man-pages/man2/sigaction.2

'\" t
.\" Copyright (c) 1994,1995 Mike Battersby <mib@deakin.edu.au>
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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
.\"
.TH SIGACTION 2 2001-12-29 "Linux 2.4" "Linux Programmer's Manual"
.SH NAME
sigaction, sigprocmask, sigpending, sigsuspend \- POSIX signal handling functions
.SH SYNOPSIS
.B #include <signal.h>
.sp 2
.BI "int sigaction(int " signum ", const struct sigaction *" act ,
.BI "struct sigaction *" oldact );
.sp
.BI "int sigprocmask(int " how ", const sigset_t *" set ,
.BI "sigset_t *" oldset );
.sp
.BI "int sigpending(sigset_t *" set );
.sp
.BI "int sigsuspend(const sigset_t *" mask );
.SH DESCRIPTION
The
.B 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
.B 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element is obsolete and should not be used.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
.I sa_sigaction
also specifies the action to be associated with
.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
or
.B SA_NOMASK
flags are 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 child
processes receive one of
.BR SIGSTOP ", " SIGTSTP ", " SIGTTIN
or
.BR SIGTTOU ")."
.TP
.BR SA_ONESHOT " or " SA_RESETHAND
Restore the signal action to the default state once the signal handler
has been called.
.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
.BR SA_NOMASK " or " SA_NODEFER
Do not prevent the signal from being received from within its own signal
handler.
.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 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.
The rest of the struct may be a union, so that one should only
read the fields that are meaningful for the given signal.
.BR kill (2),
POSIX.1b signals and SIGCHLD fill in
.IR si_pid " and " si_uid .
.BR 
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.
SIGILL, SIGFPE, SIGSEGV and SIGBUS fill in
.I si_addr
with the address of the fault.
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:timer expired
SI_MESGQ:mesq state changed
SI_ASYNCIO:AIO completed
SI_SIGIO:queued SIGIO
.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
.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

.PP
The
.B sigprocmask
call is used to change the list of currently blocked signals. The 
behaviour of the call is dependent on the value of
.IR how ,
as follows.
.RS
.TP
.B SIG_BLOCK
The set of blocked signals is the union of the current set and the
.I set
argument.
.TP
.B SIG_UNBLOCK
The signals in
.I set
are removed from the current set of blocked signals.  It is legal to
attempt to unblock a signal which is not blocked.
.TP
.B SIG_SETMASK
The set of blocked signals is set to the argument
.IR set .
.RE
.PP
If
.I oldset
is non\-null, the previous value of the signal mask is stored in
.IR oldset .
.PP
The
.B sigpending
call allows the examination of pending signals (ones which have been
raised while blocked).  The signal mask of pending signals is stored
in
.IR set .
.PP
The
.B sigsuspend
call temporarily replaces the signal mask for the process with that
given by 
.I mask
and then suspends the process until a signal is received.

.SH "RETURN VALUE"
The functions
.BR sigaction ,
.BR sigprocmask ,
and
.B sigpending
return 0 on success and \-1 on error.
The function
.B sigsuspend
always returns \-1, normally with the error
.BR EINTR .

.SH ERRORS
.TP
.B EFAULT
.IR act ", " oldact ", " set ", " oldset
or 
.I mask
point to memory which is not a valid part of the process address space.
.TP
.B EINTR
System call was interrupted.
.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.
.SH NOTES
It is not possible to block 
.BR SIGKILL " or " SIGSTOP
with the sigprocmask call.  Attempts to do so will be silently ignored.
.PP
According to POSIX, the behaviour of a process is undefined after it
ignores a SIGFPE, SIGILL, or SIGSEGV signal that was not generated
by the \fIkill()\fP or the \fIraise()\fP functions.
Integer division by zero has undefined result.
On some architectures it will generate a SIGFPE signal.
(Also dividing the most negative integer by \-1 may generate SIGFPE.)
Ignoring this signal might lead to an endless loop.
.PP
POSIX (B.3.3.1.3) disallows setting the action for SIGCHLD to SIG_IGN.
The BSD and SYSV behaviours differ, causing BSD software
that sets the action for SIGCHLD to SIG_IGN to fail on Linux.
.PP
The POSIX spec only defines
.BR SA_NOCLDSTOP .
Use of other
.I sa_flags
is non\-portable.
.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
The
.B SA_SIGINFO
flag is specified by POSIX.1b.  Support for it was added in Linux 2.2.
.PP
.B 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
See
.BR sigsetops (3)
for details on manipulating signal sets.
.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 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 killpg (2),
.BR pause (2),
.BR sigaltstack (2),
.BR signal (2),
.BR sigvec (2),
.BR raise (3),
.BR siginterrupt (3),
.BR sigsetops (3),
.BR signal (7)