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SIGACTION(2) | System Calls Manual | SIGACTION(2) |
NAME¶
sigaction — software signal facilitiesLIBRARY¶
Standard C Library (libc, -lc)SYNOPSIS¶
#include <signal.h>struct sigaction { void (*sa_handler)(int); void (*sa_sigaction)(int, siginfo_t *, void *); int sa_flags; /* see signal options below */ sigset_t sa_mask; /* signal mask to apply */ };
int
sigaction(int sig, const struct sigaction * restrict act, struct sigaction * restrict oact);
DESCRIPTION¶
The system defines a set of signals that may be delivered to a process. Signal delivery resembles the occurrence of a hardware interrupt: the signal is normally blocked from further occurrence, the current process context is saved, and a new one is built. A process may specify a handler to which a signal is delivered, or specify that a signal is to be ignored. A process may also specify that a default action is to be taken by the system when a signal occurs. A signal may also be blocked, in which case its delivery is postponed until it is unblocked. The action to be taken on delivery is determined at the time of delivery. Normally, signal handlers execute on the current stack of the process. This may be changed, on a per-handler basis, so that signals are taken on a special signal stack. Signal routines normally execute with the signal that caused their invocation blocked, but other signals may yet occur. A global signal mask defines the set of signals currently blocked from delivery to a process. The signal mask for a process is initialized from that of its parent (normally empty). It may be changed with a sigprocmask(2) call, or when a signal is delivered to the process. When a signal condition arises for a process, the signal is added to a set of signals pending for the process. If the signal is not currently blocked by the process then it is delivered to the process. Signals may be delivered any time a process enters the operating system (e.g., during a system call, page fault or trap, or clock interrupt). If multiple signals are ready to be delivered at the same time, any signals that could be caused by traps are delivered first. Additional signals may be processed at the same time, with each appearing to interrupt the handlers for the previous signals before their first instructions. The set of pending signals is returned by the sigpending(2) system call. When a caught signal is delivered, the current state of the process is saved, a new signal mask is calculated (as described below), and the signal handler is invoked. The call to the handler is arranged so that if the signal handling routine returns normally the process will resume execution in the context from before the signal's delivery. If the process wishes to resume in a different context, then it must arrange to restore the previous context itself. When a signal is delivered to a process a new signal mask is installed for the duration of the process' signal handler (or until a sigprocmask(2) system call is made). This mask is formed by taking the union of the current signal mask set, the signal to be delivered, and the signal mask associated with the handler to be invoked. The sigaction() system call assigns an action for a signal specified by sig. If act is non-zero, it specifies an action (SIG_DFL
,
SIG_IGN
, or a handler routine) and mask to be used
when delivering the specified signal. If oact is
non-zero, the previous handling information for the signal is returned to the
user.
The above declaration of struct sigaction is not literal.
It is provided only to list the accessible members. See
<sys/signal.h> for the actual
definition. In particular, the storage occupied by sa_handler and sa_sigaction
overlaps, and an application can not use both simultaneously.
Once a signal handler is installed, it normally remains installed until another
sigaction() system call is made, or an
execve(2) is performed. A signal-specific default action may
be reset by setting sa_handler to
SIG_DFL
. The defaults are process termination,
possibly with core dump; no action; stopping the process; or continuing the
process. See the signal list below for each signal's default action. If
sa_handler is SIG_DFL
, the
default action for the signal is to discard the signal, and if a signal is
pending, the pending signal is discarded even if the signal is masked. If
sa_handler is set to SIG_IGN
current and pending instances of the signal are ignored and discarded.
Options may be specified by setting sa_flags. The meaning
of the various bits is as follows:
SA_NOCLDSTOP
- If this bit is set when installing a catching function for
the
SIGCHLD
signal, theSIGCHLD
signal will be generated only when a child process exits, not when a child process stops. SA_NOCLDWAIT
- If this bit is set when calling
sigaction() for the
SIGCHLD
signal, the system will not create zombie processes when children of the calling process exit. If the calling process subsequently issues a wait(2) (or equivalent), it blocks until all of the calling process's child processes terminate, and then returns a value of -1 with errno set toECHILD
. The same effect of avoiding zombie creation can also be achieved by setting sa_handler forSIGCHLD
toSIG_IGN
. SA_ONSTACK
- If this bit is set, the system will deliver the signal to the process on a signal stack, specified with sigaltstack(2).
SA_NODEFER
- If this bit is set, further occurrences of the delivered signal are not masked during the execution of the handler.
