User Signalled Process System in Raspberry Pi -(Part 23/38)

The Raspberrypi board is powerful enough to run large operating systems like Linux, Mac and Windows. The Multi-tasking Operating Systems can run several processes at a time creating and effect of parallel processing with the help of the high speed processor. The Linux Operating Systems provides Multi-User-Multitasking. Linux operating systems especially Ubuntu is preferred for all kind of programming and development. The operating systems like Archlinux ARM, OpenELEC, Pidora, Raspbmc, RISC OS and the Raspbian and also Ubuntu versions are available for the Raspberrypi board.

The Raspberrypi is a board actually designed for helping computer education for remote schools but it is a good platform for programmers especially beginners to explore various coding techniques. The Raspberry pi board runs on the Broadcom controller chip which is a SoC (System on Chip). This SoC has the powerful ARM11 processor which runs on 700 MHz at its core. This powerful processor and the advanced controller peripherals make it a mini-computer.

A Parent process can create another process, which are called the Child process and the Parent process can use signals to control the Child process. The Parent process along with the Child process which is controlled by the Parent process forms a Process system. In such a system since the Parent process control the entire system the user can control the system by sending signals to the Parent process. This project demonstrates the implementation of a Process system in which there is a single Parent and so many Childs and the entire systems is controlled by the user with the help of signals sent to the Parent.

In this project a Parent process creates so many Child process and controls them using the signals. The Parent process creates the Child processes using the fork () function. When the Parent calls the fork (), it always returns a positive value on success. The positive value which the Parent receives is the process-id of the newly created Child process. The Child process however get a value ‘0’ from the same fork () function call.Thus the Parent can store the process-id of each and every Child process that has been created. Using this process-id the Parent process can send a signal to a particular Child process with the help of the function ‘kill ()’. The kill () is a function defined in the header file <sys/signal.h> which can be used to send a specified signal to a specified process.

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;

NAME	NUMBER	DESCRIPTION
SIGHUP	1	Linux sends a process this signal when it becomes disconnected from a terminal.
SIGINT	2	Linux sends a process this signal when the user tries to end it by pressing CTRL+C.
SIGILL	4	Linux sends a process this signal when it attempts to execute an illegal instruction.
SIGABRT	6	Linux sends a process this signal to the process when the process calls the ‘abort ()’ function
SIGFPE	8	Linux sends a process this signal when it has executed an invalid floating-point math instruction
SIGKILL	9	Linux sends a process this signal to end it immediately
SIGUSR1	10	User programs can send this signal to other process
SIGUSR2	12	User programs can send this signal to other process
SIGSEGV	11	Linux sends a process this signal when the program has attempted an invalid memory access
SIGPIPE	13	Linux sends a process this signal when the program has attempted to access a broken data stream, such as a socket connection that has been already closed
SIGALRM	14	A process can receive this signal from the Linux using the function alarm(), after a time period mentioned in its argument.
SIGTERM	15	Linux sends a process this signal requesting it to terminate
SIGCHLD	17	Linux sends a process this signal when a child process exits
SIGXCPU	24	Linux sends a process this signal when it exceeds the limit of CPU time that it can consume.
SIGVTALRM	26	A process can receive this signal from the Linux using the function setitimer (), after a time period mentioned in its argument.

Fig. 2: List of common signals for User Signalled process in Operating System

From the above table the Parent process creates so many Child process and controls them using the signal 12 which is the ‘SIGUSR2’. The user can control the Parent process by sending the signal ‘SIGUSR1’ to the Parent process from the command line. The two signals ‘SIGUSR1’ and the ‘SIGUSR2’ are reserved for the user applications and hence they can be used in this project also.

As soon as the Parent receives a ‘SIGUSR1’ signal from the user, it sends a ‘SIGUSR2’ signal to one of its Child process. The Child processes then changes the glowing state of the LED associated with it. This forms a Process System made up of several Child process and a Parent process and the entire system is controlled by the ‘SIGUSR1’ signal received by the Parent process. The diagrammatic representation of the Process System is given below;

The user can send two signals from the command line using the Keyboard shortcuts and they are SIGINT and the SIGQUIT. The SIGINT can be send by pressing the key CTRL+C and the SIGQUIT can be send by pressing the key CTRL+Y or CTRL + . Any signals other than these require the ‘kill’ command to be used with the command line. The format for sending a signal to a process using the ‘kill’ command from the command line is given below;

kill -s <command> <process-id>

For example to sends a signal ‘SIGUSR1’ to a Parent process having the process-id say ‘2770’, the following command can be used;

kill -s SIGUSR1 2770

The process-id of a particular process which has been created by executing a binary file can be found by using the command ‘ps’ with the format as shown below;

ps -C <name>

For example to find the process-id of a process created by executing a binary file of name ‘blink’, the following command can be used;

ps –C blink

***Note that in this project the latest version of library “bcm2835” is used with an old version of Raspberrypi board. It is not possible to access the pin number 13 of the old board with the latest library version and hence the pin number 24 is used to blink the 3^rd LED. The circuit diagram is also drawn accordingly. Those who have the latest version of the board can use the pin 13 without any trouble.

To find the ‘Revision’ and other important details about the Raspberrypi board, use the following command;

cat /proc/cpuinfo

*******************************************************************************

Note:

In this project the Raspberrypi board version 2 is used, but a previous version of the “bcm2835” library is installed. Accessing the GPIO pin 13 is not possible with this library and hence the 3rd IO pin is selected as pin24 of the P1 port of the Raspberrypi. The circuit and the code are modified accordingly. Those who have the version 2 of the Raspberrypi and the latest version of the “bcm2835” library can use the GPIO pin 13 using “RPI_V2_GPIO_P1_13” instead of “RPI_GPIO_P1_13” in the pin definition.

