-.\" $NetBSD: sigaction.2,v 1.7 1995/10/12 15:41:16 jtc Exp $
-.\"
.\" Copyright (c) 1980, 1990, 1993
.\" The Regents of the University of California. All rights reserved.
.\"
.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
.\" SUCH DAMAGE.
.\"
-.\" @(#)sigaction.2 8.2 (Berkeley) 4/3/94
+.\" From: @(#)sigaction.2 8.2 (Berkeley) 4/3/94
+.\" $FreeBSD: src/lib/libc/sys/sigaction.2,v 1.48 2003/03/24 16:07:19 charnier Exp $
.\"
.Dd April 3, 1994
.Dt SIGACTION 2
.Sh NAME
.Nm sigaction
.Nd software signal facilities
+.Sh LIBRARY
+.Lb libc
.Sh SYNOPSIS
-.Fd #include <signal.h>
+.In signal.h
.Bd -literal
-struct sigaction {
- void (*sa_handler)();
- sigset_t sa_mask;
- int sa_flags;
+struct sigaction {
+ union {
+ void (*__sa_handler)(int);
+ void (*__sa_sigaction)(int, struct __siginfo *, void *);
+ } __sigaction_u; /* signal handler */
+ int sa_flags; /* see signal options below */
+ sigset_t sa_mask; /* signal mask to apply */
};
+
+#define sa_handler __sigaction_u.__sa_handler
+#define sa_sigaction __sigaction_u.__sa_sigaction
.Ed
.Ft int
-.Fn sigaction "int sig" "const struct sigaction *act" "struct sigaction *oact"
+.Fo sigaction
+.Fa "int sig"
+.Fa "const struct sigaction *restrict act"
+.Fa "struct sigaction *restrict oact"
+.Fc
.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
+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
.Em handler
-to which a signal is delivered, or specify that a signal is to be
+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.
so that signals are taken on a special
.Em "signal stack" .
.Pp
-Signal routines execute with the signal that caused their
+Signal routines normally execute with the signal that caused their
invocation
.Em blocked ,
but other signals may yet occur.
-A global
+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
before their first instructions.
The set of pending signals is returned by the
.Xr sigpending 2
-function.
+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),
+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
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
-call is made).
+.Xr 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 to be delivered, and
the signal mask associated with the handler to be invoked.
.Pp
-.Fn Sigaction
-assigns an action for a specific signal.
+The
+.Fn sigaction
+system call
+assigns an action for a signal specified by
+.Fa sig .
If
.Fa act
is non-zero, it
specifies an action
-.Pf ( Dv SIG_DFL ,
+.Dv ( SIG_DFL ,
.Dv SIG_IGN ,
or a handler routine) and mask
to be used when delivering the specified signal.
-If
+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
+Once a signal handler is installed, it normally remains installed
until another
.Fn sigaction
-call is made, or an
+system call is made, or an
.Xr execve 2
is performed.
A signal-specific default action may be reset by
of the signal are ignored and discarded.
.Pp
Options may be specified by setting
-.Em sa_flags .
-If the
-.Dv SA_NOCLDSTOP
-bit is set when installing a catching function
+.Va sa_flags .
+The meaning of the various bits is as follows:
+.Bl -tag -offset indent -width SA_RESETHANDXX
+.It Dv SA_NOCLDSTOP
+If this bit is set when installing a catching function
for the
.Dv SIGCHLD
signal,
.Dv SIGCHLD
signal will be generated only when a child process exits,
not when a child process stops.
-Further, if the
-.Dv SA_ONSTACK
-bit is set in
-.Em sa_flags ,
-the system will deliver the signal to the process on a
+.It Dv SA_NOCLDWAIT
+If this bit is set when calling
+.Fn sigaction
+for the
+.Dv SIGCHLD
+signal, the system will not create zombie processes when children of
+the calling process exit. If the calling process subsequently issues
+a
+.Xr 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
+.Er ECHILD .
