Hi Alaaeldin,
I understand that you are getting an error "sldrtai" saying device not found while using a "S-function" block. A common cause of this is segmentation violation due to an incorrect input port direct feedthrough setting. Another cause can be incorrect memory accesses which occur when your code accesses beyond the end of an array. For example, if you access input port 5 and have specified only 4 input ports.
To debug your C-MEX S-function, you can enable diagnostics by compiling the S-function source with the -g flag, e.g.,
mex -g sfunction_name.c
Hope this helps!
