How to Control a Process Through Signal -(Part 18/38)

In this project the Raspberrypi board is loaded with Ubuntu and is remotely accessed using VNC. The Raspberrypi board is also connected to the internet. There are 26 connectors which can be taken out from the connector port of the Raspberrypi board. All the connector pins are taken out using 13*2 pin female connectors and at the other end of their wire 26 pin Burg stick male connectors are attached. The Burg stick male connectors allow each pin out from the Raspberrypi board to be plugged into the holes of a breadboard.  To access the pins that coming out of the Broadcom controller of the Raspberrypi board using C language, a C library is available called “bcm2835” which has been downloaded and installed.

A signal is a software interrupt that can be sent to a process which is currently executing in the Operating System. Most of the time the Operating system send signals to the processes, but process can also send signals to each other. Much like hardwareinterrupts it is a mechanism to notify a process that an event has occurred. Different signals are used to notify different events and the signals are differentiated by their signal numbers. The list of all the available signals in the OS and their signal numbers can be obtained using the following command;

kill -l

The following table gives a list of the most common signals that a process might encounter in an Operating System:

 Fig. 2: List of common signals for controlling process through signal in Operating System

Few of the above signals will end the receiving process immediately, but the rest of them the process can consider them and do the required things.The steps that the process do corresponding to a received signal is called ‘Signal Handling’. It is very much similar to the ‘Interrupt Servicing’ and just like the ‘Interrupt Service Routine’, there should be a function called ‘Signal Handler’ which can perform the necessary things in response to a signal received.

A function can be made the Signal Handler for a particular signal using the function called ‘signal ()’ and the details of the same function are mentioned below;

signal ()

The signal is a function which assigns another function as a Signal Handler for a particular signal. The prototype of the function is declared in the header file <signal.h> as;

void ( *signal(int sig, void (*handler)(int)) ) (int);

From the above prototype, it can be make out that the function signal () can receive an integer, which is the signal number provided by the OS. The function has two arguments also. The first argument is an integer which refers to the signal number to be handled when received. The second argument is a pointer to the function which needs to be act as a signal handler. The function that needs to be acts as a signal handler should have the capability to receive an integer as its argument. The function returns the same function pointer.

Assume a function; say ‘sig_handler’ is declared as follows;

voidsig_handler ( intsigno );

To make a function ‘sig_handler’ as a Signal Handler for the signal number ‘2’, use the ‘signal ()’ function as given below;

int main()

{

signal ( 2, sig_handler );

            while ( 1 ); 

}

The code will wait till the signal number 2, (SIGINT) is received and as soon as the SIGINT is received the signal number is passed to the function ‘sig_handler()’  and also the statements written inside the function ‘sig_handler()’ will get executed.

The SIGINT can be easily generated using the CTRL+C in the terminal where the program is being executed. It is suggested to write a simple

printf ( “nhellon” );

Statement inside the ‘sig_handler()’ function and try to print it by pressing CTRL+C.

The code written for this project receives the SIGINT and blinks the LEDs using the ‘sig_handler()’ function.

Project Source Code

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#include <bcm2835.h>
#include <pthread.h>
#include <unistd.h>
#include <signal.h>
 
#define PIN1 RPI_GPIO_P1_11
#define PIN2 RPI_GPIO_P1_12
#define PIN3 RPI_GPIO_P1_24
#define PIN4 RPI_GPIO_P1_15
#define PIN5 RPI_GPIO_P1_16
#define PIN6 RPI_GPIO_P1_18
#define PIN7 RPI_GPIO_P1_22
#define PIN8 RPI_GPIO_P1_07
 
void sig_handler ( int signo );
void set_pins_output ( void );
void set_all_pin_low ( void );
 
int main()
{
 
if (!bcm2835_init())
return 1;
set_pins_output ();
set_all_pin_low ();
 
signal ( 2, sig_handler );
 
while ( 1 );  
 
bcm2835_close();
return 0;
}
 
void set_all_pin_low ( void )
{
bcm2835_gpio_write(PIN1, LOW);
bcm2835_gpio_write(PIN2, LOW);
bcm2835_gpio_write(PIN3, LOW);
bcm2835_gpio_write(PIN4, LOW);
bcm2835_gpio_write(PIN5, LOW);
bcm2835_gpio_write(PIN6, LOW);
bcm2835_gpio_write(PIN7, LOW);
bcm2835_gpio_write(PIN8, LOW);
}
 
void set_pins_output ( void )
{
bcm2835_gpio_fsel(PIN1, BCM2835_GPIO_FSEL_OUTP);
bcm2835_gpio_fsel(PIN2, BCM2835_GPIO_FSEL_OUTP);
bcm2835_gpio_fsel(PIN3, BCM2835_GPIO_FSEL_OUTP);
bcm2835_gpio_fsel(PIN4, BCM2835_GPIO_FSEL_OUTP);
bcm2835_gpio_fsel(PIN5, BCM2835_GPIO_FSEL_OUTP);
bcm2835_gpio_fsel(PIN6, BCM2835_GPIO_FSEL_OUTP);
bcm2835_gpio_fsel(PIN7, BCM2835_GPIO_FSEL_OUTP);
bcm2835_gpio_fsel(PIN8, BCM2835_GPIO_FSEL_OUTP);
}
 
void sig_handler ( int signo )
{
static int i = 0;
 
set_all_pin_low ();
bcm2835_delay( 50 );
 
i ++;
 
switch ( i )
{
case  1:
bcm2835_gpio_write(PIN1, HIGH);
break;
case  2:
bcm2835_gpio_write(PIN2, HIGH);
break;
case  3:
bcm2835_gpio_write(PIN3, HIGH);
break;
case  4:
bcm2835_gpio_write(PIN4, HIGH);
break;
case  5:
bcm2835_gpio_write(PIN5, HIGH);
break;
case  6:
bcm2835_gpio_write(PIN6, HIGH);
break;
case  7:
bcm2835_gpio_write(PIN7, HIGH);
break;
case  8:
bcm2835_gpio_write(PIN8, HIGH);
i = 0;
break;
};
}
 
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Filed Under: Raspberry pi
Tagged With: Raspberry Pi
 

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