Strange MEX Compile Issue

I am currently using MATLAB 2019a (Research Licence) and am writing some MEX utility functions.

I have finished a couple of scripts today which compiled this morning, and I have used them a handful of times since. I started to work on another script for myself and am getting an unusual error when I attempt to compile:

>> mex sum_3D.c
Building with 'MinGW64 Compiler (C)'.
Error using mex
C:\Work\MY_Utilities\sum_3D.c:12:6: error: conflicting types for 'mexFunction'
 void mexFunction(int nlhs, mxArray *plhs[], int nrhs, const mxArray *prhs[])
      ^~~~~~~~~~~
In file included from C:\Work\MY_Utilities\sum_3D.c:1:0:
C:\Program Files\MATLAB\R2019a/extern/include/mex.h:356:6: note: previous declaration of 'mexFunction' was here
 void mexFunction(
 

I'm storing all my working data, etc. in

'C:\Work\'

and I have write permissions to it. I've stripped all the code I was writing out of the sum_3D.c file so tha now it just loks like this:

// Sum tissue values over each slice of a 3D matrix
#include <mex.h>
#include <matrix.h>
#include <stdio.h>
#include <stdlib.h>
#include <math.h>

void exit_now(char *err_text)
{
    mexErrMsgTxt(err_text);
}

void mexFunction(int nlhs, mxArray *plhs[], int nrhs, const mxArray *prhs[])
{
    exit_now("\nTest.\n");
}

Confusingly, the code I used this morning which is kept in the same folder and has the exact same declarations/header files/mexFunction arguments etc. recompiles successfully! I've closed down MATLAB multiple times and done a cold boot and it has not solved the problem (which, lets be honest, it usually does). Does anybody know what could be causing this?

Thanks in advance.

[Geoff Hayes]

Rowan - if you open the file C:\Program Files\MATLAB\R2019a/extern/include/mex.h, what is the signature for mexFunction? Is it identical to

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

?

[Rowan Lawrence]

Hi Geoff,

Yes, the entry-point signature is:

void mexFunction(
    int           nlhs,           /* number of expected outputs */
    mxArray       *plhs[],        /* array of pointers to output arguments */
    int           nrhs,           /* number of inputs */
    const mxArray *prhs[]         /* array of pointers to input arguments */
    );

I've actually 'solved' it; I changed to what I thought was the old syntax and it now works:

void mexFunction(int nargout, mxArray *pargout[], int nargin, const mxArray *pargin[])
{
    /*  */
}

I also changed the argument names back to those shown in the 'mex.h' file for both this code and one of the working files I compiled this morning. Everything is working fine now. I could not be more confused.