Difference between size_t and mwSize when compiling C MEX-files for Matlab

Question

I am currently working on porting some C MEX-files for 32-bit Matlab to 64-bit Matlab.

While doing so, I have encountered two types, one coming from the Matlab people, and one which is C standard.

This is what the Matlab documentation is saying about mwSize:

mwSize (C and Fortran)

Type for size values

Description

mwSize is a type that represents size values, such as array dimensions. Use this function for cross-platform flexibility. By default, mwSize is equivalent to int in C. When using the mex -largeArrayDims switch, mwSize is equivalent to size_t in C. In Fortran, mwSize is similarly equivalent to INTEGER*4 or INTEGER*8, based on platform and compilation flags.

This is what Wikipedia is saying about size_t:

size_t is an unsigned data type defined by several C/C++ standards (e.g., the C99 ISO/IEC 9899 standard) that is defined in stddef.h.[1] It can be further imported by inclusion of stdlib.h as this file internally sub includes stddef.h[2].

This type is used to represent the size of an object. Library functions that take or return sizes expect them to be of this type or have the return type of size_t. Further, the most frequently used compiler-based operator sizeof should evaluate to a value that is compatible with size_t.

The actual type of size_t is platform-dependent; a common mistake is to assume size_t is the same as unsigned int, which can lead to programming errors,[3][4] when moving from 32 to 64-bit architecture, for example.

As far as I can see, these types are actually the same. My questions are:

1. Are they?
2. If they are, which one would be considered better programming taste to use? Ideally we would like our code to be compatible with future Matlab releases as well. I am guessing that the answer is mwSize, but I am not sure.

Edit: I should add that the Matlab people are using both. For example:

size_t mxGetN(const mxArray *pm);

is a function that is retrieving the number of columns of an mxArray. However, when one creates a matrix, one uses,

mxArray *mxCreateDoubleMatrix(mwSize m, mwSize n, mxComplexity ComplexFlag);

where the input evidently should be mwSize.

Answer 1

mwSize is defined for backward compatibility and portability. As the documentation states, it maps to an int when the -largeArrayDims switch is not used during compilation, and size_t when it is. So, in the first case mwSize is signed, but in the second, it isn't.

Using mwSize in your code allows you to re-use the code on all platforms, irrespective of whether that flag is used or not.

As for the API inconsistencies you've pointed out, they are truly inconsistencies, but not ones for major concern. mxGetN() will never return a negative number, so having it return a size_t is OK. However, (I'm guessing) older versions or versions of the mex API on certain platforms expect an int to passed to mxCreateDoubleMatrix() so defining the function as taking an input of type mwSize makes it portable and / or backward compatible.

Short answer is, use mwSize and use -largeArrayDims to compile the mex function.