NAME
sigaction - software signal facilities
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <signal.h>
struct sigaction {
union {
void (*__sa_handler)(int);
void (*__sa_sigaction)(int, struct __siginfo *, void *);
} __sigaction_u; /* signal handler */
int sa_flags; /* see signal options below */
sigset_t sa_mask; /* signal mask to apply */
};
#define sa_handler __sigaction_u.__sa_handler
#define sa_sigaction __sigaction_u.__sa_sigaction
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.
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, 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. The same effect of avoiding zombie
creation can also be achieved by setting sa_handler
for SIGCHLD to SIG_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 or
SIG_IGN.
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.
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
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))
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
NOTE
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:
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);
The handler function should match the 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
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 #include
<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 or SIGSTOP.
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 ISO/IEC 9945-1:1990
(“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.
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.