SIGACTION(2) FreeBSD System Calls Manual SIGACTION(2)
NAME
sigaction - software signal facilities
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <signal.h>
struct sigaction {
void (*sa_handler)(int);)(int);
void (*sa_sigaction)(int, siginfo_t *, void *);)(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 thread
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 for a
thread, in which case it will not be delivered to that thread 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 thread. 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
thread. The signal mask for a thread is initialized from that of its
parent (normally empty). It may be changed with a sigprocmask(2) or
pthread_sigmask(3) call, or when a signal is delivered to the thread.
When a signal condition arises for a process or thread, the signal is
added to a set of signals pending for the process or thread. Whether the
signal is directed at the process in general or at a specific thread
depends on how it is generated. For signals directed at a specific
thread, if the signal is not currently blocked by the thread then it is
delivered to the thread. For signals directed at the process, if the
signal is not currently blocked by all threads then it is delivered to
one thread that does not have it blocked (the selection of which is
unspecified). Signals may be delivered any time a thread 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 thread 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 thread will resume execution in the context from before the
signal's delivery. If the thread wishes to resume in a different
context, then it must arrange to restore the previous context itself.
When a signal is delivered to a thread 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-NULL, 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-NULL, 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 it is nonsensical for an application to
attempt to 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
by each thread 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 pthread_create(3) the signal mask is inherited by the new thread
and the set of pending signals and the signal stack for the new thread
are empty.
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 or from a child
process after calling fork(2) in a multi-threaded process:
Base Interfaces:
_Exit(), _exit(), accept(), access(), alarm(), bind(), cfgetispeed(),
cfgetospeed(), cfsetispeed(), cfsetospeed(), chdir(), chmod(), chown(),
close(), connect(), creat(), dup(), dup2(), execl(), execle(), execv(),
execve(), faccessat(), fchdir(), fchmod(), fchmodat(), fchown(),
fchownat(), fcntl(), fork(), fstat(), fstatat(), fsync(), ftruncate(),
getegid(), geteuid(), getgid(), getgroups(), getpeername(), getpgrp(),
getpid(), getppid(), getsockname(), getsockopt(), getuid(), kill(),
link(), linkat(), listen(), lseek(), lstat(), mkdir(), mkdirat(),
mkfifo(), mkfifoat(), mknod(), mknodat(), open(), openat(), pause(),
pipe(), poll(), pselect(), pthread_sigmask(), raise(), read(),
readlink(), readlinkat(), recv(), recvfrom(), recvmsg(), rename(),
renameat(), rmdir(), select(), send(), sendmsg(), sendto(), setgid(),
setpgid(), setsid(), setsockopt(), setuid(), shutdown(), sigaction(),
sigaddset(), sigdelset(), sigemptyset(), sigfillset(), sigismember(),
signal(), sigpending(), sigprocmask(), sigsuspend(), sleep(),
sockatmark(), socket(), socketpair(), stat(), symlink(), symlinkat(),
tcdrain(), tcflow(), tcflush(), tcgetattr(), tcgetpgrp(), tcsendbreak(),
tcsetattr(), tcsetpgrp(), time(), times(), umask(), uname(), unlink(),
unlinkat(), utime(), wait(), waitpid(), write().
X/Open Systems Interfaces:
sigpause(), sigset(), utimes().
Realtime Interfaces:
aio_error(), clock_gettime(), timer_getoverrun(), aio_return(),
fdatasync(), sigqueue(), timer_gettime(), aio_suspend(), sem_post(),
timer_settime().
Base Interfaces not specified as async-signal safe by POSIX:
fpathconf(), pathconf(), sysconf().
Base Interfaces not specified as async-signal safe by POSIX, but planned
to be:
ffs(), htonl(), htons(), memccpy(), memchr(), memcmp(), memcpy(),
memmove(), memset(), ntohl(), ntohs(), stpcpy(), stpncpy(), strcat(),
strchr(), strcmp(), strcpy(), strcspn(), strlen(), strncat(), strncmp(),
strncpy(), strnlen(), strpbrk(), strrchr(), strspn(), strstr(),
strtok_r(), wcpcpy(), wcpncpy(), wcscat(), wcschr(), wcscmp(), wcscpy(),
wcscspn(), wcslen(), wcsncat(), wcsncmp(), wcsncpy(), wcsnlen(),
wcspbrk(), wcsrchr(), wcsspn(), wcsstr(), wcstok(), wmemchr(), wmemcmp(),
wmemcpy(), wmemmove(), wmemset().
Extension Interfaces:
accept4(), bindat(), close_range(), closefrom(), connectat(), eaccess(),
ffsl(), ffsll(), flock(), fls(), flsl(), flsll(), futimesat(), pipe2(),
strlcat(). strlcpy(), strsep().
In addition, reading or writing errno is async-signal safe.
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 is
undefined when they are called from a signal handler that interrupted an
unsafe function. 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), setitimer(2), setrlimit(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 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. 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.
FreeBSD 13.1-RELEASE-p6 June 29, 2020 FreeBSD 13.1-RELEASE-p6
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