+.It Dv SA_ONSTACK
+If this bit is set, the system will deliver the signal to the process
+on a
.Em "signal stack" ,
specified with
-.Xr sigstack 2 .
-.Pp
-Finally, the
-.Dv SA_SIGINFO
-option causes the 2nd argument for the signal handler to be a pointer
-to a
-.Em siginfo_t
-as described in
-.Pa <sys/siginfo.h> .
-The
-.Em siginfo_t
-is a part of
-.St -p1003.1b .
-and provides much more information about the causes and
-attributes of the signal that is being delivered.
+.Xr sigaltstack 2 .
+.It Dv SA_NODEFER
+If this bit is set, further occurrences of the delivered signal are
+not masked during the execution of the handler.
+.It Dv SA_RESETHAND
+If this bit is set, the handler is reset back to
+.Dv SIG_DFL
+at the moment the signal is delivered.
+.It Dv SA_RESTART
+See paragraph below.
+.It Dv SA_SIGINFO
+If this bit is set, the handler function is assumed to be pointed to by the
+.Dv sa_sigaction
+member of struct sigaction and should match the prototype shown above or as
+below in
+.Sx EXAMPLES .
+This bit should not be set when assigning
+.Dv SIG_DFL
+or
+.Dv SIG_IGN .
+.El
.Pp
If a signal is caught during the system calls listed below,
the call may be forced to terminate
with the error
-.Dv EINTR ,
+.Er 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
.Dv SA_RESTART
bit in
-.Ar sa_flags .
+.Va sa_flags .
The affected system calls include
.Xr open 2 ,
.Xr read 2 ,
all signals, the signal mask, the signal stack,
and the restart/interrupt flags are inherited by the child.
.Pp
-.Xr Execve 2
-reinstates the default
+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;
with names as in the include file
.Aq Pa signal.h :
.Bl -column SIGVTALARMXX "create core imagexxx"
-.It Sy " NAME " " Default Action " " Description"
+.It Sy "NAME Default Action Description"
.It Dv SIGHUP No " terminate process" " terminal line hangup"
.It Dv SIGINT No " terminate process" " interrupt program"
.It Dv SIGQUIT No " create core image" " quit program"
.It Dv SIGILL No " create core image" " illegal instruction"
.It Dv SIGTRAP No " create core image" " trace trap"
-.It Dv SIGABRT No " create core image" Xr abort 2
+.It Dv SIGABRT No " create core image" Ta Xr abort 3
call (formerly
.Dv SIGIOT )
.It Dv SIGEMT No " create core image" " emulate instruction executed"
.It Dv SIGKILL No " terminate process" " kill program"
.It Dv SIGBUS No " create core image" " bus error"
.It Dv SIGSEGV No " create core image" " segmentation violation"
-.It Dv SIGSYS No " create core image" " system call given invalid argument"
+.It Dv SIGSYS No " create core image" " non-existent system call invoked"
.It Dv SIGPIPE No " terminate process" " write on a pipe with no reader"
.It Dv SIGALRM No " terminate process" " real-time timer expired"
.It Dv SIGTERM No " terminate process" " software termination signal"
.It Dv SIGUSR2 No " terminate process" " User defined signal 2"
.El
.Sh NOTE
-The mask specified in
+The
+.Fa sa_mask
+field specified in
.Fa act
is not allowed to block
.Dv SIGKILL
or
.Dv SIGSTOP .
-This is done silently by the system.
-.Sh RETURN VALUES
-A 0 value indicated that the call succeeded. A \-1 return value
-indicates an error occurred and
+Any attempt to do so will be silently ignored.
