2.6.6)
SI_ASYNCIO:AIO completed
SI_SIGIO:queued SIGIO
SI_TKILL:tkill() or tgkill() (since Linux 2.4.19)
.\" FIXME? SI_DETHREAD is defined in 2.6.9 sources, but doesn't yet
.\" seem to be implemented.  Maybe it will be later?  Dec 04, MTK
.TE

.TS
tab(:) allbox;
c s
l l.
SIGILL
ILL_ILLOPC:illegal opcode
ILL_ILLOPN:illegal operand
ILL_ILLADR:illegal addressing mode
ILL_ILLTRP:illegal trap
ILL_PRVOPC:privileged opcode
ILL_PRVREG:privileged register
ILL_COPROC:coprocessor error
ILL_BADSTK:internal stack error
.TE

.TS
tab(:) allbox;
c s
l l.
SIGFPE
FPE_INTDIV:integer divide by zero
FPE_INTOVF:integer overflow
FPE_FLTDIV:floating point divide by zero
FPE_FLTOVF:floating point overflow
FPE_FLTUND:floating point underflow
FPE_FLTRES:floating point inexact result
FPE_FLTINV:floating point invalid operation
FPE_FLTSUB:subscript out of range
.TE

.TS
tab(:) allbox;
c s
l l.
SIGSEGV
SEGV_MAPERR:address not mapped to object
SEGV_ACCERR:invalid permissions for mapped object
.TE

.TS
tab(:) allbox;
c s
l l.
SIGBUS
BUS_ADRALN:invalid address alignment
BUS_ADRERR:non-existent physical address
BUS_OBJERR:object specific hardware error
.TE

.TS
tab(:) allbox;
c s
l l.
SIGTRAP
TRAP_BRKPT:process breakpoint
TRAP_TRACE:process trace trap
.TE

.TS
tab(:) allbox;
c s
l l.
SIGCHLD
CLD_EXITED:child has exited
CLD_KILLED:child was killed
CLD_DUMPED:child terminated abnormally
CLD_TRAPPED:traced child has trapped
CLD_STOPPED:child has stopped
CLD_CONTINUED:stopped child has continued (since Linux 2.6.9)
.TE

.TS
tab(:) allbox;
c s
l l.
SIGPOLL
POLL_IN:data input available
POLL_OUT:output buffers available
POLL_MSG:input message available
POLL_ERR:i/o error
POLL_PRI:high priority input available
POLL_HUP:device disconnected
.TE

.SH "RETURN VALUE"
.BR sigaction ()
returns 0 on success and \-1 on error.
.SH ERRORS
.TP
.B EFAULT
.IR act " or " oldact
points to memory which is not a valid part of the process address space.
.TP
.B EINVAL
An invalid signal was specified.  This will also be generated if an attempt
is made to change the action for
.BR SIGKILL " or " SIGSTOP ", "
which cannot be caught or ignored.
.SH NOTES
.PP
According to POSIX, the behaviour of a process is undefined after it
ignores a
.BR SIGFPE ,
.BR SIGILL ,
or
.BR SIGSEGV
signal that was not generated by
.BR kill ()
or
.BR raise ().
Integer division by zero has undefined result.
On some architectures it will generate a
.B SIGFPE
signal.
(Also dividing the most negative integer by \-1 may generate SIGFPE.)
Ignoring this signal might lead to an endless loop.
.PP
POSIX.1-1990 disallowed setting the action for
.B SIGCHLD
to
.BR SIG_IGN .
POSIX.1-2001 allows this possibility, so that ignoring
.B SIGCHLD
can be used to prevent the creation of zombies (see
.BR wait (2)).
Nevertheless, the historical BSD and System V behaviours for ignoring
.B SIGCHLD
differ, so that the only completely portable method of ensuring that
terminated children do not become zombies is to catch the
.B SIGCHLD
signal and perform a
.BR wait (2)
or similar.
.PP
POSIX.1-1990 only specified
.BR SA_NOCLDSTOP .
POSIX.1-2001 added
.BR SA_NOCLDWAIT ,
.BR SA_RESETHAND ,
.BR SA_NODEFER ,
and
.BR SA_SIGINFO .
Use of these latter values in
.I sa_flags
may be less portable in applications intended for older
Unix implementations.
.PP
Support for
.B SA_SIGINFO
was added in Linux 2.2.
.PP
The
.B SA_RESETHAND
flag is compatible with the SVr4 flag of the same name.
.PP
The
.B SA_NODEFER
flag is compatible with the SVr4 flag of the same name under kernels
1.3.9 and newer.  On older kernels the Linux implementation 
allowed the receipt of any signal, not just the one we are installing
(effectively overriding any
.I sa_mask
settings).
.\".PP
.\"The
.\".BR SA_RESETHAND " and " SA_NODEFER
.\"names for SVr4 compatibility are present only in library versions 3.0.9
.\"and greater.
.PP
.BR sigaction ()
can be called with a null second argument to query the current signal
handler. It can also be used to check whether a given signal is valid for
the current machine by calling it with null second and third arguments.
.PP
It is not possible to block 
.BR SIGKILL " or " SIGSTOP
(by specifying them in
.IR sa_mask ).
Attempts to do so are silently ignored.
.PP
See
.BR sigsetops (3)
for details on manipulating signal sets.
.SH BUGS
In kernels up to and including 2.6.13, specifying
.B SA_NODEFER
in
.I sa_flags
preventing not only the delivered signal from being masked during 
execution of the handler, but also the signals specified in
.IR sa_mask .
This bug is was fixed in kernel 2.6.14.
.SH "CONFORMING TO"
POSIX, SVr4.  SVr4 does not document the EINTR condition.
.SH UNDOCUMENTED
Before the introduction of
.B SA_SIGINFO
it was also possible to get some additional information,
namely by using a
.I sa_handler
with second argument of type
.IR "struct sigcontext".
See the relevant kernel sources for details.
This use is obsolete now.
.SH "SEE ALSO"
.BR kill (1),
.BR kill (2),
.BR pause (2),
.BR sigaltstack (2),
.BR signal (2),
.BR sigpending (2),
.BR sigprocmask (2),
.BR sigqueue (2),
.BR sigsuspend (2),
.BR wait (2),
.BR killpg (3),
.BR raise (3),
.BR siginterrupt (3),
.BR sigsetops (3),
.BR sigvec (3),
.BR signal (7)