For example to define a ‘PIN’ as 13th pin of P1 port of Raspberrypi board version2, use the following statement in the code;

#define PIN RPI_V2_GPIO_P1_13

*******************************************************************************

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 set_pins_output ( void );
void set_all_pin_low ( void );
 
void toggle1_on_signal_rx ( void );
void toggle2_on_signal_rx ( void );
void toggle3_on_signal_rx ( void );
void toggle4_on_signal_rx ( void );
void toggle5_on_signal_rx ( void );
void toggle6_on_signal_rx ( void );
void toggle7_on_signal_rx ( void );
void toggle8_on_signal_rx ( void );
 
void toggle1_sig_handler ( int signo );
void toggle2_sig_handler ( int signo );
void toggle3_sig_handler ( int signo );
void toggle4_sig_handler ( int signo );
void toggle5_sig_handler ( int signo );
void toggle6_sig_handler ( int signo );
void toggle7_sig_handler ( int signo );
void toggle8_sig_handler ( int signo );
 
void parent_signal_handler ( int signo );
 
pid_t child_id [ 8 ];
 
int main ( void )
{
int i;
 
if (!bcm2835_init())
return 1;
set_pins_output ();
set_all_pin_low ();
 
signal ( SIGUSR1, parent_signal_handler );
 
if ( ! ( child_id [ 0 ] = fork () ) )
{
toggle1_on_signal_rx (); 
_exit ( 0 );
}
else;
 
if ( ! ( child_id [ 1 ] = fork () ) )
{
toggle2_on_signal_rx (); 
_exit ( 0 );
}
else;
 
if ( ! ( child_id [ 2 ] = fork () ) )
{
toggle3_on_signal_rx (); 
_exit ( 0 );
}
else;
 
if ( ! ( child_id [ 3 ] = fork () ) )
{
toggle4_on_signal_rx (); 
_exit ( 0 );
}
else;
 
if ( ! ( child_id [ 4 ] = fork () ) )
{
toggle5_on_signal_rx (); 
_exit ( 0 );
}
else;
 
if ( ! ( child_id [ 5 ] = fork () ) )
{
toggle6_on_signal_rx (); 
_exit ( 0 );
}
else;
 
if ( ! ( child_id [ 6 ] = fork () ) )
{
toggle7_on_signal_rx (); 
_exit ( 0 );
}
else;
 
if ( ! ( child_id [ 7 ] = fork () ) )
{
toggle8_on_signal_rx (); 
_exit ( 0 );
}
else;
 
while ( 1 );
 
bcm2835_close();
 
return 0;
}
 
void parent_signal_handler ( int signo )
{
static int i = 0;
 
kill ( child_id [ i ], SIGUSR2 );
 
i ++;
 
if ( i > 7 )
i = 0;
else; 
}
 
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 toggle1_on_signal_rx ( void )
{
signal ( SIGUSR2, toggle1_sig_handler ); 
while ( 1 );
}
 
void toggle2_on_signal_rx ( void )
{
signal ( SIGUSR2, toggle2_sig_handler ); 
while ( 1 );
}
 
void toggle3_on_signal_rx ( void )
{
signal ( SIGUSR2, toggle3_sig_handler ); 
while ( 1 );
}
 
void toggle4_on_signal_rx ( void )
{
signal ( SIGUSR2, toggle4_sig_handler ); 
while ( 1 );
}
 
void toggle5_on_signal_rx ( void )
{
signal ( SIGUSR2, toggle5_sig_handler ); 
while ( 1 );
}
 
void toggle6_on_signal_rx ( void )
{
signal ( SIGUSR2, toggle6_sig_handler ); 
while ( 1 );
}
 
void toggle7_on_signal_rx ( void )
{
signal ( SIGUSR2, toggle7_sig_handler ); 
while ( 1 );
}
 
void toggle8_on_signal_rx ( void )
{
signal ( SIGUSR2, toggle8_sig_handler ); 
while ( 1 );
}
 
void toggle1_sig_handler ( int signo )
{
static int state = 0;
 
state = ~state;
 
bcm2835_gpio_write ( PIN1, state );
}
 
void toggle2_sig_handler ( int signo )
{
static int state = 0;
 
state = ~state;
 
bcm2835_gpio_write ( PIN2, state );
}
 
void toggle3_sig_handler ( int signo )
{
static int state = 0;
 
state = ~state;
 
bcm2835_gpio_write ( PIN3, state );
}
 
void toggle4_sig_handler ( int signo )
{
static int state = 0;
 
state = ~state;
 
bcm2835_gpio_write ( PIN4, state );
}
 
void toggle5_sig_handler ( int signo )
{
static int state = 0;
 
state = ~state;
 
bcm2835_gpio_write ( PIN5, state );
}
 
void toggle6_sig_handler ( int signo )
{
static int state = 0;
 
state = ~state;
 
bcm2835_gpio_write ( PIN6, state );
}
 
void toggle7_sig_handler ( int signo )
{
static int state = 0;
 
state = ~state;
 
bcm2835_gpio_write ( PIN7, state );
}
 
void toggle8_sig_handler ( int signo )
{
static int state = 0;
 
state = ~state;
 
bcm2835_gpio_write ( PIN8, state );
}
 
 
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