Get struct* from out parameter in Chibi Scheme FFI bindings - ffi

Question

Can you get a struct * from the out parameter of a C function in Chibi Scheme?
I'm trying to get a struct archive_entry * from this C function:
int archive_read_next_header(
struct archive *archive,
struct archive_entry **out_entry);
In C one would do it like this:
struct archive_entry *entry;
archive_read_next_header(archive, &entry);
My Chibi FFI code is:
(define-c-struct archive)
(define-c-struct archive_entry)
(define-c int
archive-read-next-header
(archive (result reference archive_entry)))
But it's not generating the right C code to get the archive_entry. I
think reference is the wrong thing to use. I also tried pointer
but it didn't work either.

Answer 1

I still don't know if it can be done directly.
But I was able to work around the problem by writing a custom thunk function in C:
(c-declare "
struct archive_entry *my_archive_read(struct archive *a, int *out_errcode) {
struct archive_entry *entry;
int errcode;
*out_errcode = errcode = archive_read_next_header(a, &entry);
return (errcode == ARCHIVE_OK) ? entry : NULL;
}")
(define-c archive_entry my-archive-read (archive (result int)))
So the key is that Scheme doesn't need to deal with any double-indirection (**) in this version. The C code converts double-indirection into single-indirection for Scheme so it all works out.
Example usage from a Scheme program:
(let ((archive (my-archive-open filename)))
(disp "archive" archive)
(let* ((return-values (my-archive-read archive))
(entry (car return-values))
(errcode (cadr return-values)))
(display entry)
(newline)))
I copied the technique from the chibi-sqlite3 bindings where they face a similar problem having to get a sqlite3_stmt * from an out parameter:
(c-declare
"sqlite3_stmt* sqlite3_prepare_return(sqlite3* db, const char* sql, const int len) {
sqlite3_stmt* stmt;
char** err;
return sqlite3_prepare_v2(db, sql, len, &stmt, NULL) != SQLITE_OK ? NULL : stmt;
}
")
(define-c sqlite3_stmt (sqlite3-prepare "sqlite3_prepare_return") (sqlite3 string (value (string-length arg1) int)))