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.\"	@(#)sigaction.2	8.2 (Berkeley) 4/3/94
.\"
.Dd May 22, 2018
.Dt SIGACTION 2
.Os
.Sh NAME
.Nm sigaction
.Nd software signal facilities
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In signal.h
.Ft int
.Fn sigaction "int sig" "const struct sigaction * restrict act" "struct sigaction * restrict oact"
.Sh 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 blocked from further occurrence, the current process
context is saved, and a new one is built.
A process may specify a
.Em handler
to which a signal is delivered, or specify that a signal is to be
.Em 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
.Em blocked ,
in which case its delivery is postponed until it is
.Em 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
.Em "signal stack" .
.Pp
Signal routines execute with the signal that caused their
invocation
.Em blocked ,
but other signals may yet occur.
A global
.Em "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
.Xr sigprocmask 2
call, or when a signal is delivered to the process.
Signal masks are represented using the
.Em sigset_t
type; the
.Xr sigsetops 3
interface is used to modify such data.
.Pp
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
.Em 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
.Xr 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.
.Pp
.Em "struct sigaction"
includes the following members:
.Bd -literal -offset indent
void 	  (*sa_sigaction)(int sig, siginfo_t *info, void *ctx);
void      (*sa_handler)(int sig);
sigset_t  sa_mask;
int       sa_flags;
.Ed
.Pp
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
.Xr sigprocmask 2
call is made).
This mask is formed by taking the union of the current signal mask,
the signal to be delivered, and
the signal mask associated with the handler to be invoked,
.Em sa_mask .
.Pp
.Fn sigaction
assigns an action for a specific signal.
If
.Fa act
is non-zero, it
specifies an action
.Pf ( Dv SIG_DFL ,
.Dv SIG_IGN ,
or a handler routine) and mask
to be used when delivering the specified signal.
If
.Fa oact
is non-zero, the previous handling information for the signal
is returned to the user.
.Pp
Once a signal handler is installed, it remains installed
until another
.Fn sigaction
call is made, or an
.Xr execve 2
is performed.
A signal-specific default action may be reset by
setting
.Fa sa_handler
to
.Dv 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
.Fa sa_handler
is set to
.Dv 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
.Fa sa_handler
is set to
.Dv SIG_IGN ,
current and pending instances
of the signal are ignored and discarded.
.Pp
Options may be specified by setting
.Em sa_flags .
.Bl -tag -width SA_NOKERNINFO
.It Dv SA_NODEFER
If set, then the signal that caused the handler to be executed is not added
to the list of block signals.
Please note that
.Fa sa_mask
takes precedence over
.Dv SA_NODEFER ,
so that if the specified signal is blocked in
.Fa sa_mask ,
then
.Dv SA_NODEFER
will have no effect.
.It Dv SA_NOCLDSTOP
If set when installing a catching function
for the
.Dv SIGCHLD
signal,
the
.Dv SIGCHLD
signal will be generated only when a child process exits,
not when a child process stops or continues.
.It Dv SA_NOCLDWAIT
If set, the system will not create a zombie when the child exits,
but the child process will be automatically waited for.
The same effect can be achieved by setting the signal handler for
.Dv SIGCHLD
to
.Dv SIG_IGN .
.It Dv SA_ONSTACK
If set, the system will deliver the signal to the process on a
.Em "signal stack" ,
specified with
.Xr sigaltstack 2 .
.It Dv SA_RESETHAND
If set, the default action will be reinstated when the signal
is first posted.
.It Dv SA_RESTART
Normally, if a signal is caught during the system calls listed below,
the call may be forced to terminate
with the error
.Er 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 the
.Dv SA_RESTART
bit in
.Ar sa_flags .
The affected system calls include
.Xr open 2 ,
.Xr read 2 ,
.Xr write 2 ,
.Xr sendto 2 ,
.Xr recvfrom 2 ,
.Xr sendmsg 2
and
.Xr recvmsg 2
on a communications channel or a slow device (such as a terminal,
but not a regular file)
and during a
.Xr wait 2
or
.Xr ioctl 2 .
