Multitasking on Raspberry Pi using Execve and Fork Commands -(Part 15/38)

 

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The term ‘Process’ refers to the piece of code which is currently in execution. The fork () is a function which can be used in the C code to start a new process as a ‘Child process’. The process or the code which is in execution which makes a call to the function fork () is called the ‘Parent process’ and the new process created by the fork () is called a ‘Child process’. The ‘Parent process’ and the ‘Child process’ executes parallel (high speed processor makes it feel like parallel processing) and independently and thus achieves multi-tasking. The fork () most of the time uses the execve () function to create a new process as Child.

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.

There are eight general purpose IO pins on the 13*2 pin connectors of the Raspberrypi board and to each one of them a LED is connected through 1K resistor. Separate code has been written to blink the LEDs individually and made them into executable files named blink2, blink3, blink4, blink5, blink6, blink7 and blink8. It is suggested to keep all the .c files and the executable files in a single folder for this particular project.

The user can run any of the LED blinking programs from the command line. For example to execute the file ‘blink1’, the user can use the following command;

 

./blink1

The user can perform multi-tasking on them by entering the following commands one after the other. The user can perform multi-tasking with the eight LED codes by entering the following eight commands one after the other.

 ./blink1 &

./blink2 &

./blink3 &

./blink4 &

./blink5 &

./blink6 &

./blink7 &

./blink8 &

All the above process can be generated as ‘Child processes’ of a single ‘Parent process’ with the help of fork () and execve () functions.

fork ()

As far as a Parent process is concerned it simply executes the code from top to bottom including the fork () statements. As soon as the fork () function is called a new Child process is created which starts executing the following statements from the point where the fork () is being called. The process is called a Child process because it is identical to the Parent process in terms of memory like the variables, functions etc. The Child is but independent from the Parent as it has its own process id, and the memory is not shared with the Parent process.

The fork is a function which takes no argument but has two return values unlike most of the C functions. When the Parent calls the fork (), it always returns a positive value on success. The positive value is the process id of the newly created Child process. The Child process however get a value ‘0’ from the same fork () function call. This is very helpful to limit the execution of Child process to a particular area of the main code which the Parent executes, with the help of conditional statements. The idea is to keep the entire child process inside a condition;

if ( !fork () )

{

            // execute a process here using execve //

            _exit ( 0 );

}

else;

 

The Parent process reads the condition as false and does not execute the process inside the if condition, but the Child process reads this condition as true and execute the process inside the condition as child and after execution the Child process simply exits.

To execute the ‘blink1’, ‘blink2’ … ‘blink8’ etc. as Child processes of a main process simply open up a new .c file in vim editor  and write down the following simple code;

 

#include <unistd.h>

int main ( void )

{

            if ( !fork () )

            {

                        execve ( “./blink1”, 0, 0 );       

                        _exit ( 0 );

            }

            else;

 

            if ( !fork () )

            {

                        execve ( “./blink2”, 0, 0 );       

                        _exit ( 0 );

            }

            else;

           

// similarly create other child process from blink3 to blink7 //

 

            if ( !fork () )

            {

                        execve ( “./blink8”, 0, 0 );       

                        _exit ( 0 );

            }

            else;

 

}

 

Save this file with a name, say “fork_blink_all.c” and then compile them into an executable file “fork_blink_all”.

To execute the code use the following command;

./fork_blink_all

Use the following command to view all the process with their process id, which are currently in execution;

ps -e

*** Make sure that all the executable files blink1, blink2 etc. are there in the same folder with the executable file “execve_blink_all”.

***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 in the code “blink3.c” 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

 

Circuit Diagrams

Project Components

Project Video

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Filed Under: Raspberry pi
Tagged With: execve, fork, Raspberry Pi
 

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