SA_RESETHAND
- If this bit is set, the handler is reset back to
SIG_DFL
at the moment the signal is delivered. SA_RESTART
- See paragraph below.
SA_SIGINFO
- If this bit is set, the handler function is assumed to be
pointed to by the sa_sigaction member of
struct sigaction and should match the prototype
shown above or as below in EXAMPLES.
This bit should not be set when assigning
SIG_DFL
orSIG_IGN
.
EINTR
, the call may
return with a data transfer shorter than requested, or the call may be
restarted. Restart of pending calls is requested by setting the
SA_RESTART
bit in sa_flags. The
affected system calls include open(2),
read(2), write(2),
sendto(2), recvfrom(2),
sendmsg(2) and recvmsg(2) on a
communications channel or a slow device (such as a terminal, but not a regular
file) and during a wait(2) or ioctl(2).
However, calls that have already committed are not restarted, but instead
return a partial success (for example, a short read count).
After a fork(2) or vfork(2) all signals, the
signal mask, the signal stack, and the restart/interrupt flags are inherited
by the child.
The execve(2) system call reinstates the default action for
all signals which were caught and resets all signals to be caught on the user
stack. Ignored signals remain ignored; the signal mask remains the same;
signals that restart pending system calls continue to do so.
The following is a list of all signals with names as in the include file
<signal.h>:
NAME | Default Action | Description |
SIGHUP
|
terminate process | terminal line hangup |
SIGINT
|
terminate process | interrupt program |
SIGQUIT
|
create core image | quit program |
SIGILL
|
create core image | illegal instruction |
SIGTRAP
|
create core image | trace trap |
SIGABRT
|
create core image | abort(3) call (formerly
SIGIOT ) |
SIGEMT
|
create core image | emulate instruction executed |
SIGFPE
|
create core image | floating-point exception |
SIGKILL
|
terminate process | kill program |
SIGBUS
|
create core image | bus error |
SIGSEGV
|
create core image | segmentation violation |
SIGSYS
|
create core image | non-existent system call invoked |
SIGPIPE
|
terminate process | write on a pipe with no reader |
SIGALRM
|
terminate process | real-time timer expired |
SIGTERM
|
terminate process | software termination signal |
SIGURG
|
discard signal | urgent condition present on socket |
SIGSTOP
|
stop process | stop (cannot be caught or ignored) |
SIGTSTP
|
stop process | stop signal generated from keyboard |
SIGCONT
|
discard signal | continue after stop |
SIGCHLD
|
discard signal | child status has changed |
SIGTTIN
|
stop process | background read attempted from control terminal |
SIGTTOU
|
stop process | background write attempted to control terminal |
SIGIO
|
discard signal | I/O is possible on a descriptor (see fcntl(2)) |
Dv SIGXCPU | terminate process | cpu time limit exceeded (see setrlimit(2)) |
Dv SIGXFSZ | terminate process | file size limit exceeded (see setrlimit(2)) |
Dv SIGVTALRM | terminate process | virtual time alarm (see setitimer(2)) |
Dv SIGPROF | terminate process | profiling timer alarm (see setitimer(2)) |
Dv SIGWINCH | discard signal | Window size change |
SIGINFO
|
discard signal | status request from keyboard |
SIGUSR1
|
terminate process | User defined signal 1 |
SIGUSR2
|
terminate process | User defined signal 2 |
NOTE¶
The sa_mask field specified in act is not allowed to blockSIGKILL
or
SIGSTOP
. Any attempt to do so will be silently
ignored.
The following functions are either reentrant or not interruptible by signals and
are async-signal safe. Therefore applications may invoke them, without
restriction, from signal-catching functions:
Base Interfaces:
_exit(), access(),
alarm(), cfgetispeed(),
cfgetospeed(), cfsetispeed(),
cfsetospeed(), chdir(),
chmod(), chown(),
close(), creat(), dup(),
dup2(), execle(),
execve(), fcntl(),
fork(), fpathconf(),
fstat(), fsync(),
getegid(), geteuid(),
getgid(), getgroups(),
getpgrp(), getpid(),
getppid(), getuid(),
kill(), link(), lseek(),
mkdir(), mkfifo(),
open(), pathconf(),
pause(), pipe(),
raise(), read(),
rename(), rmdir(),
setgid(), setpgid(),
setsid(), setuid(),
sigaction(), sigaddset(),
sigdelset(), sigemptyset(),
sigfillset(), sigismember(),
signal(), sigpending(),
sigprocmask(), sigsuspend(),
sleep(), stat(),
sysconf(), tcdrain(),
tcflow(), tcflush(),
tcgetattr(), tcgetpgrp(),
tcsendbreak(), tcsetattr(),
tcsetpgrp(), time(),
times(), umask(),
uname(), unlink(),
utime(), wait(),
waitpid(), write().
