Libpointers and multilevel pointers

Hi there,

I am stuck with shared library and multilevel pointers and was hoping that some of you may help. I need to use a DLL which notably provides two functions:

char ** functionOne();
[stringPtrPtr] = functionOne(); (matlab equivalent)

void functionTwo(char * Text);
functionTwo(cstring); (matlab equivalent)

The first one is recognized as returning stringPtrPtr data. Which seems allright so far. Matlab is indeed able to read the array of string returned.

Then, I need to provide functionTwo with one of the elements of the array of strings returned by functionOne.

I'm doing: TextArray = calllib(DLL_Alias , 'functionOne');

get(TextArray) shows:

• DataType = stringPtrPtr
• Value = {'Text1', 'Text2', ...}, hence a cell array of strings

Which synthax, if it is possible, do I have to use to provide Text1 or Text2's addresses to functionTwo ?

• Writing:

TAval = TextArray.Value();

calllib(DLL_Alias, TAval{1}) % for text1

calllib(DLL_Alias, TAval{2}) % for text2

fails as TAval{1} is a string and not a pointer

• Writing:

calllib(DLL_Alias, TextArray) % for text 1

calllib(DLL_Alias, TextArray+1) % for text 2

fails as TextArray or TextArray+1 are interpreted as stringPtrPtr and not cstring (nor stringPtr).

I've also tried to force the datatype to stringPtr or cstring, which Matlab seems to refuse.

Any hint?

Thanks, Matthieu

Answer 1

[Malcolm Lidierth]

Does any of this help?

http://www.mathworks.com/matlabcentral/answers/6198-attempt-to-call-external-dll-function-causing-segmentation-fault

Answer 2

[Philip Borghesani]

calllib(DLL_Alias, 'functionTwo',  TAval{1}) % for text1

Should work. Note that in your example you forgot 'functionTwo'. MATLAB strings are automatically converted into pointers (cstrings) in function calls.

Example code:

cd (fullfile(matlabroot,'extern','examples','shrlib'));
loadlibrary shrlibsample
strings={'str1','str2'};
[s1,s2]=calllib('shrlibsample','stringToUpper',strings{2})

If you still have problems please post your MATLAB version.