Salma Salma - 13 days ago 10
C Question

Free space bitmap C implementation

I trying to develop a simple file system (Linux kernel) and I'm thinking of using bitmap to keep track of used/free blocks as described in here:

https://en.wikipedia.org/wiki/Free_space_bitmap

However, I could not find any implementation of such system in C. I would like to see some examples so I could implement a similar thing to my system.

Any suggestion where I could find them?

Answer

Here's a quick implementation I made after reading this answer.

int mem_block_size;
int mem_block_count;
uint *bit_map;
char *buffer;

void init_memory_map(int block_size, int block_count)
{
    mem_block_size = block_size;
    mem_block_count = block_count;
    buffer = (char*)malloc(block_size * block_count);
    bit_map = (uint*)calloc((block_count / 32) + ((block_count % 4) != 0), 4);
}

inline
int is_allocated(int index)
{
    return (bit_map[index / 32] & (1 << (index % 32))) != 0;
}

inline
void allocate_frame(int index)
{
    bit_map[index / 32] |= 1 << (index % 32);
}

inline
void clear_frame(int index)
{
    bit_map[index / 32] &= ~(1 << (index % 32));
}

char* allocate_block(int block_count)
{
    int index = 0, free_frames = 0;
    while(index < mem_block_count)
    {
        if (!is_allocated(index))
        {
            free_frames++;
            if (free_frames == block_count)
            {
                int frame_index = index - block_count + 1;

                index = 0;
                while(index < block_count)
                    allocate_block(frame_index + index);

                return (buffer + frame_index * mem_block_size);
            }
        }
        else free_frames = 0;
    }

    perror("memory error\n");
    return 0;
}

The basic idea is, you maintain a bit map which keep tracks of allocated frames. each frame act's as a buffer of fixed size. when you are done with the frame you can mark it free by setting bit off in the bit map.

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