Dear Guile Maintainer,
I am using Guile 1.6.4 on a Debian (unstable) based i686--actually I do
not think the exact architecture is especially important since what I am
reporting is rather a problem with the documentation. I would like to
define symbols for use in Guile and C code in the C code where I also
initialise (with `scm_c_define_gsubr') the C functions that have an
interface in Guile. I have tried defining a global (in C) SCM variable
and setting the symbol's name with `scm_str2symbol' but the symbol thus
defined disappears unless I protect it (from the garbage collector?)
with `scm_gc_protect_object'. Is that right? Anyway, I have not been
able to find (in the Guile Reference Manual) an explanation of how to
define Guile symbols from the C startup code, and I think that should be
possible (to do it, and to find an explanation).
I have enclosed a complete example; it can be compiled using `gcc main.c
-lguile', and on the Guile command prompt, you can see the difference
between the symbols `exists' and `doesnt-exist' by typing
(equals-exists? 'exists)
and
(equals-doesnt-exist? 'doesnt-exist)
The functions `equals-exists?' and `equals-doesnt-exist?' are mirror
images of one another, as are the symbols `exists' and `doesnt-exist';
the only difference is that `exists' is protected using
`scm_gc_protect_object' while `doesnt-exist' is not...
Please let me know if you need more information.
Thanks,
Pascal Cedraschi
/* showcase for the use of scm_gc_protect_object */
#include <libguile.h>
/* Global variables to hold symbols; as their names indicate, the
* first one is to survive garbage collection, the second isn't.
*/
SCM exists;
SCM doesnt_exist;
/* initialise the above topology symols */
/* return #t if sym equals exists, #f otherwise */
static SCM equals_exists_q (SCM sym)
{
SCM_ASSERT(SCM_SYMBOLP(sym), sym, SCM_ARG1, "equals-exists?");
return sym==exists?SCM_BOOL_T:SCM_BOOL_F;
};
/* return #t if sym equals doesnt_exist, #f otherwise */
static SCM equals_doesnt_exist_q (SCM sym)
{
SCM_ASSERT(SCM_SYMBOLP(sym), sym, SCM_ARG1, "equals-doesnt-exist?");
return sym==doesnt_exist?SCM_BOOL_T:SCM_BOOL_F;
};
/* Here's the beef. The symbol exists is protected from the garbage
* collector whereas the symbol doesnt_exist is not. If we access the
* global variables exist and doesnt_exist from C code (invoked in
* turn from Scheme code), they behave differently since doesnt_exist
* has already been garbage collected away by the time the scheme
* interpreter is invoked.
*/
static void init (void)
{
scm_gc_protect_object(exists=scm_str2symbol("exists"));
doesnt_exist=scm_str2symbol("doesnt_exist");
scm_c_define_gsubr("equals-exists?", 1, 0, 0, equals_exists_q);
scm_c_define_gsubr("equals-doesnt-exist?", 1, 0, 0,
equals_doesnt_exist_q);
};
/* The actual main function. main (below) does not much more than
* invoke this function.
*/
static void inner_main (void *closure, int argc, char **argv)
{
init();
scm_shell(argc, argv);
};
/* Just the standard main for a Guile extended program. The actual
* work is done by inner_main above.
*/
int main (int argc, char *argv[])
{
scm_boot_guile (argc, argv, inner_main, NULL);
exit(0); /* never get here */
};
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