[cinvoke-dev] Pointer to pointer as ingoing parameters?

[Michael Zedeler]

Hi C/Invoke users.

For various reasons I am trying to interface with Apache Portable 
Runtime (apr) using C/Invoke from lua. It doesn't quite pan out the way 
I was expecting to.

I have to call the function apr_allocator_create in apr which has been 
defined as this:

apr_status_t 
<http://www.schumann.cx/docs/apr/group__APR__Error__Codes.html#a0> 
apr_allocator_create 	(  	apr_allocator_t 
<http://www.schumann.cx/docs/apr/group__APR__Pool__allocator.html#a0> 
**  	  /allocator/ 	) 


apr_status_t is just an integer and I presume that apr_allocator_t is 
supposed to be a pointer to a pointer that apr_allocator_create will 
then modify for me in order to return the newly created allocator.

This is how I do it:

require("cinvoke_lua")
apr = clibrary.new("libapr-1.so.0")
strnatcasecmp = apr:get_function(Cint, "apr_strnatcasecmp", Cstring, 
Cstring)
print("Casecmp(hello, car) = " .. strnatcasecmp("hello", "car"))
print("Casecmp(hello, hello) = " .. strnatcasecmp("hello", "hello"))

apr_allocator_create = apr:get_function( Cint, "apr_allocator_create", 
Cptr )

allocator = {{}} -- my best take, but probably wrong
apr_allocator_create( allocator )

The script produces this output:

Casecmp(hello, car) = 1
Casecmp(hello, hello) = 0
Segmentation fault (core dumped)

When looking in the manual at Cptr, I can see that Cptr is not really 
intended to be used as ingoing value, except when passing null values, 
so thats problably where the problem is. Is there any way that I can 
call functions using pointers to pointers like apr_allocator_create from 
Lua using C/Invoke?

Regards,

Michael.