sigaction
NAME
sigaction - examine and change a signal action
SYNOPSIS
#include <signal.h> I int sigaction(int signum , const struct sigaction * act , I struct sigaction * oldact );
DESCRIPTION
The
R sigaction ()
system call is used to change the action taken by a process on
receipt of a specific signal.
signum
specifies the signal and can be any valid signal except
SIGKILL
and
R SIGSTOP .
If
act
is non-null, the new action for signal
signum
is installed from
R act .
If
oldact
is non-null, the previous action is saved in
R oldact .
The
sigaction
structure is defined as something like
struct sigaction {
void (*sa_handler)(int);
void (*sa_sigaction)(int, siginfo_t *, void *);
sigset_t sa_mask;
int sa_flags;
void (*sa_restorer)(void);
}
On some architectures a union is involved: do not assign to both
sa_handler
and
R sa_sigaction .
The
sa_restorer
element is obsolete and should not be used.
POSIX does not specify a
sa_restorer
element.
sa_handler
specifies the action to be associated with
signum
and may be
SIG_DFL
for the default action,
SIG_IGN
to ignore this signal, or a pointer to a signal handling function.
This function receives the signal number as its only argument.
If
SA_SIGINFO
is specified in
R sa_flags ,
then
sa_sigaction
(instead of
R sa_handler )
specifies the signal-handling function for
R signum .
This function receives the signal number as its first argument, a
pointer to a
siginfo_t
as its second argument and a pointer to a
ucontext_t
(cast to void *) as its third argument.
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
SA_NODEFER
flag is used.
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:
SA_NOCLDSTOP
If
signum
is
R SIGCHLD ,
do not receive notification when child processes stop (i.e., when they
receive one of
R SIGSTOP , SIGTSTP , SIGTTIN
or
R SIGTTOU )
or resume (i.e., they receive
R SIGCONT )
(see
wait(2)).
SA_NOCLDWAIT
(Linux 2.6 and later)
If
signum
is
R SIGCHLD ,
do not transform children into zombies when they terminate.
See also
waitpid(2).
SA_RESETHAND
Restore the signal action to the default state once the signal handler
has been called.
SA_ONESHOT
is an obsolete, non-standard synonym for this flag.
R SA_ONSTACK
Call the signal handler on an alternate signal stack provided by
sigaltstack(2).
If an alternate stack is not available, the default stack will be used.
SA_RESTART
Provide behavior compatible with BSD signal semantics by making certain
system calls restartable across signals.
SA_NODEFER
Do not prevent the signal from being received from within its own signal
handler.
SA_NOMASK
is an obsolete, non-standard synonym for this flag.
SA_SIGINFO
The signal handler takes 3 arguments, not one.
In this case,
sa_sigaction
should be set instead of
R sa_handler .
(The
sa_sigaction
field was added in Linux 2.1.86.)
The
siginfo_t
parameter to
sa_sigaction
is a struct with the following elements
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 */
}
R si_signo , si_errno and si_code
are defined for all signals.
(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
SIGCHLD
fill in
R si_pid and si_uid .
R
*
SIGCHLD
also fills in
R si_status , si_utime and si_stime .
*
R si_int and si_ptr
are specified by the sender of the POSIX.1b signal.
See
sigqueue(2)
for more details.
*
R SIGILL ,
R SIGFPE ,
R SIGSEGV ,
and
SIGBUS
fill in
si_addr
with the address of the fault.
SIGPOLL
fills in
R si_band and si_fd .
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:
tab(:) allbox;
c s
l l.
si_code
Value:Signal origin
SI_USER:kill(2) or raise(3)
SI_KERNEL:The kernel
SI_QUEUE:sigqueue(2)
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(2) or tgkill(2) (since Linux 2.4.19)
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
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
tab(:) allbox;
c s
l l.
SIGSEGV
SEGV_MAPERR:address not mapped to object
SEGV_ACCERR:invalid permissions for mapped object
tab(:) allbox;
c s
l l.
SIGBUS
BUS_ADRALN:invalid address alignment
BUS_ADRERR:non-existent physical address
BUS_OBJERR:object specific hardware error
tab(:) allbox;
c s
l l.
SIGTRAP
TRAP_BRKPT:process breakpoint
TRAP_TRACE:process trace trap
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)
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
RETURN VALUE
R sigaction ()
returns 0 on success and -1 on error.
ERRORS
EFAULT
R act or oldact
points to memory which is not a valid part of the process address space.
EINVAL
An invalid signal was specified.
This will also be generated if an attempt
is made to change the action for
R SIGKILL or SIGSTOP ,
which cannot be caught or ignored.
CONFORMING TO
POSIX.1-2001, SVr4.
NOTES
According to POSIX, the behavior of a process is undefined after it
ignores a
R SIGFPE ,
R SIGILL ,
or
R SIGSEGV
signal that was not generated by
kill(2)
or
raise(3).
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
R SIGFPE .)
Ignoring this signal might lead to an endless loop.
POSIX.1-1990 disallowed setting the action for
SIGCHLD
to
R SIG_IGN .
POSIX.1-2001 allows this possibility, so that ignoring
SIGCHLD
can be used to prevent the creation of zombies (see
wait(2)).
Nevertheless, the historical BSD and System V behaviors for ignoring
SIGCHLD
differ, so that the only completely portable method of ensuring that
terminated children do not become zombies is to catch the
SIGCHLD
signal and perform a
wait(2)
or similar.
POSIX.1-1990 only specified
R SA_NOCLDSTOP .
POSIX.1-2001 added
R SA_NOCLDWAIT ,
R SA_RESETHAND ,
R SA_NODEFER ,
and
R SA_SIGINFO .
Use of these latter values in
sa_flags
may be less portable in applications intended for older
Unix implementations.
Support for
SA_SIGINFO
was added in Linux 2.2.
The
SA_RESETHAND
flag is compatible with the SVr4 flag of the same name.
The
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
sa_mask
settings).
R 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.
It is not possible to block
R SIGKILL or SIGSTOP
(by specifying them in
R sa_mask ).
Attempts to do so are silently ignored.
See
sigsetops(3)
for details on manipulating signal sets.
See
signal(7)
for a list of the async-signal-safe functions that can be
safely called inside from inside a signal handler.
Undocumented
Before the introduction of SA_SIGINFO it was also possible to get some additional information, namely by using a sa_handler with second argument of type R struct sigcontext. See the relevant kernel sources for details. This use is obsolete now.BUGS
In kernels up to and including 2.6.13, specifying
SA_NODEFER
in
sa_flags
preventing not only the delivered signal from being masked during
execution of the handler, but also the signals specified in
R sa_mask .
This bug is was fixed in kernel 2.6.14.
EXAMPLE
See
mprotect(2).
SEE ALSO
kill(1),
kill(2),
pause(2),
sigaltstack(2),
signal(2),
sigpending(2),
sigprocmask(2),
sigqueue(2),
sigsuspend(2),
wait(2),
killpg(3),
raise(3),
siginterrupt(3),
sigsetops(3),
sigvec(3),
core(5),
signal(7)
