Solution Security and Safety - JawaharT/Best-Practices-On-Azure-Sphere GitHub Wiki

Solutions for Security and Safety

Solution to exercise one

An example of this program run is shown below with different types of inputs and their outcomes:

#include <stdio.h>  
#include <string.h> 
#include <stdlib.h> 
#include <ctype.h> 

int end_integer_input(){ 
    // Temp variables 
    int digit=0, flag=1;
    
    // Store input, for instance from embedded system keypad
    static char number[10] = "12";

    // Loop through each char in input 
    while(number[digit] != '\0'){ 
        flag = isdigit(number[digit]); 

        // Not an Int found, otherwise keep going 
        if (!flag){ 
            break; 
        } 
        digit++; 
   } 

   if(flag) {  
       // Reusing digit variable to convert input into integer 
       digit = atoi(number); 

       // Checks if integer input is appropriate for Fibonacci program 
       if ((digit >= 1) && (digit <= 47)){ 
           return digit; 
       }else{ 
           printf("Not a Valid Integer to generate sequence\n"); 
           return 0; 
       } 
   }else{ 
       printf("Not an Integer\n"); 
       return 0; 
   } 
} 

void generate_sequence(int end_pos){  
    int fib_sequence[end_pos];  
    fib_sequence[0] = 0;  
    printf("%d ", 0);  
    if (end_pos >= 2){  
        fib_sequence[1] = 1;  
        printf("%d ", 1);  
        for(int index = 0; index < end_pos-2; index++){  
            int next_number = fib_sequence[index] + fib_sequence[index+1];  
            printf("%d ", next_number);  
            fib_sequence[index+2] = next_number;  
        }  
    }  
}  

void main(){ 
     int end_pos = end_integer_input(); 
     if(end_pos >= 1){ 
         generate_sequence(end_pos); 
     }
}

Output with generous input (12):

0 1 1 2 3 5 8 13 21 34 55 89 
#include <stdio.h>  
#include <string.h> 
#include <stdlib.h> 
#include <ctype.h> 

int end_integer_input(){ 
    // Temp variables 
    int digit=0, flag=1;
    
    // Store input 
    static char number[10] = "123";

    // Loop through each char in input 
    while(number[digit] != '\0'){ 
        flag = isdigit(number[digit]); 

        // Not an Int found, otherwise keep going 
        if (!flag){ 
            break; 
        } 
        digit++; 
   } 

   if(flag) {  
       // Reusing digit variable to convert input into integer 
       digit = atoi(number); 

       // Checks if integer input is appropriate for Fibonacci program 
       if ((digit >= 1) && (digit <= 47)){ 
           return digit; 
       }else{ 
           printf("Not a Valid Integer to generate sequence\n"); 
           return 0; 
       } 
   }else{ 
       printf("Not an Integer\n"); 
       return 0; 
   } 
} 

void generate_sequence(int end_pos){  
    int fib_sequence[end_pos];  
    fib_sequence[0] = 0;  
    printf("%d ", 0);  
    if (end_pos >= 2){  
        fib_sequence[1] = 1;  
        printf("%d ", 1);  
        for(int index = 0; index < end_pos-2; index++){  
            int next_number = fib_sequence[index] + fib_sequence[index+1];  
            printf("%d ", next_number);  
            fib_sequence[index+2] = next_number;  
        }  
    }  
}  

void main(){ 
     int end_pos = end_integer_input(); 
     if(end_pos >= 1){ 
         generate_sequence(end_pos); 
     }
}

Output with not ideal input (123):

Not a Valid Integer to generate sequence

This is the full program including generate_sequence, to showcase that the modifications do not require a whole software to be changed. The modifications made include

Inside end_integer_input:

  • Input is taken as a string
  • Loop through each individual character in the string to make sure all are digits
  • If the character is identified to not be a digit, the loop is broken otherwise continued until the end. Flag variable is used to keep track of the identification
  • “Not an Integer” is displayed if a character that is not an integer is identified
  • If it is an Integer check for only positive real numbers that are between 1 and 47 respectively otherwise exit

Inside main:

  • Extra condition is added to make sure only positive real numbers call generate_sequence

Solution to exercise two

// Button A state variable, set to button not-pressed
static GPIO_Value_Type currentButtonAState = GPIO_Value_High;

// Termination state
volatile sig_atomic_t terminationRequired = false;

static void ButtonTimerEventHandler(EventData *eventData){
    bool buttonAPressed = false;

    if (ConsumeTimerFdEvent(buttonPollTimerFd) != 0) {
        terminationRequired = true;
	return;
    }

    // Task 1: Check for button A press
    GPIO_Value_Type newButtonAState;
    int result = GPIO_GetValue(buttonAGpioFd, &newButtonAState);
    if (result != 0) {
	printf("ERROR: Could not read button GPIO: %s (%d).\n", strerror(errno), errno);
	terminationRequired = true;
	return;
    }

    // Task 2: Check if the button has GPIO_Value_Low when pressed and GPIO_Value_High when released
    if (newButtonAState != currentButtonAState) {
        if (newButtonAState == GPIO_Value_Low) {
            printf("Button A pressed!\n");
	    buttonAPressed = true;
	}else {
	    printf("Button A released!\n");
	}
	
        // Update the static variable to use next time
	currentButtonAState = newButtonAState;
   }
}

This is the full program for handling button A presses from the Azure Sphere board.

Task 1:

  • newButtonAState declaration to store the file descriptor for GPIO button
  • result will store the current button status
  • The result is then checked if the GPIO button connection is successful before use, if failed then application is terminated

Task 2:

  • First only perform the task if there is a change in button state
  • Then make sure to confirm the state of the button and update the state variable to reflect it

Azure Sphere Embedded Systems Basics and Best Practices

Disclaimer: All example code shown is only for demonstration purposes of technical best practices in terms of embedded C for Azure Sphere

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