Question

I have a very simple program, listed below, which reads a value from a .mat file (a data file from Matlab) and prints it. For some reason, I get a segfault error after exiting main() – I can run gdb my_program and step through the entire method, but as soon as main() finishes, I enter some method in a Matlab related library (libmwfl.so, a dependency of libmat.so) which throws a segfault.

I am completely new to C programming, but some reading up I suspect that I’m either somehow corrupting the stack or calling some destructor twice. However, I can’t see any of those in my code – and as I said, I can step through my code with the debugger without problems.

What am I doing wrong here?

#include <stdlib.h>
#include <stdio.h>
#include <mat.h>

int main(int argc, char *argv[]) {

    double value;
    MATFile *datafile;
    datafile = matOpen("test.mat", "r");

    mxArray *mxv;
    mxv = matGetVariable(datafile, "value");
    value = *mxGetPr(mxv);
    mxFree(mxv);
    matClose(datafile);

    printf("The value fetched from the .mat file was: %f", value);

    return 0;
}

Answer 1

The documentation recommends to use the function mxDestroyArray instead of mxFree to free an mxArray. By using mxFree you probably mess up the matlab’s heap. From the documentation

Improperly Destroying an mxArray

You cannot use mxFree to destroy an mxArray.

Warning: You are attempting to call mxFree on a <class-id> array. The destructor for mxArrays is mxDestroyArray; please call
this instead. MATLAB will attempt to fix the problem and continue,
but this will result in memory faults in future releases.

Example That Causes Warning

In the following example, mxFree does not destroy the array object.
This operation frees the structure header associated with the array,
but MATLAB will still operate as if the array object needs to be
destroyed. Thus MATLAB will try to destroy the array object, and in
the process, attempt to free its structure header again.

mxArray *temp = mxCreateDoubleMatrix(1,1,mxREAL);

  ...

mxFree(temp); /* INCORRECT */

Solution.

Call mxDestroyArray instead.

mxDestroyArray(temp); /* CORRECT */