sigaction(2) DG/UX 5.4.2 sigaction(2)
NAME
sigaction - examine and change signal action
SYNOPSIS
#include <signal.h>
int sigaction (sig, act, oact)
int sig;
const struct sigaction *act;
struct sigaction *oact;
where:
sig A signal number.
act NULL, or a new action to be installed for sig.
oact NULL, or the current action associated with sig. If act is
not NULL and the call is successful, oact will be replaced
by act.
DESCRIPTION
The sigaction() function allows the calling process to examine or
specify (or both) the action to be associated with a specific signal.
The argument sig specifies the signal; acceptable values are defined
in <signal.h>.
The structure sigaction, used to describe an action to be taken, is
defined in the header <signal.h> and includes the following members:
Member Member
Type Name Description
void (*)() sa_handler SIG_DFL, SIG_IGN, or pointer
to a function.
sigset_t sa_mask Additional set of signals to
be blocked during execution of
signal-catching function.
int sa_flags Special flags to affect
behavior of signal.
If the argument act is not NULL, it points to a structure specifying
the action to be associated with the specified signal. If the
argument oact is not NULL the action previously associated with the
signal is stored in the location pointed to by the argument oact. If
the argument act is NULL signal handling is unchanged by this
function call; thus, the call can be used to enquire about the
current handling of a given signal.
The sahandler field of the sigaction structure identifies the action
to be associated with the specified signal. It may have any of the
values specified above.
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If the sahandler field specifies a signal-catching function, the
samask field identifies a set of signals that shall be added to the
process's set of blocked signals before the signal-catching function
is invoked. In addition, the signal that caused the handler to be
invoked will be added to the set of blocked signals unless the
SA_NODEFER flag has been specified (see the saflags description
below). The SIGKILL and SIGSTOP signals shall not be added to the
signal mask using this mechanism; this restriction shall be enforced
by the system without causing an error to be indicated.
The saflags field can be used to modify the delivery of the
specified signal.
The following flags, defined in the header <signal.h>, can be set in
saflags:
Symbolic
Constant Description
SA_ONSTACK If set and the signal is caught, and an
alternate signal stack has been
declared, the signal is delivered to the
calling process using the alternate
stack. Otherwise, the signal is
delivered on the same stack as the main
program.
SA_RESETHAND If set and the signal is caught, the
action of the signal is reset to
SIG_DFL. (Note: SIGILL, SIGTRAP, and
SIGPWR cannot be automatically reset
when delivered; this restriction shall
be enforced by the system without
causing an error to be indicated.)
SA_NODEFER If set and the signal is caught, sig
will not be automatically blocked while
the handler is active.
SA_RESTART If set and the signal is caught, a
restartable system call that is
interrupted by the execution of the
signal's handler will be transparently
restarted when the handler finishes. If
this flag is not set, a system call that
is interrupted will return EINTR.
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SA_SIGINFO If this flag is not set and the signal
is caught, sig is passed as the only
argument to the signal handling
function. If this flag is set and the
signal is caught, two additional
arguments will be passed to the signal
handling function. If the second
argument is not equal to NULL, it will
point to an object of type siginfo_t,
which will explain the reason the signal
was generated (see siginfo.h). The
third argument will point to an object
of type ucontext_t, which will describe
the receiving process' context at the
time it received the signal (see
ucontext.h).
SA_NOCLDWAIT If set and sig equals SIGCHLD, the
system will clean up after the calling
processes dead children. If the calling
process subsequently calls wait(), it
will block until all of its processes
terminate and then return a value of -1
with errno set to ECHLD.
SA_NOCLDSTOP If set and sig equals SIGCHLD, sig will
not be sent to the calling process when
its child processes stop.
When a signal is caught by a signal-catching function installed by
the sigaction() function, a new signal mask is calculated and
installed for the duration of the signal-catching function (or until
a call to either the sigprocmask() or sigsuspend() function is made).
This mask is formed by taking the union of the current signal mask
and the value of the samask for the signal being delivered, and then
including the signal being delivered (unless the SA_NODEFER flag is
set, as described above). If and when the user's signal handler
returns normally, the original signal mask is restored.
Once an action is installed for a specific signal, it remains
installed until another action is explicitly requested (by another
call to the sigaction() function), or until one of the exec()
functions is called. This behavior may be modified by using the
SA_RESTART flag as described above.
If the previous action for sig had been established by the signal()
function, defined in the C Standard, the values of the fields
returned in the structure pointed to by oact are unspecified, and in
particular oact->svhandler is not necessarily the same value passed
to the signal() function. However, if a pointer to the same
structure or a copy thereof is passed to a subsequent call to the
sigaction() function via the act argument, handling of the signal
shall be as if the original call to the signal() function were
repeated.
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If the sigaction() function fails, no new signal handler is
installed.
RETURN VALUE
0 Successful completion.
-1 An error occurred. errno is set to indicate the error.
DIAGNOSTICS
If any of the following conditions occur, the sigaction() function
shall return -1 and set errno to the corresponding value:
EINVAL The value of the sig argument is an invalid or unsupported
signal number, or an attempt was made to catch a signal
that cannot be caught or to ignore a signal that cannot be
ignored. See <signal.h>.
EFAULT act or oact points to an invalid location in the user's
address space.
SEE ALSO
kill(2), sigprocmask(2), sigsuspend(2), sigsetops(3C), <signal.h>.
COPYRIGHTS
Portions of this text are reprinted from IEEE Std 1003.1-1988,
Portable Operating System Interface for Computer Environment,
copyright © 1988 by the Institute of Electrical and Electronics
Engineers, Inc., with the permission of the IEEE Standards
Department. To purchase IEEE Standards, call 800/678-IEEE.
In the event of a discrepancy between the electronic and the original
printed version, the original version takes precedence.
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