Mathlink: How do I pass arbitrary data from Mathematica to C?

*To*: mathgroup at smc.vnet.net*Subject*: [mg86837] Mathlink: How do I pass arbitrary data from Mathematica to C?*From*: Karen Bindash <KarenBindash at googlemail.com>*Date*: Sun, 23 Mar 2008 01:00:49 -0500 (EST)

I am trying to write some code to interface Mathematica to a closed- source library written in C. I'm using Mathlink for this, which is typically used to interface Mathematica to external functions - see for example http://reference.wolfram.com/mathematica/tutorial/SettingUpExternalFunctionsToBeCalledFromMathematica.html For example, if the library has a function 'f' with the following prototype: int f(int x, int y); the following in a Mathlink template (.tm file) will allow me to call this function from Mathematica by the name Foo, if I link to the library., create an executable, then install that executable in Mathematica with Install[]. :Begin: :Function: f :Pattern: Foo[x_Integer, y_Integer] :Arguments: {x, y} :ArgumentTypes: {Integer, Integer} :ReturnType: Integer :End: (There is no need for me to write the function "f", as that has been done. I just need to link to the library containing the function f, in much the same way you use the 'pow' function in C without writing it - you would just link to the maths library.). I have no problem with the above - it all works as expected. The problem occurs with a function 'g', which instead of having only integer arguments, takes a pointer to some data. The format of the data is not specified - the C function just needs to know where in memory the data is, and how many bytes there are. The C prototype is int g(const void *data, long nbytes); Does anyone know how I can write a Mathematica template so when I link with the function 'g', I can pass the data properly? Note, that since the library is closed-source, I am not able to change the calling method in any way. But I expect it should be possible to write an interface in C, such that data is passed from Mathematica in a form compatible with Mathematica, and then converts it to a from the C library accepts. But I am stuck as how to do this. If its not possible to do this with a template, can it be done by writing it all in C? If so, how? 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. Someone suggested that I might look at the C code generated by mcc to work out how to do this, (perhaps making use of MLPutInteger), but I can't work out how to do this. Does that make sense? Any ideas how to pass arbitrary data to a C program? I have a similar issue in trying to get arbitrary data from C to Mathematica, but that is another story.

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