Re: Mathlink: How do I pass arbitrary data from Mathematica to C?
- To: mathgroup at smc.vnet.net
- Subject: [mg86901] Re: Mathlink: How do I pass arbitrary data from Mathematica to C?
- From: Szabolcs Horvát <szhorvat at gmail.com>
- Date: Wed, 26 Mar 2008 04:47:38 -0500 (EST)
- Organization: University of Bergen
- References: <email@example.com>
On Mar 23, 7:00 am, Karen Bindash <KarenBind... at googlemail.com> wrote: > > int g(const void *data, long nbytes); ... > I don't want Mathematica to try to interpret the data in any way - > just to pass an address of where the data is in memory, and also the > number of bytes of data. Than the library function g will return an > integer, which I want to pass back to Mathematica. I think that there is a misunderstanding here. Everyone has assumed in their replies that you would like to transmit the data that const void *data is pointing to between the C program and Mathematica. However, it seems to me that the data is actually handled by the C program, and *never* by Mathematica. So Mathematica should only know about very simple objects that somehow *refer to* this "special" object that is stored in memory by the C program (and not Mathematica). Please clarify this important point. If this is the case, then one could use a Mathematica object someObject[ptr, len], where ptr and len are integers, and encode the pointer to the data and the length of the data. Only this someObject object would be passed to the C program (easily achievable with .tm templates), which in turn would decode the two values (ptr and len) to obtain a pointer and a length. This would work for as long as all the someObject objects are constructed by the C program. But if the user examines the structure of someObject and tries to construct someObjects with arbitrary ptr and len values, then the C program will get some invalid pointers, and will most likely crash. So I would suggest maintaining a table on the C side, which translates some identifier (e.g. a single integer) to a (ptr, len) pair. The identifier would be used on the Mathematica side, and the C program could check for validity of identifiers before translating them to pointers, to avoid invalid pointers and crashes.