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1999-08-25 Rob Browning <rlb@cs.utexas.edu> * Add this file (./ChangeLog). I'm planning to add change information here, and I invite others to do the same. For those that use emacs, 'C-x 4 a' adds a new entry. If people don't like this idea, we can drop it. * Add new scheme function gnc:choose-item-from-list-dialog. There is C side code, but it's only meant to be called from scheme. This function is quite flexible in handles selections. Eventually we might want to generalize this even more, but there are some GC issues to consider. See src/gnome/query-user.c and gnc.gwp for details, and see src/scm/extensions.scm for an example usage. * IMPORTANT: queryBox signature has changed. See the docs in src/gnome/query-user.c and in gnc.gwp for details. * Remove queryBox stubs from Motif side. Motif doesn't support it (yet), and with gnc.gwp conditional inclusion we can just ignore it on the motif side. * Add function (current-gnc-compile-flavor) to gnc.gwp so that we can have conditional blocks. Possible return values are 'gnome and 'motif. git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@1930 57a11ea4-9604-0410-9ed3-97b8803252fd
70 lines
3.0 KiB
Plaintext
70 lines
3.0 KiB
Plaintext
-*-text-*-
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This file is intended to contain information for those interested in
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working on the guile bits of GnuCash.
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I've recently added some GUI functions callable from scheme. This is
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generally pretty straightforward, and you can look in the code to see
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how I did it, but there are a few bits you have to be careful about.
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One of the main sources of useful information is "info guile-ref".
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This contains the documentation for all the guile C-side functions
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like gh_car(), gh_append(), etc. that manipulate opaque SCM objects
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from the guile side.
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Given that and a reasonable understanding of GTK/GNOME, you should be
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able to follow what I've done.
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Garbage collection:
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-------------------
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One issue to keep in mind is that of garbage collection. You cannot
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pass a scheme side item to the C side (through a SCM) and then store
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that object off somewhere on the C side such that it lives longer than
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all of it's guile side references. If you do, you're likely to get a
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crash. The problem is that guile's garbage collector only knows to
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save guile items that still have guile side pointers, or that are
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found somewhere on the current C side stack. If you store a SCM item
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off in a C data structure (say a callback pointer), and then return to
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the guile side and drop the guile-side reference to the item, guile
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may garbage collect it before it's used by the C side.
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For example, this psudeo-code is a problem:
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void gnc_some_function(SCM scm_thunk) {
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gnc_set_push_button_callback(some_button, scm_thunk);
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}
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(define (unsafe-guile-function)
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(let ((my-callback (lambda () (display "Hello\n"))))
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(gnc:some-function my-callback)))
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The problem here is that if you call unsafe-guile-function, it
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registers the pointer to the anonymous lambda created in the let
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construct with the button on the C-side and then returns. As soon as
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it returns, guile has no more references to the anonymous lambda, and
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it's not on the C stack, so guile thinks it's OK to garbage collect
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the function even though the C side has a pointer to it and may still
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use it.
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The moral of this story is that if you need to have the C side ferret
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away a scheme item for later, you must also keep at least one
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reference to that item on the guile side until the C side is finished
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with it. I believe that the guile people have recently come up with a
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nice general solution to this problem, but until that's generally
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available, there are a number of ways you can solve this.
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If nothing else, you can just create a global hash on the guile side,
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place the object in question in the hash table, and then have the
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C-side delete that item from the hash when it's finished with it.
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Guile Interrupts:
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-----------------
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Another issue that I'm not quite sure of myself yet is that of
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interrupts. Guile has the ability to protect certain segments of code
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with SCM_DEFER_INTS/SCM_ALLOW_INTS, but at the moment I'm not sure
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when this is required. If anyone gets the chance to check this out
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before I do, then please edit this file and put your findings here.
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