自己实现一个内存管理器

https://arjunsreedharan.org/post/148675821737/memory-allocators-101-write-a-simple-memory　　

#include <zconf.h>
#include <pthread.h>
#include <string.h>

typedef char ALIGN[16];

union header {
    struct {
        size_t size;
        unsigned is_free;
        union header *next;
    } s;
    ALIGN stub;
};
typedef union header header_t;

header_t *head, *tail;
pthread_mutex_t global_malloc_lock;

header_t *get_free_block(size_t size)
{
    header_t *curr = head;
    while(curr) {
        if (curr->s.is_free && curr->s.size >= size)
            return curr;
        curr = curr->s.next;
    }
    return NULL;
}

void *malloc(size_t size)
{
    size_t total_size;
    void *block;
    header_t *header;
    if (!size)
        return NULL;
    pthread_mutex_lock(&global_malloc_lock);
    header = get_free_block(size);
    if (header) {
        header->s.is_free = 0;
        pthread_mutex_unlock(&global_malloc_lock);
        return (void*)(header + 1);
    }
    total_size = sizeof(header_t) + size;
    block = sbrk(total_size);
    if (block == (void*) -1) {
        pthread_mutex_unlock(&global_malloc_lock);
        return NULL;
    }
    header = block;
    header->s.size = size;
    header->s.is_free = 0;
    header->s.next = NULL;
    if (!head)
        head = header;
    if (tail)
        tail->s.next = header;
    tail = header;
    pthread_mutex_unlock(&global_malloc_lock);
    return (void*)(header + 1);
}

void free(void *block)
{
    header_t *header, *tmp;
    void *programbreak;

    if (!block)
        return;
    pthread_mutex_lock(&global_malloc_lock);
    header = (header_t*)block - 1;

    programbreak = sbrk(0);
    if ((char*)block + header->s.size == programbreak) {
        if (head == tail) {
            head = tail = NULL;
        } else {
            tmp = head;
            while (tmp) {
                if(tmp->s.next == tail) {
                    tmp->s.next = NULL;
                    tail = tmp;
                }
                tmp = tmp->s.next;
            }
        }
        sbrk(0 - sizeof(header_t) - header->s.size);
        pthread_mutex_unlock(&global_malloc_lock);
        return;
    }
    header->s.is_free = 1;
    pthread_mutex_unlock(&global_malloc_lock);
}

void *calloc(size_t num, size_t nsize)
{
    size_t size;
    void *block;
    if (!num || !nsize)
        return NULL;
    size = num * nsize;
    /* check mul overflow */
    if (nsize != size / num)
        return NULL;
    block = malloc(size);
    if (!block)
        return NULL;
    memset(block, 0, size);
    return block;
}

void *realloc(void *block, size_t size)
{
    header_t *header;
    void *ret;
    if (!block || !size)
        return malloc(size);
    header = (header_t*)block - 1;
    if (header->s.size >= size)
        return block;
    ret = malloc(size);
    if (ret) {

        memcpy(ret, block, header->s.size);
        free(block);
    }
    return ret;
}