17-simple-memory-management/mem_man.c

/*----------------------------------------------------------
 *				HTBLA-Leonding
 * ---------------------------------------------------------
 * Title:			Simple Memory Manager.
 * Author:			Marc Tismonar
 * ----------------------------------------------------------
 * Description:
 * Implementation of a simple memory manager.
 * ----------------------------------------------------------
 */

#include "mem_man.h"
#include <stdio.h>
#include <string.h>

/**
 * The memory manager operates on a global, statically allocated, linear memory partition (array).
 * This memory is organized in memory blocks of 32 bits (4 byte), the number of totally
 * available blocks is defined by the constant 'MEMORY_BLOCK_CNT'.
 * The smallest allocatable memory unit is one block, larger memory units are allocates always
 * a multiple of one block. This means, if a memory up to 32 bit is requested, one block is allocated,
 * if a memory of size from 33 bits to 64 bits is request, two blocks are allocated, and so one.
 * 
 * Each memory block shall be initialized with the pattern '0xDEADBEEF", freed memory shall be set to
 * the pattern '0xAFFEBAFF'. Such regions are easily recognizable in a memory dump.
 *
 * The housekeeping data (start address and size) of allocated memory units
 * shall be maintained in a linked list.
 */

#define MEMORY_BLOCK_CNT 1024
#define BYTES_PER_BLOCK 4

static int memory[MEMORY_BLOCK_CNT];
static size_t allocatedBlocks = 0;
static size_t freeBlocks = MEMORY_BLOCK_CNT;

static void init_memory() {
    static int initialized = 0;
    if (!initialized) {
        for (size_t i = 0; i < MEMORY_BLOCK_CNT; i++) {
            memory[i] = 0xAFFEBAFF;
        }
        initialized = 1;
    }
}

void *my_alloc(size_t size) {
    init_memory();
    
    if (size == 0) {
        return (void*)0;
    }
    
    size_t i = 0;
    size_t start = MEMORY_BLOCK_CNT;
    size_t freePlaces = 0;
    size_t blocks_needed = (size + BYTES_PER_BLOCK - 1) / BYTES_PER_BLOCK;

    while (i < MEMORY_BLOCK_CNT) {
        if (memory[i] == 0xAFFEBAFF) {
            if (freePlaces == 0) {
                start = i;
            }
            freePlaces++;

            if (freePlaces == blocks_needed) {
                for (size_t j = 0; j < blocks_needed; j++) {
                    memory[start + j] = 0xDEADBEEF;
                }
                freeBlocks -= blocks_needed;
                allocatedBlocks += blocks_needed;
                return &memory[start];
            }
        } else {
            freePlaces = 0;
            start = MEMORY_BLOCK_CNT;
        }
        i++;
    }
    return NULL;
}

void my_free(void *p) {
    int start = get_unit_index(p);
    if (start == -1) {
        printf("Memory is not allocated\n");
        return;
    }
    
    size_t freed_blocks = 0;
    size_t i = start;
    
    while (i < MEMORY_BLOCK_CNT && memory[i] == 0xDEADBEEF) {
        memory[i] = 0xAFFEBAFF;
        freed_blocks++;
        i++;
    }
    
    freeBlocks += freed_blocks;
    allocatedBlocks -= freed_blocks;
}

MemStat mem_get_statistics() {
    init_memory();
    
    MemStat stat;
    stat.total_bytes = MEMORY_BLOCK_CNT * BYTES_PER_BLOCK;
    stat.free_bytes = freeBlocks * BYTES_PER_BLOCK;
    stat.used_bytes = allocatedBlocks * BYTES_PER_BLOCK;
    stat.free_percentage = (double)stat.free_bytes / (double)stat.total_bytes * 100;
    
    size_t units_count = 0;
    size_t i = 0;
    
    while (i < MEMORY_BLOCK_CNT) {
        if (memory[i] == 0xDEADBEEF) {
            units_count++;
            while (i < MEMORY_BLOCK_CNT && memory[i] == 0xDEADBEEF) {
                i++;
            }
        } else {
            i++;
        }
    }
    stat.allocated_units_count = units_count;
    
    size_t largest_free_unit = 0;
    size_t current_free_unit = 0;
    
    for (i = 0; i < MEMORY_BLOCK_CNT; i++) {
        if (memory[i] == 0xAFFEBAFF) {
            current_free_unit++;
        } else {
            if (current_free_unit > largest_free_unit) {
                largest_free_unit = current_free_unit;
            }
            current_free_unit = 0;
        }
    }

    if (current_free_unit > largest_free_unit) {
        largest_free_unit = current_free_unit;
    }
    
    stat.larger_free_unit_bytes = largest_free_unit * BYTES_PER_BLOCK;
    return stat;
}

void mem_print_units() {
    init_memory();
    
    size_t i = 0;
    while (i < MEMORY_BLOCK_CNT) {
        if (memory[i] == 0xDEADBEEF) {
            size_t start = i;
            size_t size = 0;
            while (i < MEMORY_BLOCK_CNT && memory[i] == 0xDEADBEEF) {
                size++;
                i++;
            }
            printf("Allocated: Index %zu; Size: %zu\n", start, size);
        } else {
            i++;
        }
    }
}

int get_unit_index(void *p_mem) {
    if (p_mem < (void*)memory || p_mem >= (void*)(memory + MEMORY_BLOCK_CNT)) {
        return -1;
    }
    
    int index = ((int*)p_mem - memory);
    
    if (memory[index] != 0xDEADBEEF) {
        return -1;
    }
    
    while (index > 0 && memory[index-1] == 0xDEADBEEF) {
        index--;
    }
    
    return index;
}

void mem_dump() {
    init_memory();
    
    for (size_t i = 0; i < MEMORY_BLOCK_CNT; i++) {
        printf("Memory: Index %zu; Content 0x%X\n", i, memory[i]);
    }
}