Why malloc memory in a function and free it outside is a bad idea?

Question

if this is a bad idea, how to allocate memory in the function?

Answer 1

It's not necessarily a bad idea to allocate memory in a function. You just have to be sure to clean it up appropriately.

The problem is that you might lose the ability to do that once you leave function scope.

Just be careful with your design. Match up malloc with free every time and you won't have memory leaks.

Answer 2

It's not a "bad idea", but rather "sometimes a bad idea", which can be said about many ideas in programming.

Allocating memory inside a function and releasing it outside may be a common design pattern, by the way. Consider:

// hashtable is a typedef-ed pointer type
hashtable ht = hashtable_new();
// .. do something with hashtable
hashtable_free(ht);

ht was allocated in a function hashtable_new and released outside it, yet you will see this pattern over and over in lots of good C code.

What it does show, however, is how the same logical unit (the hash-table ADT) takes care of allocation and de-allocation. This makes lots of sense - because the one who knows how to allocate, knows best how to deallocate. Allocating and releasing in different logical units is more often a bad idea.

Answer 3

See some previous posts:

when-should-a-c-function-return-newly-allocated-memory

where-should-i-deallocate-memory-within-functions

Answer 4

It's not a bad idea if you just keep it consistent in your own style.

A good approach is to pass the allocated memory to the caller that can then free it when its done. Something like this:

int my_new(char **obj) {
    *obj = malloc(somesize);
}

and then call this from your function like this:

char *obj;

my_new(&obj);

/* work on obj */

free(obj)

Edit: made my_new() return int. No need for double pointers otherwise