When to allocate memory in C?

Question

I've got the code bellow which it's supposed to move a bit's position inside a byte. It works, but my question is about something else?

What am i doing wrong and what am i doing right regarding the allocation of example_byte and new_byte? Is it too much hassle for this simple program? Should have i just not used malloc and let the compiler do the dirty job better?

Here some guy's opinion about this in the comments section : link.

#include <stdio.h>
#include <malloc.h>

typedef unsigned __int8 byte;

byte move(byte* our, int indexold, int indexnew)
{
byte oldvalue;
byte newvalue;
byte valuetochange;

valuetochange = 0x01 & ((*our)>>indexold);         // get the value of the bit to be moved
printf("value to change : %d\n", valuetochange);
oldvalue = (*our) & (~(1<<(indexold)));            // del the bit from position indexold
oldvalue = oldvalue & (~(1<<(indexnew)));          // del the bit from position indexnew
printf("deleted: %x\n", oldvalue);

newvalue = oldvalue | (valuetochange<<(indexnew)); // write bit in new position (indexnew)

return newvalue;
}

int main()
{
byte* example_byte;
byte* new_byte;

example_byte = (byte*)malloc(sizeof(byte));
new_byte     = (byte*)malloc(sizeof(byte));

*example_byte = 0xc3;  //  hex 0xc3 = binary 1100 0011
printf("\n");

//*****************************************************
// example 1 (move bit from position 1 to position 5)
// example_byte  1100 0011
//                 ^    ^
//               memorize bit -> valuetochange = 0x01 & ((*our)>>indexold) = 1
//               1100 0011 & 1111 1101 = 1100 0001 delete bit from oldindex (1)
//               1100 0001 & 1101 1111 = 1100 0001 delete bit from newindex (5)
// new_byte      1100 0001 | 0010 0000 = 1110 0001
*new_byte     = move(example_byte, 1, 5);

printf("old byte : %x\n", *example_byte); // 0xc3 (1100 0011)
printf("new byte : %x\n", *new_byte);     // 0xe1 (1110 0001)
printf("\n");

//*****************************************************
// example 2 (move bit from position 6 to position 3)
// example_byte  1100 0011
//                ^   ^
//               memorize bit -> valuetochange = 0x01 & ((*our)>>indexold) = 1
//               1100 0011 & 1011 1111 = 1000 0011 delete bit from oldindex (6)
//               1000 0011 & 1111 0111 = 1000 0011 delete bit ftom newindex (3)
// new_byte      1000 0011 | 0000 1000 = 1000 1011
*new_byte     = move(example_byte, 6, 3);

printf("old byte : %x\n", *example_byte); // 0xc3 (1100 0011)
printf("new byte : %x\n", *new_byte);     // 0x8b (1000 1011)
printf("\n");

//*****************************************************
// example 3 (move bit from position 2 to position 6)
// example_byte  1100 0011
//                ^    ^
//               memorize bit -> valuetochange = 0x01 & ((*our)>>indexold) = 0
//               1100 0011 & 1111 1011 = 1100 0011 delete bit from oldindex (2)
//               1100 0011 & 1011 1111 = 1000 0011 delete bit from oldindex (6)
// new_byte      1000 0011 | 0000 0000 = 1000 0011
*new_byte     = move(example_byte, 2, 6);

printf("old byte : %x\n", *example_byte); // 0xc3 (1100 0011)
printf("new byte : %x\n", *new_byte);     // 0x83 (1000 0011)
printf("\n");

//*****************************************************
// example 4 (move bit from position 2 to position 4)
// example_byte  1100 0011
//                  ^  ^
//               memorize bit -> valuetochange = 0x01 & ((*our)>>indexold) = 0
//               1100 0011 & 1111 1011 = 1100 0011 delete bit from oldindex (2)
//               1100 0011 & 1110 1111 = 1100 0011 delete bit from oldindex (4)
// new_byte      1100 0011 | 0000 0000 = 1100 0011
*new_byte     = move(example_byte, 2, 4);

printf("old byte : %x\n", *example_byte); // 0xc3 (1100 0011)
printf("new byte : %x  ", *new_byte);     // 0xc3 (1100 0011)
printf("\n");

free(new_byte);
free(example_byte);
return 0;
}

Answer 1

If it is a single byte, there is no reason to do memory allocation at all. You can just declare a local variable of type byte and pass it around.

You need to use malloc() when you need to allocate an array of elements, and you do not know the number of elements at compile time. You also need it when you need to allocate a large structure that needs to persist in memory after the function that created it exits. If you are dealing with a variable of a primitive type, then you should simply declare it as a local variable, which means that it will be allocated automatically on the stack.

Answer 2

You only need to allocate memory when you need something that will be visible outside of the scope that you declared it in (and any scopes that are nested inside of it), or if you need to dynamically allocate memory for some other reason, like creating a data structure of unknown size. Otherwise, you should just declare a local variable.