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Freeing C Functions

In C, which is best practice when it comes to freeing the memory returned from functions:

  • Provide a destructor function that encapsulates the call in free ().
  • Require users of a free () returned pointer.

For example, to open and close a file, we do:

FILE* f = fopen("blah", "w");
fclose(f);

This is preferable:

FILE* f = fopen("blah", "w");
fclose(f);
free(f);

Warning: Do not call free () in the FILE pointer. I use it only in a hypothetical implementation.

What about cases where local variables are referenced in returned memory? Is free () harmful here? (or perhaps this should never be done)

FILE f = &fopen("blah", "w");
fclose(&f);
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10 answers

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FILE* f = fopen("blah", "w"); // allocs a FILE struct
fclose(f); // The implementation of fclose() will do what necessary to 
           // free resources and if it chooses to deallocate any memory
           // previously allocated
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I not only love the functions of the destructor, but also as a Dave Hanson convention, which takes the address of the pointer and resets the pointer as the memory becomes free:

Thing_T *Thing_new(void);
void Thing_dispose(Thing_T **p);   // free *p and set *p = NULL
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