I’m having a OCaml program that calls GNU readline library via C interface. For command completion, readline has a C variable rl_attempted_completion_function which requires to be set as a pointer to a C function that the first argument is the string to be completed and the return value is an arra…

[image] LdBeth: Apparently the C caller to rl_attempted_completion_function won’t free the result for me (?*) If memory serves, you should return a freshly malloc-ed array of freshly malloc-ed strings (you signal the end with a NULL pointer) and the library will free the array after it is don…

Thank you for the reply. Does caml_string_length(s) includes the null end? That is, I’m having the following code converting the result, am I doing this right? From caml_string_is_c_safe I’d assume caml_string_length(s) does not count the null byte. static char **command_completion(const char *te…

[image] LdBeth: Does caml_string_length(s) includes the null end? OCaml strings consist of arbitrary sequences of bytes and may contain embedded NULLs. In particular they are not terminated with NULL, so you will need to add those by hand on the C side. [image] LdBeth: words = cam…

[image] nojb: OCaml strings consist of arbitrary sequences of bytes and may contain embedded NULLs. In particular they are not terminated with NULL, so you will need to add those by hand on the C side. This is slightly misleading: the actual representation of OCaml strings in memory ensures t…

[image] vlaviron: This is slightly misleading: Indeed @vlaviron ! I forgot for a moment this clever point about the encoding of strings in OCaml. Thanks! The OP still needs to reserve an extra byte for the trailing NULL though :slight_smile: Best wishes, Nicolas

Thank you for the feedback. [image] nojb: This is unsafe: you need to register the result of caml_copy_string as a GC root before calling the OCaml callback. Otherwise if the GC runs before you use that value (inside the callback), the value may be moved in the OCaml heap but the pointer won’t …

[image] beajeanm: you can have a look at linenoise Thank you for mention it, I’m actually more like to take this as an exercise learning how to interpolate OCaml with C, but this could be an implementation reference.

[image] LdBeth: Does that mean declare the result using CAMLlocal and call CAMLdrop before return? Indeed, you use CAMLlocal to declare the result, and, typically CAMLreturn and the end of the function (CAMLdrop is a low-level macro which is not documented if I’m not wrong). I should have me…

I don’t know for sure whether CAMLdrop is documented, but it works, doesn’t it? I used it with CAMLparam etc., in a C++ graphic interface that provides many, many virtual functions of which few will actually be used by an appl ication - I don’t want to go through all the motions to prepare for GC, …

C callbacks for GNU readline command completion

Learning

beajeanm March 18, 2021, 3:02pm 7

If you are more interested in the command completion rather than the readline integration, you can have a look at linenoise

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C callbacks for GNU readline command completion

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