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

*To*: mathgroup at smc.vnet.net*Subject*: [mg86897] Re: Mathlink: How do I pass arbitrary data from Mathematica to C?*From*: Karen Bindash <KarenBindash at googlemail.com>*Date*: Wed, 26 Mar 2008 04:46:53 -0500 (EST)*References*: <fs4rmb$p3s$1@smc.vnet.net> <fs7ilu$gll$1@smc.vnet.net>

On 24 Mar, 06:45, Jens-Peer Kuska <ku... at informatik.uni-leipzig.de> wrote: > Hi, > > a) MathLink is a protocol, i.e., it *must* transmit data Understood > b) MathLink is always buffered, i.e., you will *never* > get the true address of a pointer in the Mathematica kernel Understood > c) only the kernel developer know how a data structure is organized > in the kernel, i.e., a array of integers *may* be a memory block > of successive integers but it can be also an array of pointers > to integers OK, I can see I cant make such assumptions. > d) for a general expression you have to build your own tree like > structure of the Mathematica expression by setting the argument > type to Manual Do you have an example of this, where the format of the data is not known in advance? It *must* be considered just a collection of bytes of data. The data can be any length from one byte to about a MB. It might be binary data, but will probably be text. The data will be passed to some hardware. The format of the data depends on the hardware. Hence I just want to pass what is a collection of bytes. I cant assume they will be integers (they might be), I cant assume they will be floating point values (they might be). > > Regards > Jens > > 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.