The
sigaction()
system call is used to change the action taken by a process on
receipt of a specific signal.
(See
signal(7)
for an overview of signals.)
signum
specifies the signal and can be any valid signal except
SIGKILL
and
SIGSTOP.
If
act
is non-NULL, the new action for signal
signum
is installed from
act.
If
oldact
is non-NULL, the previous action is saved in
oldact.
The
sigaction
structure is defined as something like:
On some architectures a union is involved: do not assign to both
sa_handler
and
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element is obsolete and should not be used.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
sa_flags,
then
sa_sigaction
(instead of
sa_handler)
specifies the signal-handling function for
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
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
SA_NODEFER
flag is used.
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:
SA_NOCLDSTOP
If
signum
is
SIGCHLD,
do not receive notification when child processes stop (i.e., when they
receive one of
SIGSTOP, SIGTSTP, SIGTTIN
or
SIGTTOU)
or resume (i.e., they receive
SIGCONT)
(see
wait(2)).
This flag is only meaningful when establishing a handler for
SIGCHLD.
SA_NOCLDWAIT (since Linux 2.6)
If
signum
is
SIGCHLD,
do not transform children into zombies when they terminate.
See also
waitpid(2).
This flag is only meaningful when establishing a handler for
SIGCHLD,
or when setting that signal's disposition to
SIG_DFL.
If the
SA_NOCLDWAIT
flag is set when establishing a handler for
SIGCHLD,
POSIX.1 leaves it unspecified whether a
SIGCHLD
signal is generated when a child process terminates.
On Linux, a
SIGCHLD
signal is generated in this case;
on some other implementations, it is not.
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.
SA_NOMASK
is an obsolete, nonstandard synonym for this flag.
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.
This flag is only meaningful when establishing a signal handler.
SA_RESETHAND
Restore the signal action to the default state once the signal handler
has been called.
This flag is only meaningful when establishing a signal handler.
SA_ONESHOT
is an obsolete, nonstandard synonym for this flag.
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
signal(7)
for a discussion of system call restarting.
SA_SIGINFO (since Linux 2.2)
The signal handler takes 3 arguments, not one.
In this case,
sa_sigaction
should be set instead of
sa_handler.
This flag is only meaningful when establishing a signal handler.
The
siginfo_t
argument 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 */
int si_trapno; /* Trap number that caused
hardware-generated signal
(unused on most architectures) */
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 */
void *si_addr; /* Memory location which caused fault */
long si_band; /* Band event (was int 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) */
}
si_signo, si_errno and si_code
are defined for all signals.
(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:
*
Signals sent with
kill(2)
and
sigqueue(2)
fill in
si_pid and si_uid.
In addition, signals sent with
sigqueue(2)
fill in
si_int and si_ptr
with the values specified by the sender the signal;
see
sigqueue(2)
for more details.
*
Signals sent by POSIX.1b timers (since Linux 2.6) fill in
si_overrun
and
si_timerid.
The
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
timer_create(2).
The
si_overrun
field is the timer overrun count;
this is the same information as is obtained by a call to
timer_getoverrun(2).
These fields are nonstandard Linux extensions.
*
Signals sent for message queue notification (see the description of
SIGEV_SIGNAL
in
mq_notify(3))
fill in
si_int/si_ptr,
with the
sigev_value
supplied to
mq_notify(3);
si_pid,
with the process ID of the message sender; and
si_uid,
with the real user ID of the message sender.
*
SIGCHLD
fills in
si_pid, si_uid, si_status, si_utime and si_stime,
providing information about the child.
The
si_pid
field is the process ID of the child;
si_uid
is the child's real user ID.
The
si_status
field contains the exit status of the child (if
si_code
is
CLD_EXITED),
or the signal number that caused the process to change state.
The
si_utime
and
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
getrusage(2)
and
time(2)).
In kernels up to 2.6, and since 2.6.27, these fields report
CPU time in units of
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
time(7)).
*
SIGILL,
SIGFPE,
SIGSEGV,
SIGBUS,
and
SIGTRAP
fill in
si_addr
with the address of the fault.
On some architectures,
these signals also fill in the
si_trapno
filed.
Some suberrors of
SIGBUS,
in particular
BUS_MCEERR_AO
and
BUS_MCEERR_AR,
also fill in
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,
si_addr_lsb
contains
log2(sysconf(_SC_PAGESIZE)).
BUS_MCERR_*
and
si_addr_lsb
are Linux-specific extensions.
*
SIGPOLL/SIGIO
fills in
si_band and si_fd.
The
si_band
event is a bit mask containing the same values as are filled in the
revents
field by
poll(2).
The
si_fd
field indicates the file descriptor for which the I/O event occurred.
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
si_code
for any signal, along with reason that the signal was generated.
The following values can be placed in
si_code
for a
SIGILL
signal:
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
The following values can be placed in
si_code
for a
SIGFPE
signal:
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
The following values can be placed in
si_code
for a
SIGSEGV
signal:
SEGV_MAPERR
address not mapped to object
SEGV_ACCERR
invalid permissions for mapped object
The following values can be placed in
si_code
for a
SIGBUS
signal:
BUS_ADRALN
invalid address alignment
BUS_ADRERR
nonexistent physical address
BUS_OBJERR
object-specific hardware error
BUS_MCEERR_AR (since Linux 2.6.32)
Hardware memory error consumed on a machine check; action required.
BUS_MCEERR_AO (since Linux 2.6.32)
Hardware memory error detected in process but not consumed; action optional.
The following values can be placed in
si_code
for a
SIGTRAP
signal:
TRAP_BRKPT
process breakpoint
TRAP_TRACE
process trace trap
TRAP_BRANCH (since Linux 2.4)
process taken branch trap
TRAP_HWBKPT (since Linux 2.4)
hardware breakpoint/watchpoint
The following values can be placed in
si_code
for a
SIGCHLD
signal:
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)
The following values can be placed in
si_code
for a
SIGPOLL
signal:
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
sigaction()
returns 0 on success and -1 on error.
ERRORS
EFAULT
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
SIGKILL or SIGSTOP,
which cannot be caught or ignored.
CONFORMING TO
POSIX.1-2001, SVr4.
NOTES
A child created via
fork(2)
inherits a copy of its parent's signal dispositions.
During an
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
SIGFPE,
SIGILL,
or
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
SIGFPE.)
Ignoring this signal might lead to an endless loop.
POSIX.1-1990 disallowed setting the action for
SIGCHLD
to
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
SA_NOCLDSTOP.
POSIX.1-2001 added
SA_NOCLDWAIT,
SA_RESETHAND,
SA_NODEFER,
and
SA_SIGINFO.
Use of these latter values in
sa_flags
may be less portable in applications intended for older
Unix implementations.
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).
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
SIGKILL or SIGSTOP
(by specifying them in
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
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
prevents not only the delivered signal from being masked during
execution of the handler, but also the signals specified in
sa_mask.
This bug was fixed in kernel 2.6.14.
This page is part of release 3.27 of the Linux
man-pages
project.
A description of the project,
and information about reporting bugs,
can be found at
http://www.kernel.org/doc/man-pages/.