I don't know how this could have ever worked. In the first place, the arguments to sprintf are wrong. E.g. the 1st argument needs to point to a writable buffer to hold a printed result and you've got a constant character string there. 2nd argument is wrong also. In the second place, the function result of sprintf is an integer value giving the number of characters written, while mexErrMsgTxt is expecting a string argument. So I will give you one simple way to deal with this, and then point out that this code probably isn't even necessary in a mex routine.
You could change this:
if (plhs[0] == NULL)
mexErrMsgTxt(sprintf("Error allocating %dx%d result matrix",wid,wid));
To something like this:
if (plhs[0] == NULL) {
mexPrintf("Error allocating %dx%d result matrix",wid,wid);
mexErrMsgTxt("plhs[0] is NULL");
}
However, in a mex routine, if a memory allocation fails you don't drop down to the next line ... the mex routine simply terminates control back to the caller with an error. So if plhs[0] is the result of some mxCreateWatever call, and that call fails because of a memory allocation error, you WILL NOT drop down to the next line where you can test the plhs[0] value for NULL. So in all likelihood you do not need that if-test at all. If you got there, plhs[0] is guaranteed to not be NULL and that test will never be true (again assuming that it was the result of a mxCreateWhatever call).
