内存回收算法c语言实现代码

内存回收算法有很多种，其中最常见的是标记-清除算法和引用计数算法。下面是它们的C语言实现代码：

1. 标记-清除算法

void mark(void* p) {
    unsigned long* ptr = (unsigned long*)p;
    if (*ptr & 1) {
        return;
    }
    *ptr |= 1;
    void** next = (void**)(ptr + 1);
    while (*next) {
        mark(*next);
        next++;
    }
}

void sweep() {
    void* p = heap_start;
    while (p < heap_end) {
        unsigned long* ptr = (unsigned long*)p;
        if (*ptr & 1) {
            *ptr &= ~1;
        } else {
            free(p);
        }
        p += (*ptr & ~1) + sizeof(unsigned long);
    }
}

void collect() {
    void* stack_top = get_stack_top();
    void* stack_bottom = get_stack_bottom();
    for (void** p = stack_bottom; p < stack_top; p++) {
        if (*p >= heap_start && *p < heap_end) {
            mark(*p);
        }
    }
    sweep();
}

2. 引用计数算法

struct object {
    int ref_count;
    void* data;
};

void* allocate(size_t size) {
    struct object* obj = malloc(sizeof(struct object) + size);
    obj->ref_count = 1;
    obj->data = (void*)(obj + 1);
    return obj->data;
}

void release(void* p) {
    struct object* obj = (struct object*)((char*)p - sizeof(struct object));
    obj->ref_count--;
    if (obj->ref_count == 0) {
        free(obj);
    }
}

void retain(void* p) {
    struct object* obj = (struct object*)((char*)p - sizeof(struct object));
    obj->ref_count++;
}

void collect() {
    void* stack_top = get_stack_top();
    void* stack_bottom = get_stack_bottom();
    for (void** p = stack_bottom; p < stack_top; p++) {
        if (*p >= heap_start && *p < heap_end) {
            retain(*p);
        }
    }
    void* p = heap_start;
    while (p < heap_end) {
        struct object* obj = (struct object*)p;
        if (obj->ref_count == 0) {
            p += sizeof(struct object) + obj->size;
            free(obj);
        } else {
            p += sizeof(struct object) + obj->size;
        }
    }
}

其中，`get_stack_top`和`get_stack_bottom`是获取当前线程的栈顶和栈底的函数，`heap_start`和`heap_end`是堆的起始地址和结束地址，可以通过`sbrk`等函数获取。