我想知道malloc和free是怎么工作的。

int main() {
    unsigned char *p = (unsigned char*)malloc(4*sizeof(unsigned char));
    memset(p,0,4);
    strcpy((char*)p,"abcdabcd"); // **deliberately storing 8bytes**
    cout << p;
    free(p); // Obvious Crash, but I need how it works and why crash.
    cout << p;
    return 0;
}

如果答案是在记忆层面上的深度,如果可能的话,我会非常感激。


当前回答

正如aluser在这个论坛上说的:

Your process has a region of memory, from address x to address y, called the heap. All your malloc'd data lives in this area. malloc() keeps some data structure, let's say a list, of all the free chunks of space in the heap. When you call malloc, it looks through the list for a chunk that's big enough for you, returns a pointer to it, and records the fact that it's not free any more as well as how big it is. When you call free() with the same pointer, free() looks up how big that chunk is and adds it back into the list of free chunks(). If you call malloc() and it can't find any large enough chunk in the heap, it uses the brk() syscall to grow the heap, i.e. increase address y and cause all the addresses between the old y and the new y to be valid memory. brk() must be a syscall; there is no way to do the same thing entirely from userspace.

Malloc()依赖于系统/编译器,所以很难给出一个具体的答案。基本上,它会跟踪它所分配的内存,这取决于它是如何做的,所以你对free的调用可能失败或成功。

malloc()和free()并不是在每个O/S上都以相同的方式工作。

其他回答

malloc/free的一个实现如下所示:

通过sbrk() (Unix调用)从操作系统获取一块内存。 在该内存块周围创建一个页眉和页脚,并提供一些信息,如大小、权限以及下一个和上一个块的位置。 当传入对malloc的调用时,引用一个指向适当大小的块的列表。 然后返回这个块,页眉和页脚也相应地更新。

这与malloc和free没有特别的关系。你的程序在复制字符串后表现出未定义的行为——它可能在那一点或之后的任何一点崩溃。即使您从未使用malloc和free,并在堆栈上或静态地分配char数组,也会出现这种情况。

您的strcpy行尝试存储9个字节,而不是8个字节,因为有NUL结束符。它调用未定义的行为。

The call to free may or may not crash. The memory "after" the 4 bytes of your allocation might be used for something else by your C or C++ implementation. If it is used for something else, then scribbling all over it will cause that "something else" to go wrong, but if it isn't used for anything else, then you could happen to get away with it. "Getting away with it" might sound good, but is actually bad, since it means your code will appear to run OK, but on a future run you might not get away with it.

使用调试风格的内存分配器,您可能会发现在那里写了一个特殊的保护值,free会检查该值,如果没有找到它就会惊慌失措。

否则,您可能会发现接下来的5个字节包括属于某个尚未分配的其他内存块的链接节点的一部分。释放块很可能涉及将其添加到可用块的列表中,并且由于您在列表节点中乱写了代码,该操作可能会解除对具有无效值的指针的引用,从而导致崩溃。

这完全取决于内存分配器——不同的实现使用不同的机制。

这很难说,因为不同的编译器/运行时之间的实际行为是不同的。即使是调试/发布版本也有不同的行为。VS2005的调试版本将在分配之间插入标记来检测内存损坏,因此它将在free()中断言,而不是崩溃。

同样重要的是要意识到,简单地使用brk和sbrk移动程序断点指针实际上并不分配内存,它只是设置了地址空间。例如,在Linux上,当访问该地址范围时,内存将由实际的物理页“备份”,这将导致页错误,并最终导致内核调用页分配器以获得备份页。