SIGACTION(2) | System Calls Manual | SIGACTION(2) |
sigaction
—
software signal facilities
Standard C Library (libc, -lc)
#include
<signal.h>
struct sigaction { union __sigaction_u __sigaction_u; /* signal handler */ sigset_t sa_mask; /* signal mask to apply */ int sa_flags; /* see signal options below */ }; union __sigaction_u { void (*__sa_handler)(int); void (*__sa_sigaction)(int, siginfo_t *, void *); }; #define sa_handler __sigaction_u.__sa_handler #define sa_sigaction __sigaction_u.__sa_sigaction
sigaction
(int sig,
const struct sigaction *restrict act,
struct sigaction *restrict 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) 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
SIGCHLD
signal, the
SIGCHLD
signal will be generated only when a child
process exits, not when a child process stops.SA_NOCLDWAIT
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
SA_NODEFER
SA_RESETHAND
SIG_DFL
at the moment the signal is
delivered.SA_RESTART
SA_SIGINFO
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 |
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
(),
sigpause
(),
aio_return
(),
aio_suspend
(),
sem_post
(),
sigset
().
ANSI C Interfaces:
strcpy
(),
strcat
(),
strncpy
(),
strncat
(),
and perhaps some others.
Extension Interfaces:
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.
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.
There are three possible prototypes the handler may match:
handler
(int);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 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 uap argument to a POSIX SA_SIGINFO handler points to an instance of ucontext_t.
The sigaction
() system call will fail and
no new signal handler will be installed if one of the following occurs:
EFAULT
]EINVAL
]EINVAL
]SIGKILL
or SIGSTOP
.EINVAL
]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.
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), siginterrupt(3), sigsetops(3), ucontext(3), tty(4)
September 18, 2008 | macOS 14.6 |