Day 18: Dynamic Memory Allocation | Bluetooth

Lab

Background Info/Notes

• Standardized protocol for sending/receiving data via 2.4GHz wireless link
• Good for short-range, low-power, low-cost, wireless transmissions

How it Works

• Use master/slave model
• Master can send and request data from slaves
• Slaves can only transmit to and receive from master
• Can't talk to other slaves
• Addresses and Names
• Each device has unique 48-bit address

That's all theoretical. No matter what I did, I couldn't get the bluetooth to connect to my laptop. I tried resetting stuff, restarting my computer, using different code, using different hardware, nothing worked. I even took my laptop outside to try to connect it, but it simply wouldn't connect. 

Software: Dynamic Memory Allocation

Dynamic memory allocation allows for allocation of memory when program is executed instead of when it is compiled. Especially important when program uses array whose size is not determined until program is executed.

It is specified with either malloc or calloc function, which stand for "memory allocation" or "cleared allocation" and both reserve group of memory locations.

My cipher project from a previous homework assignment used dynamic memory allocation, so that it could reallocate memory according to the length of the user input.

Code:
#ifndef _CIPHERH_
#define _CIPHERH_

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

char *getInput(void)
{
  int mem=16; 
  char *message=malloc(mem); //Preallocate memory
  printf("Input Message\n");
fgets(message,mem,stdin); //Take user input 

while(message[strlen(message)-1]!='\n') //Check if there's still space
{
mem*=2; 
message=realloc(message,mem); //Double amount of space
fgets(message + mem/2 - 1, mem/2 + 1, stdin); //Read the rest of the line
}
return message; //return character array message
}

int EncryptMessage(void)
{
  char *message=getInput(); //call getInput function
  int ch; //initialize variable ch
  
  int shift; //initialize variable shift
  
  printf("\nInput number of characters to shift: \n");
  scanf("%i",&shift); //ask user to choose shift for encryption
  
  char newarray[strlen(message)]; //initialize array that is the same length as the message to be encrypted
  
  for(int k=0; k<=strlen(message); k++)
  {
    ch=*(message+k); //set ch equal to the current value in the array
    if(ch>='a' && ch<='z')
    {
      ch=ch+shift; //encrypt letter
      if(ch>'z')
      {
        ch=ch-26; //if the value goes past z, subtract 26
      }
      if(ch<'a')
      {
        ch=ch+26; //if value goes past a, add 26;
      }
      newarray[k]=ch; //write new value into array
    }
    
    if(ch>='A' && ch<='Z')
    {
      ch=ch+shift;
      if(ch>'Z')
      {
        ch=ch-26;
      }
      if(ch<'A')
      {
        ch=ch+26;
      }
      newarray[k]=ch;
    }
    
    else
    {
      newarray[k]=ch; //ignore non-letters
      continue; //continue
    }
  }
  printf("Encrypted Message: \n %s\n",newarray); //print encrypted message
}

#endif

I used malloc to pre-allocate memory, and realloc to reallocate the memory.