我们都被教导必须释放每个已分配的指针。不过,我有点好奇不释放内存的真正代价。在一些明显的情况下,比如在循环内部或线程执行的一部分调用malloc()时,释放是非常重要的,这样就不会有内存泄漏。但是考虑下面两个例子:

首先,如果我有这样的代码:

int main()
{
    char *a = malloc(1024);
    /* Do some arbitrary stuff with 'a' (no alloc functions) */
    return 0;
}

真正的结果是什么?我的想法是进程死亡,然后堆空间也消失了,所以错过对free的调用没有什么坏处(然而,我确实认识到无论如何拥有它对于闭包、可维护性和良好实践的重要性)。我这样想对吗?

Second, let's say I have a program that acts a bit like a shell. Users can declare variables like aaa = 123 and those are stored in some dynamic data structure for later use. Clearly, it seems obvious that you'd use some solution that will calls some *alloc function (hashmap, linked list, something like that). For this kind of program, it doesn't make sense to ever free after calling malloc because these variables must be present at all times during the program's execution and there's no good way (that I can see) to implement this with statically allocated space. Is it bad design to have a bunch of memory that's allocated but only freed as part of the process ending? If so, what's the alternative?


当前回答

如果您正在从头开始开发一个应用程序,那么您可以在何时调用free方面做出一些明智的选择。您的示例程序很好:它分配内存,也许您让它工作几秒钟,然后关闭,释放它所要求的所有资源。

但是,如果您正在编写其他任何东西——服务器/长时间运行的应用程序,或供其他人使用的库,则应该期望对malloc的所有内容调用free。

暂时忽略实用主义的一面,遵循更严格的方法,并强迫自己释放您malloc的所有内容要安全得多。如果您没有在编写代码时监视内存泄漏的习惯,那么很容易就会出现一些内存泄漏。换句话说,是的,你可以没有它;不过,请小心。

其他回答

是的,你是对的,你的例子没有造成任何伤害(至少在大多数现代操作系统上没有)。进程退出后,操作系统将恢复进程分配的所有内存。

来源:分配和GC神话(PostScript警告!)

Allocation Myth 4: Non-garbage-collected programs should always deallocate all memory they allocate. The Truth: Omitted deallocations in frequently executed code cause growing leaks. They are rarely acceptable. but Programs that retain most allocated memory until program exit often perform better without any intervening deallocation. Malloc is much easier to implement if there is no free. In most cases, deallocating memory just before program exit is pointless. The OS will reclaim it anyway. Free will touch and page in the dead objects; the OS won't. Consequence: Be careful with "leak detectors" that count allocations. Some "leaks" are good!

也就是说,您应该尽量避免所有内存泄漏!

第二个问题:你的设计还可以。如果你需要存储一些东西直到你的应用程序退出,那么使用动态内存分配是可以的。如果您事先不知道所需的大小,就不能使用静态分配的内存。

几乎每个现代操作系统都会在程序退出后恢复所有已分配的内存空间。我能想到的唯一例外可能是像Palm OS这样的东西,其中程序的静态存储和运行时内存几乎是一样的,所以不释放可能会导致程序占用更多的存储空间。(我只是在猜测。)

所以一般来说,这样做没有什么坏处,除了存储容量超过所需的运行时成本。当然,在您给出的示例中,您希望保留可能被使用的变量的内存,直到它被清除。

然而,一旦你不再需要内存,就立即释放内存,并在程序退出时释放你仍然拥有的内存,这被认为是一种很好的方式。它更像是一种了解您正在使用哪些内存的练习,并考虑是否仍然需要它。如果不进行跟踪,可能会出现内存泄漏。

On the other hand, the similar admonition to close your files on exit has a much more concrete result - if you don't, the data you wrote to them might not get flushed, or if they're a temp file, they might not get deleted when you're done. Also, database handles should have their transactions committed and then closed when you're done with them. Similarly, if you're using an object oriented language like C++ or Objective C, not freeing an object when you're done with it will mean the destructor will never get called, and any resources the class is responsible might not get cleaned up.

It depends on the OS environment the program is running in, as others have already noted, and for long running processes, freeing memory and avoiding even very slow leaks is important always. But if the operating system deals with stuff, as Unix has done for example since probably forever, then you don't need to free memory, nor close files (the kernel closes all open file descriptors when a process exits.) If your program allocates a lot of memory, it may even be beneficial to exit without "hesitation". I find that when I quit Firefox, it spends several !minutes ! paging in gigabytes of memory in many processes. I guess this is due to having to call destructors on C++ objects. This is actually terrible. Some might argue, that this is necessary to save state consistently, but in my opinion, long-running interactive programs like browsers, editors and design programs, just to mention a few, should ensure that any state information, preferences, open windows/pages, documents etc is frequently written to permanent storage, to avoid loss of work in case of a crash. Then this state-saving can be performed again quickly when the user elects to quit, and when completed, the processes should just exit immediately.

我认为你的两个例子实际上只有一个:free()应该只在进程结束时发生,正如你指出的那样,这是无用的,因为进程正在终止。

但在第二个示例中,唯一的区别是允许未定义的malloc()数量,这可能导致内存耗尽。处理这种情况的唯一方法是检查malloc()的返回代码并采取相应的行动。

不释放变量并没有真正的危险,但是如果将一个内存块的指针分配给另一个内存块而不释放第一个块,第一个块将不再可访问,但仍然占用空间。这就是所谓的内存泄漏,如果您经常这样做,那么您的进程将开始消耗越来越多的内存,从其他进程占用系统资源。

如果进程是短期的,那么通常可以这样做,因为当进程完成时,所有分配的内存都会被操作系统回收,但我建议养成释放所有不再使用的内存的习惯。