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Raise and Signal - C++ Example

Updated on June 13, 2016

1. Signal and Raise Introduction

The signal and raise can be used in combination to perform interrupt on the C++ program. When a program is running, the interrupt can be done by making a call to the Raise function. Before we make a call to the raise function, we should register the Signal handling function. The function Signal() can be used to register the signal handling function.

The Signal function will register the signal type with the Signal handler function. Signal types are shown in the below screenshot:

Signal Types
Signal Types | Source

In our example, we are going to use SIGINT (Signal Interrupt) and its handler function.

2. The Signal Function & Handler

The signal function takes is used to register the signal type with the corresponding handler function. The signature of the signal and handler function is shown in the below picture:

Signal and Handler function
Signal and Handler function | Source

In the above screenshot, it can be observed that the first parameter passed is the type of the signal and the second parameter passed is the actual handler function. The handler function should accept an integer as a parameter and return nothing (i.e.) returns void.

3. Raise Function

The raise function is used to Raise the signal by specifying the signal type number as an input parameter. The signature of the Raise function is given below:

raise( int);

Remember, in the previous section, we saw the Signal function which registers a signal number with a handler function. When we call the raise function, the registered handler function gets called. Now let us explore the signal number SIGINT, used to specify program interrupted. The handle will get called when the Raise function is called with SIGINT.

4. Example Program

The Signal handling functions are available in the CSignal.h and hence this header file is included in the program. The code is given below:


//Sample 01: Include statements
#include <iostream>
#include <csignal>
using namespace std;

Once the required header file is included, a delay method is written. This method will print periods (.) in the console output window. The delay for each period depends on the processor speed. The delay function is shown below:

//Sample 02: The delay function which makes the program a long running one
void Delay(int delayNo)
{
	for (int i = 1; i < (delayNo * 100000000)+1; i++)
	{
		if (i % 100000000  == 0)
			printf(".");
		else
			continue;
	}
}

In the main function, using the call to a signal function, the interrupt handler function “InterruptHandler” is registered. The first parameter SIGINT passed-in makes an association of the signal SIGINT with the handler InterruptHandler. After this, the main program makes a call to the delay, and thereby periods are printed in the console window. Finally, we make a call to the function raise by passing the SIGINT as a parameter. This will make the program get interrupted and the final printf message will never get printed. Code is given below:


//Sample 04: Program main which makes a delay of 20 units
void main ()
{
    signal(SIGINT, InterruptHandler);  
	Delay(20);
	raise(SIGINT);
	printf("I may not get a chance to get printted");
}

The interrupt handler function will print the message stating the program is interrupted and quitting. Also, it makes a call to the exit thereby removing the process from the Operating system. The interrupt handler function is given below:

//Sample 03: Signal Handler function
void InterruptHandler( int IntNumber )
{
	printf("\nProgram Interrupted. Interrupt No: %d\n", IntNumber);
	printf("Quitting...\n");
	exit(IntNumber);  
}

Now when you run the program you will get the following output:

....................

Program Interrupted. Interrupt No: 2

Quitting...

In the above output, you can see all the 20 periods and after that, we are explicitly making program interrupt call to quit the application.

5. Closing Notes

Here we saw the example for the Signal "SIGINT" and processing that signal with a signal, raise and the corresponding handler function. We can also process other signals (Shown in Picture 1) the same way we did it for SIGINT.

You can also test the program output by interrupting the program through ctrl+c when the program is printing the periods. Pressing control ctrl+c when the program is running is also a kind of interrupt (Not an interrupt through raise call) and this interrupt is triggered by the user. Here, in this case also, we can make sure that interrupt handler gets called.

The below screen shot shows user-interrupt and output:


Program Iterrupt

Ctrl+C Interrupt and Signal SIGINT process
Ctrl+C Interrupt and Signal SIGINT process | Source

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