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NAME
sigaction — software signal facilities
LIBRARY
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, 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 <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))
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 <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:
[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), sigpending(2), sigprocmask(2), sigsuspend(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.