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

在那方面你完全正确。在小的程序中,变量必须存在,直到程序结束,释放内存并没有真正的好处。

事实上,我曾经参与过一个项目,在这个项目中,程序的每次执行都非常复杂,但时间相对较短,决定只是保持分配内存,而不是因为错误地释放内存而破坏项目的稳定。

话虽如此,在大多数程序中,这并不是一个真正的选项,或者它会导致内存耗尽。

其他回答

您是正确的,当进程退出时,内存将自动释放。有些人在进程终止时尽量不进行广泛的清理,因为它将全部移交给操作系统。然而,当你的程序运行时,你应该释放未使用的内存。如果不这样做,如果您的工作集太大,最终可能会耗尽或导致过度分页。

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。

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.

如果一个程序在退出前忘记释放几兆字节,操作系统将释放它们。但是如果你的程序一次运行几个星期,并且程序内部的循环忘记在每次迭代中释放几个字节,你将会有一个巨大的内存泄漏,它将耗尽你计算机中所有可用的内存,除非你定期重新启动它=>如果程序用于一个非常大的任务,即使很小的内存泄漏也可能是不好的,即使它最初并不是为它设计的。

如果您正在使用已分配的内存,那么您没有做错任何事情。当你编写的函数(main以外的)分配内存而不释放它,并且没有使它对程序的其余部分可用时,这就会成为一个问题。然后您的程序继续运行分配给它的内存,但没有办法使用它。您的程序和其他正在运行的程序将被剥夺该内存。

编辑:说其他正在运行的程序被剥夺了该内存并不是100%准确。操作系统总是可以让他们使用它,代价是把你的程序交换到虚拟内存(</ hand招手>)。关键是,如果您的程序释放了它不使用的内存,那么虚拟内存交换就不太可能是必要的。