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MATLAB returns empty string from C-MEX file

Asked by Zoltán Csáti on 19 Nov 2018
Latest activity Commented on by Zoltán Csáti on 22 Nov 2018
I have an external C function which takes a double pointer and returns a char. The problem is that when I interface that function, I cannot extract the character. Always an empty string is returned by MATLAB. What is the problem? The mex compiler returns no errors and warnings.
Here is a minimum working example:
#include "mex.h"
char mwe(double* a)
{
return 'h';
}
/* Gateway function */
void mexFunction( int nlhs, mxArray *plhs[],
int nrhs, const mxArray *prhs[])
{
char *code;
double *input;
input = mxGetPr(prhs[0]);
*code = mwe(input);
plhs[0] = mxCreateString(code);
}

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1 Answer

Answer by James Tursa
on 19 Nov 2018
Edited by James Tursa
on 20 Nov 2018
 Accepted Answer

It is hard to believe that this code did not crash MATLAB. The issues:
char *code; <-- This is an uninitialized pointer ... it contains a garbage address
double *input;
input = mxGetPr(prhs[0]);
*code = mwe(input); <-- this copies a char into the garbage address (should have crashed MATLAB here)
plhs[0] = mxCreateString(code); <-- Attempts to create a MATLAB char array from the bytes at that garbage address
That mxCreateString( ) call could easily have crashed MATLAB as well. If you want to copy characters into a pointer, then you need to allocate that pointer first. E.g.,
code = mxMalloc(2);
:
code[0] = mwe(input);
code[1] = '\0'; /* Null terminate the string */
:
mxFree(code);
It is not really clear what your ultimate intent is passing a double pointer into mwe and getting a char passed back, so I can't give you any advice there unless you give more details.

  5 Comments

Most C functions that have a (char *) type as an input argument are expecting a Null terminated string. This is true for C in general, not just MATLAB C/C++ API functions. So anytime you see that as an input agrument you should be expecting to supply a Null terminated string unless there is specific documentation otherwise. The terminology "C-style string" or simply the single word "string" in C means "Null-terminated string."
In the mxCreateString( ) doc they use the word "string" when describing the input argument str. Although not explicitly stated, just the use of the word "string" in a C context means "Null-terminated string."
Contrast that with the doc for mxGetString( ). Here, the description for the argument variable str simply calls it a "starting location." I.e., on input you are not expected to supply a C-style Null-terminated string ... you are simply supplying a memory address. The mxGetString( ) function will actually write charaters into this memory and Null-terminate it (i.e., it will create a C-style Null-terminated string at that memory location as an output of the function).
If you are always only getting a single char back and you want to create an mxArray string from it, I would just avoid that mxMalloc( ) and mxFree( ) stuff entirely. E.g., just do this instead:
char code[2] = {0,0}; /* Includes the null termination */
double *input;
input = mxGetPr(prhs[0]);
code[0] = mwe(input);
plhs[0] = mxCreateString(code);
Yes, the C function I must interface always returns one char. Thanks for the suggestion.

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