Problem building a Matlab equivalent to C-type Char* pointer to a list of strings

Hi,

I need to interface my Matlab code to a C code through the Matlab-C interface.

In the C code there's a function that expects as input a pointer to a list of strings where each string is 30 characters long including terminator.

So in my Matlab code I create the following:

Say the values I want to pass from Matlab to the C code are given in the "labelsArray" variable which is an 8x1 cell array.

say it looks something like this:

labelsArray =

  8×1 cell array

    {'label1'       }
    {'label2'       }
    {'label3'       }
    {'label4'       }
    {'label5'       }
    {'label6'       }
    {'label7'       }
    {'label8'       }

I do the following:

% Convert labelsArray to string array, pad every entry up to 29 characters,
% and convert the entire thing to a char array
parameter_labels_asChars = char(pad(string(labelsArray),29));
% add the null terminator to each entry (making it 30 characters long including
% termminator) for each entry
parameter_labels_null = obj.nullTerminatedCharArray(parameter_labels_asChars);
% reshape the char array from 8x30 to 1x240
parameter_labels = reshape(parameter_labels_null.',1,[]);

in the end what I send in the calllib function is parameter_labels. and it looks like:

parameter_labels =

    'label1                        label2                        label3                        label4                        label5                        label6                        label7                        label8                        '

where there are 30 cahracters (including spaces and null terminator) between the beginning of each 2 words (including the words themselves).

However, the C application, upon receiving this input, is unhappy, saying it can't find 'label1' parameter.

Now, the execution of the calllib function on the Matlab side is successful. Also, If I run the C function from within the C application with a normal C Char* pointer to a list of strings, the function works fine - so no internal problem with this function.

It seems there's a problem with how I build the Matlab equivalent of the required pointer to a list of strings.

Your help will be very appreciated.

Answer 1

[Benjamin Thompson]

You may want to review the documentation article "Pass Arguments to Shared C Library Functions". For a NULL terminated string, use char[] as the type in the C function. For a cell array of character vectors, use *char[]. To get more help we may need you to post your C code and C function prototype plus supporting MATLAB code.