+.Pp
+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:
+.Pp
+Base Interfaces:
+.Pp
+.Fn _exit ,
+.Fn access ,
+.Fn alarm ,
+.Fn cfgetispeed ,
+.Fn cfgetospeed ,
+.Fn cfsetispeed ,
+.Fn cfsetospeed ,
+.Fn chdir ,
+.Fn chmod ,
+.Fn chown ,
+.Fn close ,
+.Fn creat ,
+.Fn dup ,
+.Fn dup2 ,
+.Fn execle ,
+.Fn execve ,
+.Fn fcntl ,
+.Fn fork ,
+.Fn fpathconf ,
+.Fn fstat ,
+.Fn fsync ,
+.Fn getegid ,
+.Fn geteuid ,
+.Fn getgid ,
+.Fn getgroups ,
+.Fn getpgrp ,
+.Fn getpid ,
+.Fn getppid ,
+.Fn getuid ,
+.Fn kill ,
+.Fn link ,
+.Fn lseek ,
+.Fn mkdir ,
+.Fn mkfifo ,
+.Fn open ,
+.Fn pathconf ,
+.Fn pause ,
+.Fn pipe ,
+.Fn raise ,
+.Fn read ,
+.Fn rename ,
+.Fn rmdir ,
+.Fn setgid ,
+.Fn setpgid ,
+.Fn setsid ,
+.Fn setuid ,
+.Fn sigaction ,
+.Fn sigaddset ,
+.Fn sigdelset ,
+.Fn sigemptyset ,
+.Fn sigfillset ,
+.Fn sigismember ,
+.Fn signal ,
+.Fn sigpending ,
+.Fn sigprocmask ,
+.Fn sigsuspend ,
+.Fn sleep ,
+.Fn stat ,
+.Fn sysconf ,
+.Fn tcdrain ,
+.Fn tcflow ,
+.Fn tcflush ,
+.Fn tcgetattr ,
+.Fn tcgetpgrp ,
+.Fn tcsendbreak ,
+.Fn tcsetattr ,
+.Fn tcsetpgrp ,
+.Fn time ,
+.Fn times ,
+.Fn umask ,
+.Fn uname ,
+.Fn unlink ,
+.Fn utime ,
+.Fn wait ,
+.Fn waitpid ,
+.Fn write .
+.Pp
+Realtime Interfaces:
+.Pp
+.Fn aio_error ,
+.Fn clock_gettime ,
+.Fn sigpause ,
+.Fn timer_getoverrun ,
+.Fn aio_return ,
+.Fn fdatasync ,
+.Fn sigqueue ,
+.Fn timer_gettime ,
+.Fn aio_suspend ,
+.Fn sem_post ,
+.Fn sigset ,
+.Fn timer_settime .
+.Pp
+ANSI C Interfaces:
+.Pp
+.Fn strcpy ,
+.Fn strcat ,
+.Fn strncpy ,
+.Fn strncat ,
+and perhaps some others.
+.Pp
+Extension Interfaces:
+.Pp
+.Fn strlcpy ,
+.Fn strlcat .
+.Pp
+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.
+.Pp
+Also, it is good practice to make a copy of the global variable
.Va errno
-is set to indicated the reason.
-.Sh EXAMPLE
-The handler routine can be declared:
-.Bd -literal -offset indent
-void handler(sig, sip, scp)
-int sig;
-siginfo_t *sip;
-struct sigcontext *scp;
-.Ed
+and restore it before returning from the signal handler.
+This protects against the side effect of
+.Va errno
+being set by functions called from inside the signal handler.
+.Sh RETURN VALUES
+.Rv -std sigaction
+.Sh EXAMPLES
+There are three possible prototypes the handler may match:
+.Bl -tag -offset indent -width short
+.It ANSI C:
+.Ft void
+.Fn handler int ;
+.It POSIX SA_SIGINFO:
+.Ft void
+.Fn handler int "siginfo_t *info" "ucontext_t *uap" ;
+.El
+.Pp
+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
+.Dv sa_sigaction
+member of
+.Dv struct sigaction .
+Note that you should not assign SIG_DFL or SIG_IGN this way.
+.Pp
+If the SA_SIGINFO flag is not set, the handler function should match
+either the ANSI C or traditional
+.Bx
+prototype and be pointed to by
+the
+.Dv sa_handler
+member of
+.Dv struct sigaction .
