Interfacing with C

Interfacing with C
- 1 Dec 2011
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At last night's meetup Niki told us about the glory and the horror of getting OCaml and C talking with each other. It's a little frustrating that OCaml exposes so much of its internal representation in the C API but if you want to write bindings for C libraries you have to know this stuff.
- PDF of the presentation
- Respository of code examples
Other resources
- A more detailed tutorial
- The C interface section of "OCaml for Scientific Computation"
- The manual's chapter on Interfacing C wth OCaml
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5 months ago Markus Mottl responded:

It's great that this hairy topic got a decent presentation, but to save others some headaches, there is one important issue that I think should be fixed about it:
The talk states that the "Field"-macro can be used to both get and set fields. Though this is true in a narrow sense, it does not hold true in general. I think the subsequent examples in the presentation will confuse people about how to correctly set fields of structured blocks.
The "record example" uses "caml_alloc_tuple" to first allocate a block of size 2 and then fill in values using the "Field"-macro. The only reason why this particular example will always work is, because the size of the record is small and there is no subsequent allocation. This ensures that the block is in the minor heap (= "young generation"), and it is always safe to set fields of blocks in the minor heap with the "Field"-macro. But if the record were large (more than 256 fields (= Max_young_wosize), admittedly unlikely in practice), or if there were a subsequent allocation that could trigger the GC, the above example could segfault.
Especially the "array example" is likely to confuse people, too. Arrays can often be larger than 256 elements, which will put them on the major heap. Overwriting an element of arrays (blocks) on the major heap with "Field" is only safe in a certain situation: the written element must not be a block in the minor heap and the overwritten element must not be a block in the major heap. Your "caml_create_array" example is confusing, because it only deals with integer arrays, for which the "Field"-macro will always work. You could run into segfaults with other arrays.
Long story short: when in doubt, always use the "Store_field"-macro to set values in blocks. Unless you are really (really!) comfortable with the "low-level interface" in the OCaml-manual for interfacing C, please do not use "Field" for storing values in blocks. Even seasoned OCaml-hackers will only use it if performance matters and have to reason very carefully about the code then.
Anyway, thanks for helping people with this tricky topic!

Alex R

Contributed by Alex R
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NYC OCaml

Interfacing with C

Alex R

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