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

*To*: mathgroup at smc.vnet.net*Subject*: [mg86908] Re: Mathlink: How do I pass arbitrary data from Mathematica to C?*From*: Karen Bindash <KarenBindash at googlemail.com>*Date*: Wed, 26 Mar 2008 04:48:58 -0500 (EST)*References*: <fs4rmb$p3s$1@smc.vnet.net> <fsa5b1$a9u$1@smc.vnet.net>

On 25 Mar, 06:16, David Bailey <dave at Remove_Thisdbailey.co.uk> wrote: > Karen Bindash wrote: > > 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/SettingUpExternalFu... > > > 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. > > When you pass data over MathLink, you are communicating between two > processes - so no pointers can be passed. It certainly sounds as though > you need to write an interface routine in C - as you suggest. Presumably > you know the structure of this data, so perhaps you could transmit it > from Mathematica in pieces and assemble it in the required format in the > interface routine. > > Perhaps you should tell us the structure of this data, and how big it is. > > David Baileyhttp://www.dbaileyconsultancy.co.uk The data will be passed to some hardware via a library routine written in C. For example, the first bit of data might bea command like "RESET", the next might be "LOADFILE" The next bit of data might be a binary file. Hence I need to find a way of passing an arbitrary collection of bytes. It might be one byte, two bytes, three bytes ... up to a couple MB or so. I will know the exact length. It it was a file I wanted to send, then I assume something like data=BinaryReadList["filenane.dat",Character8] would be suitable to read it, then I would need to send the contents of 'data' But equally the data might just be a bit of text.