Hello guys, welcome back to my blog. In this article, I will discuss the difference between a Mex and M-script file in MATLAB, performance comparison, and use cases.

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Difference Between A MEX and M Script File in MATLAB

MATLAB is primarily known for its matrix computations, algorithm development, modeling, simulation, and data analysis. At its core, MATLAB code is written in .m files, also known as M scripts or M functions.

However, sometimes, MATLAB’s interpreted nature (meaning it reads and executes line-by-line) may cause performance bottlenecks for large-scale or computationally intensive tasks.

To overcome this, MATLAB allows integration with compiled languages like C, C++, or Fortran using MEX (MATLAB Executable) files. Both M Scripts and MEX Files serve their own purposes, and selecting one depends on your application’s requirements.

What is an M Script?

An M Script is a plain text file containing a series of MATLAB commands with a .m extension. It is interpreted and executed by MATLAB in real-time.

There are two types of M files:

Scripts: A script is a file containing a sequence of MATLAB statements. It operates on data in the workspace.
Functions: Functions are M files that accept input arguments and return outputs. They have their own local workspaces.

Characteristics of M Scripts:

Simple and easy to write.
Platform-independent (can run on any OS where MATLAB is installed).
Excellent for prototyping and rapid development.
Slower for computation-heavy tasks compared to compiled languages.
Easy to debug using the MATLAB IDE.

Example of an M Script:

% This is a simple M script to add two numbers
a = 5;
b = 10;
c = a + b;
disp(['Sum is: ', num2str(c)]);

What is a MEX File?

MEX stands for MATLAB Executable. An MEX file is a C, C++, or Fortran subroutine that is compiled into a dynamically linked library (DLL), which can be called directly from MATLAB as if it were a built-in function.

Characteristics of MEX Files:

Written in C, C++, or Fortran.
Requires compilation into a platform-specific binary (.mexw64 for Windows 64-bit, .mexa64 for Linux, .mexmaci64 for macOS).
Typically much faster than M Scripts for computation-intensive operations.
Enables integration of legacy code or third-party libraries with MATLAB.
Requires knowledge of C/C++ or Fortran along with MATLAB’s MEX API.

Example of a Simple MEX Function (C Code):

#include "mex.h"

void mexFunction(int nlhs, mxArray *plhs[], int nrhs, const mxArray *prhs[]) {
    double *x, *y;
    x = mxGetPr(prhs[0]);
    y = mxGetPr(prhs[1]);
    
    plhs[0] = mxCreateDoubleScalar(*x + *y);
}

To compile:

mex addNumbers.c

Then use in MATLAB:

result = addNumbers(5, 10);
disp(result); % Output: 15

Key Differences Between MEX and M Script

Performance Comparison

One of the main reasons developers use MEX files is performance.

MATLAB scripts, being interpreted, have overheads at runtime.
MEX files, being compiled, have minimal runtime overhead and direct machine code execution, making them significantly faster, sometimes 10x to 100x faster, depending on the task.

Performance Comparison Example:

A matrix multiplication in MATLAB script might take 10 seconds.
A matrix multiplication using optimized MEX code can complete in less than 1 second.

Important Note: Not every M Script will benefit greatly by converting into MEX. Overheads like disk I/O, plotting, or MATLAB-internal functions are already optimized.

Advantages and Disadvantages

M Script Advantages:

Easy to write and read.
Best for prototyping.
Highly portable.
Easy integration with MATLAB GUI and Simulink.

M Script Disadvantages:

Slower for computation-heavy tasks.
Limited access to low-level system resources.

MEX File Advantages:

Extremely fast for numerically heavy or loop-intensive algorithms.
Access to low-level system features.
Reuse existing C/C++/Fortran codebases.

MEX File Disadvantages:

Complex to develop and debug.
Not portable across operating systems without recompilation.
Requires careful memory management (risk of crashes).

Use Cases

When to Use M Scripts:

Quick algorithm development.
Data visualization.
Signal processing and analysis.
GUI creation and simple automations.
Scripting repetitive tasks.

When to Use MEX Files:

Real-time data processing.
Heavy mathematical computations (e.g., image processing, FFTs, optimization problems).
Integrating C/C++ libraries.
Enhancing speed of bottleneck MATLAB functions.

How to Create MEX Files

Steps to Create a MEX File:

a. Install a supported C/C++ Compiler:

MATLAB automatically detects supported compilers (mex -setup).

b. Write C/C++ code using MEX API.

c. Compile using mex command:

mex filename.c

d. Call the compiled function directly from MATLAB.

MEX API Basics:

Every MEX file must define a function:

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

Where:

nrhs = number of input arguments.
prhs = array of input arguments.
nlhs = number of output arguments.
plhs = array of output arguments.

Common Challenges and Solutions

Best Practices for Using MEX and M Scripts

Prototype in M Scripts first, then convert to MEX if performance is insufficient.
Profile your MATLAB code using profile on and profile viewer to find bottlenecks.
Minimize data copies between MATLAB and MEX; work on pointers when possible.
Validate inputs carefully inside MEX files to avoid segmentation faults.
Use try-catch blocks and error handling properly in MEX code.
Document your MEX functions properly, just like MATLAB functions.
Separate computation-heavy tasks into MEX, and keep data visualization in M Scripts.
Always test across different MATLAB versions if distributing code.

Conclusion

Both M Scripts and MEX Files are integral parts of MATLAB’s ecosystem, each suited for specific needs.

M Scripts are ideal for flexibility, rapid development, and prototyping.
MEX Files provide critical performance advantages for time-sensitive, heavy computations and allow MATLAB users to tap into the power of compiled C, C++, and Fortran code.

Choosing between them depends on:

The complexity of the task.
Performance requirements.
Developer expertise.
Project timelines.

In many professional MATLAB projects, developers start with M Scripts for clarity and fast iteration and optimize critical parts using MEX when needed.

Thus, a strong grasp of both will make you a far more efficient and capable MATLAB developer.

This was about “Difference Between A MEX and M Script File in MATLAB: A Complete Guide“. Thank you for reading.

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Chetan Shidling

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