[mercury-users] C-Interface (Memory Management)
fjh at cs.mu.OZ.AU
Tue Nov 10 17:53:11 AEDT 1998
On 09-Nov-1998, Peter Herkenrath <P.Herkenrath at rrz.uni-koeln.de> wrote:
> Hi folks,
> as an old Perl addict I missed pattern matching in Mercury.
> I wanted to reuse an Sicstus interface to the regex library (prolog-rx
> by Gerd van Noord).
> The C function regex_match should return the matched substrings as a
> list(string) type. Such a case is not mentioned in the Language
> and not demonstrated in samples/c_interface.
> After much try and error I finally managed to make a (mostly) working
> interface to the regex library. The strings were allocated via
> make_many(), 4cons4ed by an Mercury function, ... .
> The general questions:
> Is this the 4right4 way to allocate memory for an object which will be
> used by Mercury?
> When (and how) should such Objects be deallocated (via oldmem?)?
The most reliable way of allocating lists and other complex data
structures, if you need to do it in C code rather than in Mercury code,
is to define constructor functions in Mercury and export these to C
using `pragma export':
:- func cons(T, list(T)) = list(T).
:- pragma export (cons/2, "cons").
cons(H, T) = [H | T].
:- func nil = list(T).
:- pragma export (nil/0, "nil").
nil = .
Then you can allocate memory by just calling the functions cons() and nil()
from your C code. The memory will be recovered by garbage collection
(if you have GC enabled) or on backtracking (if you're not using
Strings are a little more complicated. I suppose the most reliable,
implementation-independent way to allocate strings would be to make
list of characters using the technique above, and then call the Mercury
predicate string__from_char_list/2. However, that would be rather
inefficient! Fortunately there are more efficient alternatives.
Unfortunately they are a bit more implementation-specific;
that is, the details of exactly how to do depend on what compilation
grade you select, and they might change a bit in future versions of Mercury.
But nevertheless, here's how to do it. Rather than using make_many(),
you should use make_aligned_string_copy(). If the string you are
returning is guaranteed to survive for as long as Mercury will need it,
e.g. it is a string literal, then you can use make_aligned_string(),
which will often be more efficient, instead of make_aligned_string_copy().
See the comments in mercury_string.h for documentation on these macros.
If you're using conservative GC, that's all you have to worry about.
But if you're not, then there is an additional constraint to observe:
the make_aligned_string() and make_aligned_string_copy() macros should
only be called from `pragma c_code' fragments declared with
Memory allocated using make_aligned_string_copy() will be allocated on
the Mercury heap and will be recovered on backtracking or garbage
collection as usual.
Fergus Henderson <fjh at cs.mu.oz.au> | "Binaries may die
WWW: <http://www.cs.mu.oz.au/~fjh> | but source code lives forever"
PGP: finger fjh at 18.104.22.168 | -- leaked Microsoft memo.
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