Manpages

NAME

FBB::Signal - Signal Handler

SYNOPSIS

#include <bobcat/signal>
Linking option: -lbobcat

DESCRIPTION

Signals have the well known drawback that signals arrive free of context. E.g., assume a program runs a flow control loop like this:

void Class::run()
{
while (d_continue)
handleTasks();
cleanup();
}

then if the program needs to recognize a termination signal then the typical signal handler looks like this:

void signalHandler(int signal)
{
// perform required actions
}

Since the signalHandler is called asynchronically, there is no context available, and the usual way of communicating between objects and signal handlers is via static variables, like this:

// declared as static bool s_continue;
bool Class::s_continue = true;

void Class::run()
{
while (s_continue)
handleTasks();
cleanup();
}

// declared as static void signalHander(int signal);
void Class::signalHandler(int signal)
{
s_continue = false;
}

The class Signal allows the signal handler to operate in the context of a class. The advantage of this is that static data members are no longer required and that the signal may be used to control data members of individual objects.

The signal is now handled by an object, whose class must define a member

void signalHandler(size_t signum) override;

and this function is responsible for handling the received signal. Since it is a member function it may affect its object’s local variables and it may call its object’s member functions. Static data members are not required anymore (see below for an example).

Note that, as the signal may arrive at unpredicable times data members that can be modified by signalHandler should be declared using the volatile modifier. Moreover, data that can be modified by the signalHandler member and by other class members should be protected by mutexes (cf. the C++-11 class std::mutex or pthread_mutex_lock(3posix)).

NAMESPACE

FBB
All constructors, members, operators and manipulators, mentioned in this man-page, are defined in the namespace FBB.

INHERITS FROM

Signal is not derived from other classes, but the classes for which signals must be handled by Signal must themselves publicly be derived from the class FBB::SignalHandler and must implement a member

void signalHandler(size_t signum) override;

handling the received signal.

CONSTRUCTORS AND OVERLOADED OPERATORS

Signal is defined as a singleton, and does not offer public or protected constructors, nor does it offer overloaded operators.

STATIC MEMBER FUNCTION

This static member can be used to access a reference to the program’s single Signal object.

MEMBER FUNCTIONS

All of Signal’s member functions can only be called through a reference to the program’s Signal object, returning a reference to the program’s single Signal object:

SignalHandler object is activated on arrival of signal signum. If multiple SignalHandler objects must be called then multiple Signal::add calls can be provided, and the various SignalHandler::signalHandler members are called in the same sequence as their respective Signal::add calls. If one of the earlier signalHandler members terminates the program then later signalHandler members are not activated anymore. If Signal::add is called by, e.g., an object’s constructor, then its destructor should call Signal::remove to prevent the object’s signal handler from being called after its destruction.

SignalHandler object for signal signum is removed from the Signal object. It is the responsibility of object to deregister itself from Signal just before object goes out of scope. Objects can only deregister themselves if they’ve previously registered themselves using add.

Any previously installed SignalHandler object is no longer activated on arrival of signal signum. In addition, if possible, signal signum is completely ignored (some signals cannot be caught, blocked, of ignored, like SIGKILL and SIGSTOP (cf. signal(7))).

Any previously installed SignalHandler object is no longer activated on arrival of signal signum. In addition, the default action the program takes on arrival of signal signum is reinstalled (cf. signal(7)).

If the signum value that is passed to Signal’s members is not a defined signal value, then an FBB::Exception exception is thrown.

EXAMPLE

#include <sys/types.h>
#include <unistd.h>

#include <iostream>

#include "../signal"

class SignalDemo: public FBB::SignalHandler
{
volatile size_t d_signal;
volatile bool d_continue;
pid_t d_pid;

public:
SignalDemo();
void run();

private:
void signalHandler(size_t signum) override;
};

using namespace std;
using namespace FBB;

SignalDemo::SignalDemo()
:
d_signal(0),
d_continue(true),
d_pid(getpid())
{}

void SignalDemo::run()
{
while (d_continue)
{
cout << "Send a SIGINT (2) or SIGTERM (15) ... to process " <<
d_pid << endl;

sleep(1);
}
cout << "Ending ’run’ after receiving signal " << d_signal << endl;
}

void SignalDemo::signalHandler(size_t signal)
{
if (signal == SIGINT)
cout << "Process " << d_pid << " received SIGINT" << endl;
else if (signal == SIGTERM)
{
cout << "Process " << d_pid << " received SIGTERM" << endl;
d_signal = SIGTERM;
d_continue = false;
}
}

int main()
{
SignalDemo signalDemo;

Signal::instance().add(SIGINT, signalDemo);
Signal::instance().add(SIGTERM, signalDemo);

signalDemo.run();
}

FILES

bobcat/signal - defines the class interface

SEE ALSO

bobcat(7), pthread_mutex_lock(3posix), signal(7),
and the C++-11 class std::mutex.

BUGS

None Reported.

BOBCAT PROJECT FILES

BOBCAT

Bobcat is an acronym of ’Brokken’s Own Base Classes And Templates’.

COPYRIGHT

This is free software, distributed under the terms of the GNU General Public License (GPL).

AUTHOR

Frank B. Brokken (f.b.brokken [AT] rug.nl).