sigaction — examine and change a signal action
#include <signal.h>
int
sigaction( |
int | signum, |
const struct sigaction * | act, | |
struct sigaction * | oldact) ; |
The 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 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
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 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element is obsolete and should not be used.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
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
isSIGCHLD
, do not receive notification when child processes stop (i.e., when they receive one ofSIGSTOP
,SIGTSTP
,SIGTTIN
orSIGTTOU
) or resume (i.e., they receiveSIGCONT
) (see wait(2)).SA_NOCLDWAIT
(Linux 2.6 and later) If
signum
isSIGCHLD
, 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.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 ofsa_handler
. (Thesa_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 */ }
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 si_pid
and si_uid
.
SIGCHLD
also fills in
si_status
,
si_utime
and
si_stime
.
si_int
and
si_ptr
are
specified by the sender of the POSIX.1b signal. See
sigqueue(2) for more
details.
SIGILL
, SIGFPE
, SIGSEGV
, and SIGBUS
fill in si_addr
with the
address of the fault. SIGPOLL
fills in 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:
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) |
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 |
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 |
SIGSEGV | |
SEGV_MAPERR | address not mapped to object |
SEGV_ACCERR | invalid permissions for mapped object |
SIGBUS | |
BUS_ADRALN | invalid address alignment |
BUS_ADRERR | non-existent physical address |
BUS_OBJERR | object specific hardware error |
SIGTRAP | |
TRAP_BRKPT | process breakpoint |
TRAP_TRACE | process trace trap |
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) |
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 |
act
or
oldact
points
to memory which is not a valid part of the process
address space.
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.
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.
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).
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.
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 sa_mask
. This bug is was
fixed in kernel 2.6.14.
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)
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