sigaction
—
software signal facilities
Standard C Library (libc, -lc)
#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);
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 |
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
(),
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.
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:
- 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.
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
.
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)
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
.