However, calls that have already committed are not restarted,
but instead return a partial success (for example, a short read count).
.Pp
After a
.Xr fork 2
or
.Xr vfork 2
all signals, the signal mask, the signal stack,
and the restart/interrupt flags are inherited by the child.
.Pp
The
.Xr 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.
.Pp
See
.Xr signal 7
for comprehensive list of supported signals.
.It Dv SA_SIGINFO
If set, the signal handler function will receive additional information
about the caught signal.
An alternative handler that gets passed additional arguments will
be called which is named
.Fa sa_sigaction .
The
.Ar sig
argument of this handler contains the signal number that was caught.
The
.Ar info
argument contains additional signal specific information which
is listed in
.Xr siginfo 2 .
The
.Ar ctx
argument
is a pointer to the
.Xr ucontext 2
context where the signal handler will return to.
.It Dv SA_NOKERNINFO
This flag is relevant only to
.Dv SIGINFO ,
and turns off printing kernel messages on the tty.
It is similar to the
.Dv NOKERNINFO
flag in
.Xr termios 4 .
.El
.Pp
If the signal handler is called due to signal delively resulting from reasons
other than direct calls to
.Xr kill 2
or
.Xr _lwp_kill 2
or indirect calls to
.Xr _lwp_kill 2
via
.Xr abort 3
or
.Xr raise 3
any activity (such as calling functions or assigning variables in the global
or static scopes) other than setting a variable of the type
.Vt volatile sig_atomic_t
is undefined.
.Ss Signal-safe functions
Only functions that are guaranteed to be async-signal-safe can safely
be used in signal handlers.
These are functions that are either reentrant or non-interruptible.
(These functions are also the only functions that may be used in a
child process after doing
.Xr fork 2
in a threaded program.)
.Pp
The following functions are async-signal-safe.
Any function not listed
below is unsafe to use in signal handlers.
.Pp
.Xr _Exit 2 ,
.Xr _exit 2 ,
.Xr abort 3 ,
.Xr accept 2 ,
.Xr access 2 ,
.\" .Xr aio_error
.\" .Xr aio_return
.\" .Xr aio_suspend
.Xr alarm 3 ,
.Xr bind 2 ,
.Xr cfgetispeed 3 ,
.Xr cfgetospeed 3 ,
.Xr cfsetispeed 3 ,
.Xr cfsetospeed 3 ,
.Xr chdir 2 ,
.Xr chmod 2 ,
.Xr chown 2 ,
.Xr clock_gettime 2 ,
.Xr close 2 ,
.Xr connect 2 ,
.Xr creat 3 ,
.Xr dup 2 ,
.Xr dup2 2 ,
.Xr execle 3 ,
.Xr execve 2 ,
.Xr fchmod 2 ,
.Xr fchown 2 ,
.Xr fcntl 2 ,
.Xr fdatasync 2 ,
.Xr fork 2 ,
.Xr fpathconf 2 ,
.Xr fstat 2 ,
.Xr fsync 2 ,
.Xr ftruncate 2 ,
.Xr getegid 2 ,
.Xr geteuid 2 ,
.Xr getgid 2 ,
.Xr getgroups 2 ,
.Xr getpeername 2 ,
.Xr getpgrp 2 ,
.Xr getpid 2 ,
.Xr getppid 2 ,
