SIGACTION(2) BSD Programmer's Manual SIGACTION(2)
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 invo-
cation blocked, but other signals may yet occur. A global signal mask de-
fines 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 sig-
nals are ready to be delivered at the same time, any signals that could
be caused by traps are delivered first. Additional signals may be pro-
cessed 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 han-
dling 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 previ-
ous 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 de-
fault 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 func-
tion 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 execu-
tion 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 struc-
ture is a part of IEEE Std 1003.1b ("POSIX.1b"). 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 list-
ed 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 af-
fected 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 al-
ready committed are not restarted, but instead re-
turn 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 sig-
nals 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), readv(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), writev(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 modifica-
tion 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.1").
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 in-
tended 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 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(), get-
pid(), getppid(), getuid(), kill(), link(), lseek(), mkdir(), mkfifo(),
open(), pathconf(), pause(), pipe(), raise(), read(), rename(), rmdir(),
setgid(), setpgid(), setsid(), setuid(), sigaction(), sigaddset(), sig-
delset(), sigemptyset(), sigfillset(), sigismember(), signal(), sigpend-
ing(), 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.
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