Realtime Interfaces:
aio_error(), clock_gettime(),
sigpause(), timer_getoverrun(),
aio_return(), fdatasync(),
sigqueue(), timer_gettime(),
aio_suspend(), sem_post(),
sigset(), timer_settime().
ANSI C Interfaces:
strcpy(), strcat(),
strncpy(), strncat(), and perhaps some
others.
Extension Interfaces:
strlcpy(), strlcat().
All functions not in the above lists are considered to be unsafe with respect to
signals. That is to say, the behaviour of such functions when called from a
signal handler is undefined. In general though, signal handlers should do
little more than set a flag; most other actions are not safe.
Also, it is good practice to make a copy of the global variable
errno and restore it before returning from the signal
handler. This protects against the side effect of errno
being set by functions called from inside the signal handler.
RETURN VALUES¶
The sigaction() function returns the value 0 if successful; otherwise the value -1 is returned and the global variable errno is set to indicate the error.EXAMPLES¶
There are three possible prototypes the handler may match:- ANSI C:
- void handler(int);
- Traditional BSD style:
- void handler(int, int code, struct sigcontext *scp);
- POSIX
SA_SIGINFO
: - void handler(int, siginfo_t *info, ucontext_t *uap);
SA_SIGINFO
prototype if the SA_SIGINFO
bit is set in
sa_flags. It then should be pointed to by the
sa_sigaction member of struct
sigaction. Note that you should not assign
SIG_DFL
or SIG_IGN
this way.
If the SA_SIGINFO
flag is not set, the handler function
should match either the ANSI C or traditional BSD
prototype and be pointed to by the sa_handler member of
struct sigaction. In practice,
FreeBSD always sends the three arguments of the latter
and since the ANSI C prototype is a subset, both will work. The
sa_handler member declaration in
FreeBSD include files is that of ANSI C (as required
by POSIX), so a function pointer of a BSD-style
function needs to be casted to compile without warning. The traditional
BSD style is not portable and since its capabilities
are a full subset of a SA_SIGINFO
handler, its use is
deprecated.
The sig argument is the signal number, one of the
SIG...
values from
<signal.h>.
The code argument of the BSD-style
handler and the si_code member of the
info argument to a SA_SIGINFO
handler contain a numeric code explaining the cause of the signal, usually one
of the SI_...
values from
<sys/signal.h> or codes specific to
a signal, i.e., one of the FPE_...
values for
SIGFPE
.
The scp argument to a BSD-style
handler points to an instance of struct sigcontext.
The uap argument to a POSIX
SA_SIGINFO
handler points to an instance of
ucontext_t.
ERRORS¶
The sigaction() system call will fail and no new signal handler will be installed if one of the following occurs:- [
EFAULT
] - Either act or oact points to memory that is not a valid part of the process address space.
- [
EINVAL
] - The sig argument is not a valid signal number.
- [
EINVAL
] - An attempt is made to ignore or supply a handler for
SIGKILL
orSIGSTOP
.
SEE ALSO¶
kill(1), kill(2), ptrace(2), sigaltstack(2), sigblock(2), sigpause(2), sigpending(2), sigprocmask(2), sigsetmask(2), sigsuspend(2), sigvec(2), wait(2), fpsetmask(3), setjmp(3), siginfo(3), siginterrupt(3), sigsetops(3), ucontext(3), tty(4)STANDARDS¶
The sigaction() system call is expected to conform to IEEE Std 1003.1-1990 (“POSIX.1”). TheSA_ONSTACK
and SA_RESTART
flags are Berkeley extensions, as are the signals,
SIGTRAP
, SIGEMT
,
SIGBUS
, SIGSYS
,
SIGURG
, SIGIO
,
SIGXCPU
, SIGXFSZ
,
SIGVTALRM
, SIGPROF
,
SIGWINCH
, and SIGINFO
. Those
signals are available on most BSD-derived systems. The
SA_NODEFER
and SA_RESETHAND
flags are intended for backwards compatibility with other operating systems.
The SA_NOCLDSTOP
, and
SA_NOCLDWAIT
flags are featuring options commonly
found in other operating systems. The flags are approved by
Version 2 of the Single UNIX Specification
(“SUSv2”), along with the option to avoid zombie creation
by ignoring SIGCHLD
.April 18, 2010 | Debian |