path: root/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.
.\" 2010-06-11 Andi Kleen, add hwpoison signal extensions
.\" 2010-06-11 mtk, improvements to discussion of various siginfo_t fields.
.\"
.TH SIGACTION 2 2013-02-12 "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
.sp
.in -4n
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.in
.sp
.ad l
.BR sigaction ():
_POSIX_C_SOURCE\ >=\ 1 || _XOPEN_SOURCE || _POSIX_SOURCE

.IR siginfo_t :
_POSIX_C_SOURCE >= 199309L
.ad b
.SH DESCRIPTION
The
.BR sigaction ()
system call is used to change the action taken by a process on
receipt of a specific signal.
(See
.BR signal (7)
for an overview of signals.)
.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
.in +4n
.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
.in
.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 \fIvoid\ *\fP) as its third argument.
(Commonly, the handler function doesn't make any use of the third argument.
See
.BR getcontext (3)
for further information about
.IR ucontext_t .)
.PP
.I sa_mask
specifies a mask of signals which should be blocked
(i.e., added to the signal mask of the thread in which
the signal handler is invoked)
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.
It is formed by the bitwise OR of zero or more of the following:
.RS 4
.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)).
This flag is only meaningful when establishing a handler for
.BR SIGCHLD .
.TP
.BR SA_NOCLDWAIT " (since Linux 2.6)"
.\" 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).
This flag is only meaningful when establishing a handler for
.BR SIGCHLD ,
or when setting that signal's disposition to
.BR SIG_DFL .

If the
.B SA_NOCLDWAIT
flag is set when establishing a handler for
.BR SIGCHLD ,
POSIX.1 leaves it unspecified whether a
.B SIGCHLD
signal is generated when a child process terminates.
On Linux, a
.B SIGCHLD
signal is generated in this case;
on some other implementations, it is not.
.TP
.B SA_NODEFER
Do not prevent the signal from being received from within its own signal
handler.
This flag is only meaningful when establishing a signal handler.
.B SA_NOMASK
is an obsolete, nonstandard 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.
This flag is only meaningful when establishing a signal handler.
.TP
.BR SA_RESETHAND
Restore the signal action to the default upon entry to the signal handler.
This flag is only meaningful when establishing a signal handler.
.B SA_ONESHOT
is an obsolete, nonstandard synonym for this flag.
.TP
.B SA_RESTART
Provide behavior compatible with BSD signal semantics by making certain
system calls restartable across signals.
This flag is only meaningful when establishing a signal handler.
See
.BR signal (7)
for a discussion of system call restarting.
.TP
.BR SA_SIGINFO " (since Linux 2.2)"
The signal handler takes three arguments, not one.
In this case,
.I sa_sigaction
should be set instead of
.IR sa_handler .
This flag is only meaningful when establishing a signal handler.
.\" (The
.\" .I sa_sigaction
.\" field was added in Linux 2.1.86.)
.RE
.PP
The
.I siginfo_t
argument to
.I sa_sigaction
is a struct with the following elements:
.sp
.in +4n
.nf
siginfo_t {
    int      si_signo;    /* Signal number */
    int      si_errno;    /* An errno value */
    int      si_code;     /* Signal code */
    int      si_trapno;   /* Trap number that caused
                             hardware-generated signal
                             (unused on most architectures) */
.\" FIXME
.\" si_trapno seems to be only used on SPARC and Alpha;
.\" this page could use a little more detail on its purpose there.
    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 */
    int      si_overrun;  /* Timer overrun count; POSIX.1b timers */
    int      si_timerid;  /* Timer ID; POSIX.1b timers */
.\" In the kernel: si_tid
    void    *si_addr;     /* Memory location which caused fault */
    long     si_band;     /* Band event (was \fIint\fP in
                             glibc 2.3.2 and earlier) */
    int      si_fd;       /* File descriptor */
    short    si_addr_lsb; /* Least significant bit of address
                             (since kernel 2.6.32) */
}
.fi
.in

