man-pages/man2/sigaction.2

'\" t
.\" Copyright (c) 1994,1995 Mike Battersby <mib@deakin.edu.au>
.\" and Copyright 2004, 2005 Michael Kerrisk <mtk.manpages@gmail.com>
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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@gmail.com>
.\"	Added mention of SIGCONT under SA_NOCLDSTOP
.\"	Added SA_NOCLDWAIT
.\" Modified 2004-11-17 by Michael Kerrisk <mtk.manpages@gmail.com>
.\"	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 2005-07-08 "Linux" "Linux Programmer's Manual"
.SH NAME
sigaction \- examine and change a signal action
.SH SYNOPSIS
.nf
.B #include <signal.h>
.sp
.BI "int sigaction(int " signum ", const struct sigaction *" act ,
.BI "              struct sigaction *" oldact );
.fi
.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element is obsolete and should not be used.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 behavior 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
.B 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 behavior 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
siginfo_t {
.\" FIXME si_tid and si_overrun are not documented.
.\" FIXME si_trapno is not documented; is it actually used?
    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_errno
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:
.IP * 2
POSIX.1b signals and
.B SIGCHLD
fill in
.IR si_pid " and " si_uid .
.BR
.IP *
.B SIGCHLD
also fills in
.IR si_status ", " si_utime " and " si_stime .
.IP *
.IR si_int " and " si_ptr
are specified by the sender of the POSIX.1b signal.
See
.BR sigqueue (2)
for more details.
.IP *
.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 .
.PP
.I si_code
is a value (not a bit mask)
indicating why this signal was sent.
The following list shows the values can be placed in
.I si_code
for any signal, along with reason that the signal was generated.
.RS 4
.TP 15
.B SI_USER
.BR kill (2)
or
.BR raise (3)
.TP
.B SI_KERNEL
Sent by the kernel.
.TP
.B SI_QUEUE
.BR sigqueue (2)
.TP
.B SI_TIMER
POSIX timer expired
.TP
.B SI_MESGQ
POSIX message queue state changed (since Linux 2.6.6); see
.BR mq_notify (3)
.TP
.B SI_ASYNCIO
AIO completed
.TP
.B SI_SIGIO
queued SIGIO
.TP
.B SI_TKILL
.BR tkill (2)
or
.BR tgkill (2)
(since Linux 2.4.19)
.\" SI_DETHREAD is defined in 2.6.9 sources, but isn't implemented
.\" It appears to have been an idea that was tried during 2.5.6
.\" through to 2.5.24 and then was backed out.
.RE
.PP
The following values can be place in
.I si_code
for a
.B SIGILL
signal:
.RS 4
.TP 15
.B ILL_ILLOPC
illegal opcode
.TP
.B ILL_ILLOPN
illegal operand
.TP
.B ILL_ILLADR
illegal addressing mode
.TP
.B ILL_ILLTRP
illegal trap
.TP
.B ILL_PRVOPC
privileged opcode
.TP
.B ILL_PRVREG
privileged register
.TP
.B ILL_COPROC
coprocessor error
.TP
.B ILL_BADSTK
internal stack error
.RE
.PP
The following values can be place in
.I si_code
for a
.B SIGFPE
signal:
.RS 4
.TP 15
.B FPE_INTDIV
integer divide by zero
.TP
.B FPE_INTOVF
integer overflow
.TP
.B FPE_FLTDIV
floating point divide by zero
.TP
.B FPE_FLTOVF
floating point overflow
.TP
.B FPE_FLTUND
floating point underflow
.TP
.B FPE_FLTRES
floating point inexact result
.TP
.B FPE_FLTINV
floating point invalid operation
.TP
.B FPE_FLTSUB
subscript out of range
.RE
.PP
The following values can be place in
.I si_code
for a
.B SIGSEGV
signal:
.RS 4
.TP 15
.B SEGV_MAPERR
address not mapped to object
.TP
.B SEGV_ACCERR
invalid permissions for mapped object
.RE
.PP
The following values can be place in
.I si_code
for a
.B SIGBUS
signal:
.RS 4
.TP 15
.B BUS_ADRALN
invalid address alignment
.TP
.B BUS_ADRERR
non-existent physical address
.TP
.B BUS_OBJERR
object specific hardware error
.RE
.PP
The following values can be place in
.I si_code
for a
.B SIGTRAP
signal:
.RS 4
.TP 15
.B TRAP_BRKPT
process breakpoint
.TP
.B TRAP_TRACE
process trace trap
.RE
.PP
The following values can be place in
.I si_code
for a
.B SIGCHLD
signal:
.RS 4
.TP 15
.B CLD_EXITED
child has exited
.TP
.B CLD_KILLED
child was killed
.TP
.B CLD_DUMPED
child terminated abnormally
.TP
.B CLD_TRAPPED
traced child has trapped
.TP
.B CLD_STOPPED
child has stopped
.TP
.B CLD_CONTINUED
stopped child has continued (since Linux 2.6.9)
.RE
.PP
The following values can be place in
.I si_code
for a
.B SIGPOLL
signal:
.RS 4
.TP 15
.B POLL_IN
data input available
.TP
.B POLL_OUT
output buffers available
.TP
.B POLL_MSG
input message available
.TP
.B POLL_ERR
i/o error
.TP
.B POLL_PRI
high priority input available
.TP
.B POLL_HUP
device disconnected
.RE
.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 "CONFORMING TO"
POSIX.1-2001, SVr4.
.\" SVr4 does not document the EINTR condition.
.SH NOTES
.PP
According to POSIX, the behavior of a process is undefined after it
ignores a
.BR SIGFPE ,
.BR SIGILL ,
or
.B SIGSEGV
signal that was not generated by
.BR kill (2)
or
.BR raise (3).
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
.BR 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 behaviors 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.
.PP
See
.BR signal (7)
for a list of the async-signal-safe functions that can be
safely called inside from inside a signal handler.
.SS 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 BUGS
In kernels up to and including 2.6.13, specifying
.B SA_NODEFER
in
.I sa_flags
prevents not only the delivered signal from being masked during
execution of the handler, but also the signals specified in
.IR sa_mask .
This bug was fixed in kernel 2.6.14.
.SH EXAMPLE
See
.BR mprotect (2).
.SH "SEE ALSO"
.BR kill (1),
.BR kill (2),
.BR killpg (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 raise (3),
.BR siginterrupt (3),
.BR sigsetops (3),
.BR sigvec (3),
.BR core (5),
.BR signal (7)