When should a C function return newly allocated memory?

Question

In a response elsewhere, I found the following snippet:

In general it is nicer in C to have the caller allocate memory, not the callee - hence why str

Answer 1

The main advantage of having the caller allocate the memory is that it simplifies the interface, and it's completely unambiguous that the caller owns the memory. As your create/destroy example shows, the simplification is not very great.

I prefer the create/destroy convention established by Dave Hanson in C Interfaces and Implementations:

struct foo *foo_new(...);   // returns result of malloc()
void foo_free(struct foo **foop); // free *foop's resources and set *foop = NULL

You follow the convention thus:

struct foo *a = foo_new();
...
foo_free(&a);
// now `a` is guaranteed to be NULL

This convention makes it a little less likely you will leave a dangling pointer.