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6.18.2.1 Upgrading from scm_must_malloc et al.

Version 1.6 of Guile and earlier did not have the functions from the previous section. In their place, it had the functions scm_must_malloc, scm_must_realloc and scm_must_free. This section explains why we want you to stop using them, and how to do this.

The functions scm_must_malloc and scm_must_realloc behaved like scm_gc_malloc and scm_gc_realloc do now, respectively. They would inform the GC about the newly allocated memory via the internal equivalent of scm_gc_register_allocation. However, scm_must_free did not unregister the memory it was about to free. The usual way to unregister memory was to return its size from a smob free function.

This disconnectedness of the actual freeing of memory and reporting this to the GC proved to be bad in practice. It was easy to make mistakes and report the wrong size because allocating and freeing was not done with symmetric code, and because it is cumbersome to compute the total size of nested data structures that were freed with multiple calls to scm_must_free. Additionally, there was no equivalent to scm_malloc, and it was tempting to just use scm_must_malloc and never to tell the GC that the memory has been freed.

The effect was that the internal statistics kept by the GC drifted out of sync with reality and could even overflow in long running programs. When this happened, the result was a dramatic increase in (senseless) GC activity which would effectively stop the program dead.

The functions scm_done_malloc and scm_done_free were introduced to help restore balance to the force, but existing bugs did not magically disappear, of course.

Therefore we decided to force everybody to review their code by deprecating the existing functions and introducing new ones in their place that are hopefully easier to use correctly.

For every use of scm_must_malloc you need to decide whether to use scm_malloc or scm_gc_malloc in its place. When the memory block is not part of a smob or some other Scheme object whose lifetime is ultimately managed by the garbage collector, use scm_malloc and free. When it is part of a smob, use scm_gc_malloc and change the smob free function to use scm_gc_free instead of scm_must_free or free and make it return zero.

The important thing is to always pair scm_malloc with free; and to always pair scm_gc_malloc with scm_gc_free.

The same reasoning applies to scm_must_realloc and scm_realloc versus scm_gc_realloc.


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