NAME
sigaction
—
software signal facilities
SYNOPSIS
#include
<signal.h>
struct sigaction { union { /* signal handler */ void (*__sa_handler)(int); void (*__sa_sigaction)(int, siginfo_t *, void *); } __sigaction_u; sigset_t sa_mask; /* signal mask to apply */ int sa_flags; /* see signal options below */ };
#define sa_handler __sigaction_u.__sa_handler
#define sa_sigaction __sigaction_u.__sa_sigaction
int
sigaction
(int
sig, const struct
sigaction *act, struct
sigaction *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) function. 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) 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
sa_mask
associated with the handler to be invoked, but always excluding
SIGKILL
and SIGSTOP
.
sigaction
()
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
()
call is made, or an
execve(2) is performed. The value of sa_handler
(or, if the SA_SIGINFO
flag is set, the value of
sa_sigaction instead) indicates what action should be
performed when a signal arrives. A signal-specific default action may be
reset by setting sa_handler to
SIG_DFL
. Alternately, if the
SA_RESETHAND
flag is set the default action will be
reinstated when the signal is first posted. 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. If sig is
SIGCHLD
and sa_handler is set
to SIG_IGN
, the SA_NOCLDWAIT
flag (described below) is implied.
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, theSIGCHLD
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 theSIGCHLD
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 toECHILD
. 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_SIGINFO
- If this bit is set, the 2nd argument of the handler is set to be a pointer to a siginfo_t structure as described in ⟨sys/siginfo.h⟩. The siginfo_t structure is a part of IEEE Std 1003.1b (“POSIX.1b”). It provides much more information about the causes and attributes of the signal that is being delivered.
SA_RESTART
- 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. Restarting of pending calls is requested by setting theSA_RESTART
bit in sa_flags. The affected system calls include 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.
execve(2) 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 (cannot be caught or ignored) |
SIGBUS |
create core image | bus error |
SIGSEGV |
create core image | segmentation violation |
SIGSYS |
create core image | system call given invalid argument |
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 |
SIGTHR |
discard signal | thread AST |
RETURN VALUES
A 0 value indicates that the call succeeded. A -1 return value indicates an error occurred and errno is set to indicate the reason.
EXAMPLES
The handler routine can be declared:
void handler(int sig)
SA_SIGINFO
option is enabled, the canonical way
to declare it is:
void handler(int sig, siginfo_t *sip, struct sigcontext *scp)
Here sig is the signal number, into which
the hardware faults and traps are mapped. If the
SA_SIGINFO
option is set, sip
is a pointer to a siginfo_t
as described in
⟨sys/siginfo.h⟩. If
SA_SIGINFO
is not set, this pointer will be
NULL
instead. The function specified in
sa_sigaction will be called instead of the function
specified by sa_handler (Note that in some
implementations these are in fact the same). scp is a
pointer to the sigcontext structure (defined in
⟨signal.h⟩), used to restore the
context from before the signal.
ERRORS
sigaction
() 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
] - sig is not a valid signal number.
- [
EINVAL
] - An attempt is made to ignore or supply a handler for
SIGKILL
orSIGSTOP
.
SEE ALSO
kill(1), kill(2), ptrace(2), sigaltstack(2), sigprocmask(2), sigsuspend(2), wait(2), setjmp(3), sigblock(3), sigpause(3), sigsetops(3), sigvec(3), tty(4)
STANDARDS
The sigaction
() function conforms 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
. These
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
,
SA_NOCLDWAIT
, and SA_SIGINFO
flags are options commonly found in other operating systems. 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
(), _exit
(),
accept
(), access
(),
alarm
(), bind
(),
cfgetispeed
(),
cfgetospeed
(),
cfsetispeed
(),
cfsetospeed
(), chdir
(),
chmod
(), chown
(),
clock_gettime
(), close
(),
connect
(), creat
(),
dup
(), dup2
(),
execl
(), execle
(),
execv
(), execve
(),
fchdir
(), fchmod
(),
fchown
(), fcntl
(),
fork
(), fpathconf
(),
fstat
(), fsync
(),
ftruncate
(), futimes
(),
getegid
(), geteuid
(),
getgid
(), getgroups
(),
getpeername
(), getpgrp
(),
getpid
(), getppid
(),
getsockname
(), getsockopt
(),
getuid
(), kill
(),
link
(), listen
(),
lseek
(), lstate
(),
mkdir
(), mkfifo
(),
mknod
(), open
(),
pathconf
(), pause
(),
pipe
(), poll
(),
raise
(), read
(),
readlink
(), recv
(),
recvfrom
(), recvmsg
(),
rename
(), rmdir
(),
select
(), send
(),
sendmsg
(), sendto
(),
setgid
(), setpgid
(),
setsid
(), setsockopt
(),
setuid
(), shutdown
(),
sigaction
(), sigaddset
(),
sigdelset
(), sigemptyset
(),
sigfillset
(), sigismember
(),
signal
(), sigpause
(),
sigpending
(), sigprocmask
(),
sigsuspend
(), sleep
(),
socket
(), socketpair
(),
stat
(), symlink
(),
sysconf
(), tcdrain
(),
tcflow
(), tcflush
(),
tcgetattr
(), tcgetpgrp
(),
tcsendbreak
(), tcsetattr
(),
tcsetpgrp
(), time
(),
times
(), umask
(),
uname
(), unlink
(),
utime
(), utimes
(),
wait
(), waitpid
(),
write
().
ANSI C Interfaces:
strcat
(),
strcpy
(), strncat
(),
strncpy
(), and perhaps some others.
Extension Interfaces:
chflags
(),
fchflags
(), getresgid
(),
getresuid
(), setresgid
(),
setresuid
(), strlcat
(),
strlcpy
(), wait3
(),
wait4
().
In addition, access and updates to errno are guaranteed to be safe. Most 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, ideally of type volatile sig_atomic_t; most other actions are not safe.
Additionally, it is advised that signal handlers guard against modification of the external symbol errno by the above functions, saving it at entry and restoring it on return, thus:
void handler(int sig) { int save_errno = errno; ... errno = save_errno; }
The functions below are async-signal-safe in OpenBSD except when used with floating-point arguments or directives, but are probably unsafe on other systems:
snprintf
()- Safe.
vsnprintf
()- Safe.
syslog_r
()- Safe if the syslog_data struct is initialized as a local variable.