.Xr getsockname 2 ,
.Xr getsockopt 2 ,
.Xr getuid 2 ,
.Xr kill 2 ,
.Xr link 2 ,
.Xr listen 2 ,
.Xr lseek 2 ,
.Xr lstat 2 ,
.Xr mkdir 2 ,
.Xr mkfifo 2 ,
.Xr open 2 ,
.Xr pathconf 2 ,
.Xr pause 3 ,
.Xr pipe 2 ,
.Xr poll 2 ,
.\" .Xr posix_trace_event 2
.\" .Xr pselect 2
.Xr pthread_mutex_unlock 3 ,
.Xr raise 3 ,
.Xr read 2 ,
.Xr readlink 2 ,
.Xr recv 2 ,
.Xr recvfrom 2 ,
.Xr recvmsg 2 ,
.Xr rename 2 ,
.Xr rmdir 2 ,
.Xr select 2 ,
.Xr sem_post 3 ,
.Xr send 2 ,
.Xr sendmsg 2 ,
.Xr sendto 2 ,
.Xr setgid 2 ,
.Xr setpgid 2 ,
.Xr setsid 2 ,
.Xr setsockopt 2 ,
.Xr setuid 2 ,
.Xr shutdown 2 ,
.Xr sigaddset 3 ,
.Xr sigdelset 3 ,
.Xr sigemptyset 3 ,
.Xr sigfillset 3 ,
.Xr sigismember 3 ,
.Xr sleep 3 ,
.Xr signal 3 ,
.Xr sigpause 3 ,
.Xr sigpending 2 ,
.Xr sigprocmask 2 ,
.\" .Xr sigqueue
.Xr sigset 3 ,
.Xr sigsuspend 2 ,
.Xr sockatmark 3 ,
.Xr socket 2 ,
.Xr socketpair 2 ,
.Xr stat 2 ,
.Xr symlink 2 ,
.Xr sysconf 3 ,
.Xr tcdrain 3 ,
.Xr tcflow 3 ,
.Xr tcflush 3 ,
.Xr tcgetattr 3 ,
.Xr tcgetpgrp 3 ,
.Xr tcsendbreak 3 ,
.Xr tcsetattr 3 ,
.Xr tcsetpgrp 3 ,
.Xr time 3 ,
.Xr timer_getoverrun 2 ,
.Xr timer_gettime 2 ,
.Xr timer_settime 2 ,
.Xr times 3 ,
.Xr umask 2 ,
.Xr uname 3 ,
.Xr unlink 2 ,
.Xr utime 3 ,
.Xr wait 2 ,
.Xr waitpid 2 ,
.Xr write 2 .
.Sh NOTES
The mask specified in
.Fa act
is not allowed to block
.Dv SIGKILL
or
.Dv SIGSTOP .
This is enforced silently by the system.
.Sh RETURN VALUES
A 0 value indicates that the call succeeded.
A \-1 return value indicates an error occurred and
.Va errno
is set to indicate the reason.
.Sh ERRORS
.Fn sigaction
will fail and no new signal handler will be installed if one
of the following occurs:
.Bl -tag -width Er
.It Bq Er EFAULT
Either
.Fa act
or
.Fa oact
points to memory that is not a valid part of the process
address space.
.It Bq Er EINVAL
.Fa sig
is not a valid signal number;
or an attempt is made to ignore or supply a handler for
.Dv SIGKILL
or
.Dv SIGSTOP ;
or the
.Em sa_flags
word contains bits other than
.Dv SA_NOCLDSTOP ,
.Dv SA_NOCLDWAIT ,
.Dv SA_NODEFER ,
.Dv SA_ONSTACK ,
.Dv SA_RESETHAND ,
.Dv SA_RESTART ,
and
.Dv SA_SIGINFO .
.El
.Sh SEE ALSO
.Xr kill 1 ,
.Xr kill 2 ,
.Xr ptrace 2 ,
.Xr sigaltstack 2 ,
.Xr sigprocmask 2 ,
.Xr sigstack 2 ,
.Xr sigsuspend 2 ,
.Xr fpgetmask 3 ,
.Xr fpsetmask 3 ,
.Xr setjmp 3 ,
.Xr sigblock 3 ,
.Xr siginterrupt 3 ,
.Xr signal 3 ,
.Xr sigpause 3 ,
.Xr sigsetmask 3 ,
.Xr sigsetops 3 ,
.Xr tty 4
.Sh STANDARDS
The
.Fn sigaction
function conforms to
.St -p1003.1-90 .
The
.Dv SA_ONSTACK
and
.Dv SA_RESTART
flags are Berkeley extensions, available on most
.Bx Ns \-derived
systems.