Create and resize dynamic array without C libraries?

malloc is not just a library, it is the way you interface with the Operating System to ask for more memory for the running process. Well, you could ask more memory and manage free/occupied memory yourself, but it would be wrong on many levels.

But, I am inclined to believe that your project is going to run in some kind of platform which does not have an operating system, is it?¹ In that case, the faster solution is to first allocate statically some memory in a big global array, and every time you need memory you would ask for a manager responsible for this big array.

Let me give you an example, for the sake of simplicity it will be tiny and not very functional, but it is a very good quick start.

typedef char bool;

#define BLOCK_SIZE 1024 //Each allocation must have in max 1kb
#define MAX_DATA 1024*1024*10  //Our program statically allocates 10MB
#define BLOCKS (MAX_DATA/BLOCK_SIZE)

typedef char Scott_Block[BLOCK_SIZE];

Scott_Block Scott_memory[BLOCKS];
bool Scott_used_memory[BLOCKS];

void* Scott_malloc(unsigned size) {
    if( size > BLOCK_SIZE )
        return NULL;
    unsigned int i;
    for(i=0;i<BLOCKS;++i) {
        if( Scott_used_memory[i] == 0 ) {
            Scott_used_memory[i] = 1;
            return Scott_memory[i];
        }
    }
    return NULL;
}

void Scott_free(void* ptr) {
    unsigned int pos = ((char*)(ptr)-Scott_memory[0])/BLOCK_SIZE;
    printf("Pos %d\n",pos);
    Scott_used_memory[pos] = 0;
}

I wrote this code to show how to emulate a memory manager. Let me point out a few improvements that may be done to it.

First, the Scott_used_memory could be a bitmap instead of a bool array. Second, it does not allocate memory bigger than BLOCK_SIZE, it should search for consecutives blocks to create a bigger block. But for that you would need more control data to tell how much blocks an allocated void* occupies. Third, the way free memory is searched (linearly) is very slow, usually the blocks creates a link list of free blocks.

But, like I said, this is a great quick start. And depending on your needs this may fulfill it very well.

¹ If not, then you have absolutely no reason to not use malloc.

Well why not this program (C99)

#include <stdio.h>

int main(int argc, char *argv[])
{
   int sizet, i;
   printf("Enter size:");
   scanf("%d",&sizet);
   int array[sizet];
   for(i = 0; i < sizet; i++){ 
     array[i] = i;
   }
   for(i = 0; i < sizet; i++){
     printf("%d", array[i]);
   }
   return 0;
}

Like a boss! :-)