MATLAB: Is it possible to store a array of single precision type into unsigned char

data conversiondata types

In my existing parsing file there is a assignment

plhs[0] = mxCreateNumericMatrix( nof_data_values, elem_count, mxSINGLE_CLASS ,mxREAL);

where the number of data values is 6 and element count depends upon the no. of samples reading in single precision and in later in that same code another assignment is there

unsigned char* output_mat = reinterpret_cast< unsigned char* >(mxGetData(plhs[0]));

where after reinterpret casting of plhs[0] is getting stored in a matrix.

How is it possible I tried the same but failed using matlab.

Best Answer

No, you cannot do that directly in MATLAB. MATLAB does not have unsigned char. MATLAB has char(), which is a non-numeric datatype (neither signed nor unsigned) which is 16 bits wide per element. char() are automatically promoted to double when used in arithmetic statement as if they were small positive integers.

If you want to convert an array of single into uint8 then use

typecast(YourArrayOfSingle, 'uint8')

If you want to convert an array of single into char then use either

char(typecast(YourArrayOfSingle, 'uint16'))

char(typecast(YourArrayOfSingle, 'uint8'))

Remember, though, that it is common for bytes inside a single precision number to map into unicode code points for which there is no standard glyph.

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MATLAB: Using structures in MEX files

I see the following issues:

1) You don't check your inputs before you use them. I would recommend putting in code to check that nrhs == 2, that prhs[0] is a structure, that prhs[0] has at least one element, that prhs[0] has the expected field names (i.e., that the results of your mxGetField calls are not NULL), that prhs[1] is a numeric scalar variable.

2) You use the result of a mxGetChars call, which returns type (mxChar * ), cast it to a (char * ) type, and then downstream attempt to use these pointers to access the data as char data. This will not work. The mxChar type is typcially a 2-byte per character type (MATLAB stores character data as 2-bytes each) whereas a char in C/C++ is 1-byte. You need to use mxArrayToString instead and convert the mxArray to a C-style character string, then use that for your downstream string processing in C, then use mxFree on that pointer to free the dynamically allocated C-style string you just created with mxArrayToString.

3) You cannot get a pointer from a field of an input mxArray (e.g., bufferArray) and simply attach it to a field of an output mxArray (i.e. plhs[0]) as you are doing. The MATLAB memory manager will likely get confused and crash later on since you have probably not told it everything it needs to know about how many copies of the data are around in memory. What you will probably need to do is either duplicate bufferArray with mxDuplicateArray and then attach that to the plhs[0] field, or use the undocumented function mxCreateSharedDataCopy to create a shared data copy of bufferArray and then attach that to the plhs[0] field.

4) You use BUFFER_SIZE for your copying, but you don't check to see that the inputs have at least that many characters to copy. Also, this won't copy a null character at the end since MATLAB strings do not terminate with null characters. Another reason to use the mxArrayToString function.

5) You are writing into bufferDataPointer, which is a data area of an input mxArray prhs[0]. This data memory can be shared at the MATLAB level with other variables, so changing it directly like you are doing has the potential unwanted side-effect of changing other MATLAB variables at the same time. This is generally not a good programming practice and can lead to strange downstream errors at the MATLAB level.

MATLAB: Same Same But Different? mxGetPr and mxGetData…

mxGetData returns a (void *) type to the address of the data memory block. It can be used with a cast to get a pointer to whatever data type is in the data memory.

mxGetPr is equivalent to (double *)mxGetData. That is, it returns the same address ad mxGetData but cast as a pointer to double. E.G.,

mxArray *mx, *my;
double *pr, *qr;
int *sr;
mx = mxCreateDoubleMatrix(2,3,mxREAL);
pr = mxGetPr(mx); // pointer to first double element of mx
qr = (double *)mxGetData(mx); // pointer to first double element of mx
my = mxCreateNumericMatrix(2,3,mxINT32_CLASS,mxREAL);
sr = (int *)mxGetData(my); // pointer to first int element of my

pr and qr are essentially the same. Each one points to the first double element of mx. sr is different, it points to the first int element of my.

mxCreateDoubleMatrix creates 2D matrices where the data block contains double type data. mxCreateNumericMatrix creates 2D matrices where the data block contains a user selectable data type. mxCreateNumericArray creates nD matrices where the data block contains a user selectable data type.

mxArray *mx, *my, *mz;
mwSize ndim = 3;
mwSize dims[] = {3,4,5};
mx = mxCreateDoubleMatrix(2,3,mxREAL); // 2x3 mxArray of doubles
my = mxCreateNumericMatrix(2,3,mxINT32_CLASS,mxREAL); // 2x3 mxArray of int
mz = mxCreateNumericMatrix(ndim,dims,mxINT32_CLASS,mxREAL); // 3x4x5 mxArray of int

Best Answer

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