How to fix Matlab mex error: You can't use 'macro parameter character #' in vertical mode.

Problem

While trying to compile your Matlab or Simulink S-Function using a command such as

mex mysfunction.cpp

you get the following error:

This is pdfTeX, Version 3.141592653-2.6-1.40.26 (TeX Live 2024) (preloaded format=mex)
 restricted \write18 enabled.
entering extended mode
(./sine_wave_generator.cpp
This is MeX  Version 1.05  18 XII 1993  (B. Jackowski & M. Ry\'cko)
! You can't use `macro parameter character #' in vertical mode.
l.1 #
     define S_FUNCTION_NAME  sine_wave_generator
? 

Solution

The issue here is that you are not using mex from Matlab, but a different TeX-based program called mex. This is likely due to the mex binary being in your PATH environment variable before the Matlab binary directory.

To fix this, you can either:

• Use the full path to the mex binary in your Matlab installation, e.g. /usr/local/MATLAB/R2021b/bin/mex, or
• Change the order of directories in your PATH environment variable so that the Matlab binary directory comes before the other mex binary directory.
• Use the mex command from the Matlab command line, which should automatically use the correct binary.