[Mono-devel-list] P/Invoke question

[Julio David Quintana]

Thanks for the tips and links.  Looking at the code that calls 
rrd_graph(), prdata (calcpr) looks to be a pointer to an array of 
strings for holding some messages generated during the graph creation 
process.  xsize and ysize are filled during the process as well..  I am 
not interested in any of this returned info (calcpr, xsize, ysize), so 
I'd like to just fill the spots with dummy vars.

----- BEGIN rrd-tool.c -----
char **calcpr;
int xsize, ysize;
int i;
int tostdout = (strcmp(argv[2],"-") == 0);
if( rrd_graph(argc-1, &argv[1], &calcpr, &xsize, &ysize) != -1 ) {
            if (!tostdout)
                printf ("%dx%d\n",xsize,ysize);
            if (calcpr) {
                for(i=0;calcpr[i];i++){
                    if (!tostdout)
                        printf("%s\n",calcpr[i]);
                    free(calcpr[i]);
                }
                free(calcpr);
}
----- END -----

>Also important: if `prdata' is an "out" array of strings, what is the
>memory allocator?  malloc(3)?  g_malloc()?  GlobalAlloc()? 
>CoTaskMemAlloc()?


It is, and malloc is used.  I checked out your P/Invoke doc, but most of 
it went over my head. :)


Marcus:
Thanks for the code snippets.  However, I could not get them to work 
properly.  data.cpp compiles and runs fine standalone, and compiles fine 
as a shared lib, however, whenever data.cs calls it, garbage is 
returned.  Maybe I'm doing something silly.


Thanks again for all the help.

JDQ



Jonathan Pryor wrote:

>On Sat, 2004-08-21 at 18:56, Julio David Quintana wrote:
>  
>
>>I'm trying to write bindings for rrdtool (librrd.so) and have run across 
>>a triple-pointer situation that I'm not quite sure how to handle.
>>
>>Code:
>>int rrd_graph(int argc, char **argv, char ***prdata, int *xsize, int *ysize)
>>    
>>
>
>Well, you could always use IntPtr, or `ref IntPtr', or...
>
>A better question is this: what does `prdata' represent?  Is it an "out"
>pointer to an array of strings?  Is it a triply-indirected single
>character pointer (unlikely, but possible).  Is it an "in" pointer to a
>3-D matrix?
>
>You need to know what you're dealing with to properly P/Invoke with it.
>
>Also important: if `prdata' is an "out" array of strings, what is the
>memory allocator?  malloc(3)?  g_malloc()?  GlobalAlloc()? 
>CoTaskMemAlloc()?
>
>For more information, see:
>
>  - The "Managed and Unmanaged Code Integration" guide, in monodoc
>    (/Mono Handbook/Advanced Topics/Interoperability/Platform Invoke),
>    or http://www.jprl.com/~jon/interop.html.
>
>  - Interop Marshaling at MSDN (beware line wrapping):
>    http://msdn.microsoft.com/library/default.asp?url=/library/en-us/cpguide/html/cpconinteropmarshaling.asp
>
> - Jon
>
>
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