如果我们释放已经免费或没有malloc的内存，会发生什么？

Question

When I free the malloc ed memory which are not allocated using malloc than valgraind tool gives me error like:

Invalid free() / delete / delete[]

Is it dangerous in embedded C programming?

In code whenever malloc fails than i cannot judge which malloc ed pointer i have to release.

Answer 1

This will lead to undefined behavior, most likely heap corruption and/or crashing your program. It's dangerous in any kind of system.

Answer 2

Your program could crash (as everyone replied), and even worse, it could crash much latter than when you have incorrectly called malloc . So finding these kind of bugs is hard.

A possible solution, assuming you have access to all your source code and can change it (ie assuming that you don't use third parties libraries which you cannot alter) might be to use Boehm conservative garbage collector , by replacing every occurrence of malloc , calloc etc ... with GC_MALLOC or GC_CALLOC after having #include d <gc/gc.h> (and then, you won't bother calling free anymore).

Boehm's garbage collector being conservative, it does have a small probability to leak (because it handles every word on your call stack as a possible pointer, even if that word in fact don't carry a dereferencable pointer, but eg an integer or a float).

And you did not explain what embedded C programming means for you. In some industries and contexts, critical embedded C code (like the one making your aircraft flying) is not even allowed to call malloc or to use dynamically allocated heap data.

Answer 3

如果你在没有用malloc ， calloc或realloc分配的内存上调用free ，或者你在已经释放的内存上调用它，你将得到未定义的行为。

Answer 4

Yes, it is dangerous for embedded programming. It is almost always crash you software.

Answer 5

You will get undefined behaviour, typically memory corruption by freeing memory that hasn't been allocated.

I don't see how you can say that you don't know which pointer result from a malloc operation has failed?

It's really quite simple.

Declare your pointers and set them to null.

Use malloc to allocate your memory. Only use malloc on pointers that are already null (to prevent memory leaks due to reassigning a pointer to a memory block to another one). If the malloc fails then the pointer will remain null.

When it comes to freeing memory allocated by a malloc operation, only perform the free operation if the pointer is not null and once freed set that pointer to null.