.IR si_signo ", " si_errno " and " si_code
are defined for all signals.
.RI ( si_errno
is generally 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
Signals sent with
.BR kill (2)
and
.BR sigqueue (3)
fill in
.IR si_pid " and " si_uid .
In addition, signals sent with
.BR sigqueue (3)
fill in
.IR si_int " and " si_ptr
with the values specified by the sender of the signal;
see
.BR sigqueue (3)
for more details.
.IP *
Signals sent by POSIX.1b timers (since Linux 2.6) fill in
.I si_overrun
and
.IR si_timerid .
The
.I si_timerid
field is an internal ID used by the kernel to identify
the timer; it is not the same as the timer ID returned by
.BR timer_create (2).
The
.I si_overrun
field is the timer overrun count;
this is the same information as is obtained by a call to
.BR timer_getoverrun (2).
These fields are nonstandard Linux extensions.
.IP *
Signals sent for message queue notification (see the description of
.B SIGEV_SIGNAL
in
.BR mq_notify (3))
fill in
.IR si_int / si_ptr ,
with the
.I sigev_value
supplied to
.BR mq_notify (3);
.IR si_pid ,
with the process ID of the message sender; and
.IR si_uid ,
with the real user ID of the message sender.
.IP *
.B SIGCHLD
fills in
.IR si_pid ", " si_uid ", " si_status ", " si_utime " and " si_stime ,
providing information about the child.
The
.I si_pid
field is the process ID of the child;
.I si_uid
is the child's real user ID.
The
.I si_status
field contains the exit status of the child (if
.I si_code
is
.BR CLD_EXITED ),
or the signal number that caused the process to change state.
The
.I si_utime
and
.I si_stime
contain the user and system CPU time used by the child process;
these fields do not include the times used by waited-for children (unlike
.BR getrusage (2)
and
.BR time (2)).
In kernels up to 2.6, and since 2.6.27, these fields report
CPU time in units of
.IR sysconf(_SC_CLK_TCK) .
In 2.6 kernels before 2.6.27,
a bug meant that these fields reported time in units
of the (configurable) system jiffy (see
.BR time (7)).
.\" FIXME .
.\" When si_utime and si_stime where originally implemented, the
.\" measurement unit was HZ, which was the same as clock ticks
.\" (sysconf(_SC_CLK_TCK)).  In 2.6, HZ became configurable, and
.\" was *still* used as the unit to return the info these fields,
.\" with the result that the field values depended on the the
.\" configured HZ.  Of course, the should have been measured in
.\" USER_HZ instead, so that sysconf(_SC_CLK_TCK) could be used to
.\" convert to seconds.  I have a queued patch to fix this:
.\" http://thread.gmane.org/gmane.linux.kernel/698061/ .
.\" This patch made it into 2.6.27.
.\" But note that these fields still don't return the times of
.\" waited-for children (as is done by getrusage() and times()
.\" and wait4()).  Solaris 8 does include child times.
.IP *
.BR SIGILL ,
.BR SIGFPE ,
.BR SIGSEGV ,
.BR SIGBUS ,
and
.BR SIGTRAP
fill in
.I si_addr
with the address of the fault.
.\" FIXME SIGTRAP also sets the following for ptrace_notify() ?
.\"     info.si_code = exit_code;
.\"     info.si_pid = task_pid_vnr(current);
.\"     info.si_uid = current_uid();  /* Real UID */
On some architectures,
these signals also fill in the
.I si_trapno
filed.
Some suberrors of
.BR SIGBUS ,
in particular
.B BUS_MCEERR_AO
and
.BR BUS_MCEERR_AR ,
also fill in
.IR si_addr_lsb .
This field indicates the least significant bit of the reported address
and therefore the extent of the corruption.
For example, if a full page was corrupted,
.I si_addr_lsb
contains
.IR log2(sysconf(_SC_PAGESIZE)) .
.B BUS_MCERR_*
and
.I si_addr_lsb
are Linux-specific extensions.
.IP *
.BR SIGIO / SIGPOLL
(the two names are synonyms on Linux)
fills in
.IR si_band " and " si_fd .
The
.I si_band
event is a bit mask containing the same values as are filled in the
.I revents
field by
.BR poll (2).
The
.I si_fd
field indicates the file descriptor for which the I/O event occurred.
.PP
.I si_code
is a value (not a bit mask)
indicating why this signal was sent.
The following list shows the values which 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)
.TP
.B SI_KERNEL
Sent by the kernel.
.TP
.B SI_QUEUE
.BR sigqueue (3)
.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
.B SIGIO
(only in kernels up to Linux 2.2; from Linux 2.4 onward
.BR SIGIO / SIGPOLL
fills in
.I si_code
as described below).
.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 placed 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 placed 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 placed 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 placed in
.I si_code
for a
.B SIGBUS
signal:
.RS 4
.TP 15
.B BUS_ADRALN
invalid address alignment
.TP
.B BUS_ADRERR
nonexistent physical address
.TP
.B BUS_OBJERR
object-specific hardware error
.TP
.BR BUS_MCEERR_AR " (since Linux 2.6.32)"
Hardware memory error consumed on a machine check; action required.
.TP
.BR BUS_MCEERR_AO " (since Linux 2.6.32)"
Hardware memory error detected in process but not consumed; action optional.
.RE
.PP
The following values can be placed 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
.TP
.BR TRAP_BRANCH " (since Linux 2.4)"
process taken branch trap
.TP
.BR TRAP_HWBKPT " (since Linux 2.4)"
hardware breakpoint/watchpoint
.RE
.PP
The following values can be placed 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 placed in
.I si_code
for a
.BR SIGIO / 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
A child created via
.BR fork (2)
inherits a copy of its parent's signal dispositions.
During an
.BR execve (2),
the dispositions of handled signals are reset to the default;
the dispositions of ignored signals are left unchanged.

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
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
.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 Linux 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 signalfd (2),
.BR sigpending (2),
.BR sigprocmask (2),
.BR sigsuspend (2),
.BR wait (2),
.BR raise (3),
.BR siginterrupt (3),
.BR sigqueue (3),
.BR sigsetops (3),
.BR sigvec (3),
.BR core (5),
.BR signal (7)