Region-based memory management

main.c

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

typedef struct Region Region;

struct Region {
    Region *next;
    size_t capacity;
    size_t size;
    char buffer[];
};

Region *region_create(size_t capacity)
{
    const size_t region_size = sizeof(Region) + capacity;
    Region *region = malloc(region_size);
    memset(region, 0, region_size);
    region->capacity = capacity;
    return region;
}

typedef struct {
    size_t capacity;
    Region *first;
    Region *last;
} Arena;

void *arena_alloc(Arena *arena, size_t size)
{
    if (arena->last == NULL) {
        assert(arena->first == NULL);

        Region *region = region_create(
            size > arena->capacity ? size : arena->capacity);

        arena->last = region;
        arena->first = region;
    }

    while (arena->last->size + size > arena->last->capacity &&
           arena->last->next) {
        arena->last = arena->last->next;
    }

    if (arena->last->size + size > arena->last->capacity) {
        Region *region = region_create(
            size > arena->capacity ? size : arena->capacity);

        arena->last->next = region;
        arena->last = region;
    }

    void *result = arena->last->buffer + arena->last->size;
    arena->last->size += size;
    return result;
}

void *arena_clean(Arena *arena)
{
    for (Region *iter = arena->first;
         iter != NULL;
         iter = iter->next)
    {
        iter->size = 0;
    }

    arena->last = arena->first;
}

void arena_free(Arena *arena)
{
    Region *iter = arena->first;
    while (iter != NULL) {
        Region *next = iter->next;
        free(iter);
        iter = next;
    }
    arena->first = NULL;
    arena->last = NULL;
}

void arena_summary(Arena *arena)
{
    if (arena->first == NULL) {
        printf("[empty]");
    }

    for (Region *iter = arena->first;
         iter != NULL;
         iter = iter->next)
    {
        printf("[%d/%d] -> ", iter->size, iter->capacity);
    }
    printf("\n");
}

int main(int argc, char *argv[])
{
    Arena arena = {.capacity = 640 * 1000};
    arena_summary(&arena);

    arena_alloc(&arena, 35);
    arena_summary(&arena);

    arena_alloc(&arena, 5);
    arena_summary(&arena);

    arena_alloc(&arena, 29);
    arena_summary(&arena);

    arena_alloc(&arena, 1);
    arena_summary(&arena);

    arena_alloc(&arena, 70);
    arena_summary(&arena);

    arena_clean(&arena);
    arena_summary(&arena);

    arena_alloc(&arena, 35);
    arena_summary(&arena);

    arena_free(&arena);

    return 0;
}