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?
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.
真正的结果是什么?
你的程序泄露了内存。根据您的操作系统，它可能已经恢复。
大多数现代桌面操作系统确实会在进程终止时恢复泄漏的内存，这使得忽略这个问题变得很常见(从这里可以看到许多其他答案)。
但是您依赖的是一个不属于该语言的安全特性，您不应该依赖它。您的代码可能运行在这样一个系统上，该行为下次会导致“硬”内存泄漏。
你的代码最终可能会在内核模式下运行，或者在老式/嵌入式操作系统上运行，这些操作系统不采用内存保护作为权衡。(mmu占用芯片空间，内存保护成本额外的CPU周期，并且要求程序员自己清理并不过分)。
您可以以任何您喜欢的方式使用和重用内存(和其他资源)，但请确保在退出之前释放了所有资源。
