我每次返回指针时都必须使用malloc吗？

Question

I'm working on a program that has few undiscovered memory leaks. The code is literally decades old and written by an unknown person. I have functions that return char * and I was wondering if I need to free the memory.
Specifically I have a function that replaces a substring in a string, I return the pointer to the newly malloc ed memory. Do I free the original string ?

Original version:

return(findAndReplace(str, "•",rstring));

New version:

char *result = findAndReplace(str, "•", rstring);
free(str);
return result;

Questions being: Can I be 100% sure that since I had a pointer returned from somewhere else that memory has been allocated? Is it safe to free that memory (assuming it's not used anywhere else)?

Edit: str comes from here:

str = axiom_element_get_text(element, env, messageDataNode);

Answer 1

No. You need clearly-defined contracts for your functions' pre and post conditions. In your "old version", the contract seems to be that the caller provides a pointer str to an arbitrary string; storage and lifetime of this string are up to the caller. In the "new version", you've changed it so that the caller must provide a pointer to a string obtained by malloc , and so that the pointer is no longer valid (the object has been freed) after your function returns.

Your core problem is most likely lack of any documentation of the contracts in the existing codebase , meaning you have to reverse-engineer the original author's intents. But you shouldn't go trying to change any of thus until you've documented the current situation, decided if a change makes sense, and ensured that you can safely change every point in the existing code that was assuming the original undocumented contracts.

Answer 2

No, it is not safe the free memory that was not allocated by malloc in the first place. You can use pointers without malloc/free but in a non-trivial program you are likely to be using malloc and free at certain, but not all, places.

If you are trying to detect memory leaks in your program, a tool like valgrind might help.