Table of Contents
struct sigaction
{ void (*sa_handler)();
sigset_t sa_mask;
int sa_flags;
};
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 to which a signal
is delivered, or specify that a signal is to be 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 in which case its delivery is postponed until it
is 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 routines execute with the signal that caused their invocation
but other signals may yet occur. A global 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 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 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 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 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. assigns an action for a specific signal. If is non-zero,
it specifies an action or a handler routine) and mask to be used when
delivering the specified signal. If is non-zero, the previous handling
information for the signal is returned to the user. Once a signal handler
is installed, it remains installed until another call is made, or an
is performed. A signal-specific default action may be reset by setting
to 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 is 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 is set to current and pending
instances of the signal are ignored and discarded. Options may be specified
by setting If the bit is set when installing a catching function for
the signal, the signal will be generated only when a child process exits,
not when a child process stops. Further, if the bit is set in the system
will deliver the signal to the process on a specified with If a signal
is caught during the system calls listed below, the call may be forced
to terminate with the error 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 bit in The affected system calls include
and on a communications channel or a slow device (such as a terminal,
but not a regular file) and during a or However, calls that have already
committed are not restarted, but instead return a partial success (for
example, a short read count). After a or all signals, the signal mask,
the signal stack, and the restart/interrupt flags are inherited by the
child. 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 call (formerly
is possible on a descriptor (see
The mask specified
in is not allowed to block or This is done silently by the system.
A 0 value indicated that the call succeeded. A -1 return value indicates
an error occurred and is set to indicated the reason.
The handler
routine can be declared: void handler(sig, code, scp) int sig, code; struct
sigcontext *scp; Here is the signal number, into which the hardware
faults and traps are mapped. is a parameter that is either a constant or
the code provided by the hardware. is a pointer to the structure (defined
in used to restore the context from before the signal.
will fail
and no new signal handler will be installed if one of the following occurs:
Either or points to memory that is not a valid part of the process
address space. is not a valid signal number. An attempt is made to ignore
or supply a handler for or
The function is defined by The
and flags are Berkeley extensions, as are the signals, and
Those signals are available on most systems.
Table of Contents