+In practice,
+.Fx
+always sends the three arguments of the latter and since the ANSI C
+prototype is a subset, both will work.
+The
+.Dv sa_handler
+member declaration in
+.Fx
+include files is that of ANSI C (as required by POSIX),
+so a function pointer of a
+.Bx Ns -style
+function needs to be casted to
+compile without warning.
+The traditional
+.Bx
+style is not portable and since its capabilities
+are a full subset of a SA_SIGINFO handler,
+its use is deprecated.
.Pp
-Here
+The
.Fa sig
-is the signal number, into which the hardware faults and traps are
-mapped.
-If the
-.Dv SA_SIGINFO
-option is set,
-.Fa sip
-is a pointer to a
-.Dv siginfo_t
-as described in
-.Pa <sys/siginfo.h> .
-If
-.Dv SA_SIGINFO
-is not set, this is NULL.
-.Fa Scp
-is a pointer to the
-.Fa sigcontext
-structure (defined in
-.Aq Pa signal.h ) ,
-used to restore the context from before the signal.
+argument is the signal number, one of the
+.Dv SIG...
+values from <signal.h>.
+.Pp
+The
+.Fa code
+argument of the
+.Bx Ns -style
+handler and the
+.Dv si_code
+member of the
+.Dv info
+argument to a SA_SIGINFO handler contain a numeric code explaining the
+cause of the signal, usually one of the
+.Dv SI_...
+values from
+<sys/signal.h> or codes specific to a signal, i.e. one of the
+.Dv FPE_...
+values for SIGFPE.
+.Pp
+The
+.Fa uap
+argument to a POSIX SA_SIGINFO handler points to an instance of
+ucontext_t.
.Sh ERRORS
-.Fn Sigaction
+The
+.Fn sigaction
+system call
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
+or
.Fa oact
points to memory that is not a valid part of the process
address space.
+.\" ===========
.It Bq Er EINVAL
-.Fa Sig
+The
+.Fa sig
+argument
is not a valid signal number.
+.\" ===========
.It Bq Er EINVAL
An attempt is made to ignore or supply a handler for
.Dv SIGKILL
or
.Dv SIGSTOP .
+.\" ===========
+.It Bq Er EINVAL
+An attempt was made to set the action to SIG_DFL
+for a signal that cannot be caught or ignored (or both).
.El
.Sh STANDARDS
The
-.Nm sigaction
-function is defined by
-.St -p1003.1-88 .
+.Fn sigaction
+system call is expected to conform to
+.St -p1003.1-90 .
The
.Dv SA_ONSTACK
and
and
.Dv SIGINFO .
Those signals are available on most
-.Tn BSD Ns \-derived
+.Bx Ns \-derived
systems.
+The
+.Dv SA_NODEFER
+and
+.Dv SA_RESETHAND
+flags are intended for backwards compatibility with other operating
+systems. The
+.Dv SA_NOCLDSTOP ,
+and
+.Dv SA_NOCLDWAIT
+.\" and
+.\" SA_SIGINFO
+flags are featuring options commonly found in other operating systems.
.Sh SEE ALSO
.Xr kill 1 ,
-.Xr ptrace 2 ,
.Xr kill 2 ,
-.Xr sigaction 2 ,
-.Xr sigprocmask 2 ,
-.Xr sigsuspend 2 ,
+.Xr ptrace 2 ,
+.Xr sigaltstack 2 ,
.Xr sigblock 2 ,
-.Xr sigsetmask 2 ,
.Xr sigpause 2 ,
-.Xr sigstack 2 ,
-.Xr sigvec 3 ,
+.Xr sigpending 2 ,
+.Xr sigprocmask 2 ,
+.Xr sigsetmask 2 ,
+.Xr sigsuspend 2 ,
+.Xr sigvec 2 ,
+.Xr wait 2 ,
+.Xr fpsetmask 3 ,
.Xr setjmp 3 ,
.Xr siginterrupt 3 ,
.Xr sigsetops 3 ,
+.Xr ucontext 3 ,
.Xr tty 4
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