sigaction - software signal facilities
#include <signal.h>
struct sigaction {
union { /* signal handler */
void (*__sa_handler)(int);
void (*__sa_sigaction)(int, siginfo_t *,
void *);
} __sigaction_u;
sigset_t sa_mask; /* signal mask to apply
*/
int sa_flags; /* see signal options below */
};
#define sa_handler __sigaction_u.__sa_handler
#define sa_sigaction __sigaction_u.__sa_sigaction
int
sigaction(int sig, const struct sigaction *act, struct
sigaction *oact);
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) function.
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) 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 sa_mask associated with the handler to be invoked.
sigaction() 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.
Once a signal handler is installed, it normally remains installed until
another sigaction() call is made, or an execve(2) is performed. The value
of sa_handler (or, if the SA_SIGINFO flag is set, the
value of
sa_sigaction instead) indicates what action should be performed when a
signal arrives. A signal-specific default action may be reset by setting
sa_handler to SIG_DFL. Alternately, if the SA_RESETHAND
flag is set the
default action will be reinstated when the signal is first
posted. 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. If sig is SIGCHLD and
sa_handler is
set to SIG_IGN, the SA_NOCLDWAIT flag (described below) is
implied.
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, the
SIGCHLD 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 to
ECHILD.
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_SIGINFO If this bit is set, the 2nd argument
of the handler
is set to be a pointer to a siginfo_t
structure as
described in <sys/siginfo.h>. The
siginfo_t structure
is a part of IEEE Std 1003.1b
(``POSIX''). It
provides much more information about
the causes and
attributes of the signal that is being
delivered.
SA_RESTART If a signal is caught during the system calls listed
below, the call may be forced to
terminate with
the error EINTR, the call may return
with a data
transfer shorter than requested, or
the call may be
restarted. Restarting of pending
calls is requested
by setting the SA_RESTART bit in
sa_flags. The
affected system calls include 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.
execve(2) 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 (cannot
be caught or
ignored)
SIGBUS create core image bus error
SIGSEGV create core image segmentation violation
SIGSYS create core image system call given
invalid
argument
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))
SIGXCPU terminate process CPU time limit exceeded (see
setrlimit(2))
SIGXFSZ terminate process file size limit exceeded (see
setrlimit(2))
SIGVTALRM terminate process virtual time alarm
(see
setitimer(2))
SIGPROF terminate process profiling timer
alarm (see
setitimer(2))
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
The sa_mask field specified in act is not allowed to block
SIGKILL 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:
_exit(2), access(2), alarm(3), cfgetispeed(3), cfgetospeed(3),
cfsetispeed(3), cfsetospeed(3), chdir(2), chmod(2),
chown(2),
close(2), creat(3), dup(2), dup2(2), execle(3), execve(2),
fcntl(2), fork(2), fpathconf(2), fstat(2), fsync(2),
getegid(2),
geteuid(2), getgid(2), getgroups(2), getpgrp(2), getpid(2),
getppid(2), getuid(2), kill(2), link(2), lseek(2),
mkdir(2),
mkfifo(2), open(2), pathconf(2), pause(3), pipe(2),
raise(3),
read(2), rename(2), rmdir(2), setgid(2), setpgid(2),
setsid(2),
setuid(2), sigaction(2), sigaddset(3), sigdelset(3),
sigemptyset(3), sigfillset(3), sigismember(3), signal(3),
sigpause(3), sigpending(2), sigprocmask(2), sigsuspend(2),
sleep(3), stat(2), sysconf(3), tcdrain(3), tcflow(3),
tcflush(3),
tcgetattr(3), tcgetpgrp(3), tcsendbreak(3), tcsetattr(3),
tcsetpgrp(3), time(3), times(3), umask(2), uname(3),
unlink(2),
utime(3), wait(2), waitpid(2), write(2).
Please see signal(3) for a more detailed list.
All functions not in the above list 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.
Additionally, it is advised that signal handlers guard
against modification
of the external symbol errno by the above functions,
saving it at
entry and restoring it on return, thus:
void
handler(sig)
{
int save_errno = errno;
...
errno = save_errno;
}
A 0 value indicates that the call succeeded. A -1 return
value indicates
an error occurred and errno is set to indicate the reason.
The handler routine can be declared:
void
handler(sig)
int sig;
If the SA_SIGINFO option is enabled, the canonical way to
declare it is:
void
handler(sig, sip, scp)
int sig;
siginfo_t *sip;
struct sigcontext *scp;
Here sig is the signal number, into which the hardware
faults and traps
are mapped. If the SA_SIGINFO option is set, sip is a
pointer to a
siginfo_t as described in <sys/siginfo.h>. If SA_SIGINFO is
not set,
this pointer will be NULL instead. The function specified
in
sa_sigaction will be called instead of the function specified by
sa_handler (Note that in some implementations these are in
fact the
same). scp is a pointer to the sigcontext structure (defined in
<signal.h>), used to restore the context from before the
signal.
sigaction() 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] sig is not a valid signal number.
[EINVAL] An attempt is made to ignore or supply a handler for
SIGKILL or SIGSTOP.
kill(1), kill(2), ptrace(2), sigaltstack(2), sigprocmask(2),
sigsuspend(2), wait(2), setjmp(3), sigblock(3), sigpause(3),
sigsetops(3), sigvec(3), tty(4)
The sigaction() function conforms to IEEE Std 1003.1-1990
(``POSIX'').
The SA_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. These 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, SA_NOCLDWAIT, and SA_SIGINFO flags are options
commonly
found in other operating systems. The following functions
are either
reentrant or not interruptible by signals and are async-signal safe.
Therefore applications may invoke them, without restriction,
from signalcatching
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().
ANSI C Interfaces:
strcat(), strcpy(), strncat(), strncpy(), and perhaps some
others.
Extension Interfaces:
strlcat(), strlcpy().
Most 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.
Additionally, inside the signal handler it is also considered safer to
make a copy of the global variable errno and restore it before returning
from the signal handler.
A few other functions are signal race safe in OpenBSD but
probably not on
other systems:
snprintf() Safe.
vsnprintf() Safe.
syslog_r() Safe if the syslog_data struct is initialized as a
local variable.
OpenBSD 3.6 April 3, 1994
[ Back ] |
