Converting Parameter from mxArray to a C-Style String

I have a mexfunction where I input a total of 3 parameters: two double floating point values and one with a string of characters. My question is how do I take the string parameter and convert it to a C or C++ string so I can use it in the C++ file?

[James Tursa]

Is the string parameter a char type using single quotes ' ' (in which case the answer will be easy), or is it a string class type using double quotes " " (in which case the answer is more work)?

[Jerome Richards]

I am passing it in double quotes. In MATLAB, I have:

mex my_mexfile.cpp

my_mexfile(x, y, "My String");

and in my C++ Mexfile, I have:

void mexFunction(int nlhs, mxArray *plhs[], int nrhs, const mxArray *prhs[])

{

const mxArray *pm = prhs[2];

char* str = (char*)malloc(15);

mwSize len = 15;

int v = mxGetString(pm, str, len);

printf("%s\n", str);

}

prhs[2] is what contains the string of characters. Whey I try reading and printing it, it displays completely random characters. What am I doing wrong?

Answer 1

[James Tursa]

If it is a single quote ' ' char array, then just

char *cp;
cp = mxArrayToString(prhs[2]);

If it is a double quote " " string class variable, then a bit more work:

mxArray *mx;
char *cp;
if( mexCallMATLAB(1, &mx, 1, (mxArray **)(prhs+2), "char") ) {
    mexErrMsgTxt("Unable to convert input string to char");
}
cp = mxArrayToString(mx);
mxDestroyArray(mx);

The way to tell if an input is one or the other is:

if( mxIsChar(prhs[2]) ) {
    /* do the char stuff */
} else if( mxIsClass( prhs[2], "string" )  ) {
    /* do the string stuff */
} else {
    mexErrMsgTxt("Third input needs to be char or string");
}

cp will point to a C-style string. When you are done using it, it is good programming practice to free it:

mxFree(cp);