How can I pass a constant C string to a function in the code generated from Embedded MATLAB in Real-Time Workshop Embedded Coder 5.1 (R2008a)?

In an Embedded MATLAB function, I can call a custom C function with EML.CEVAL.

It I pass a string as an argument, the generated code use a pointer to an array of char. I would like to pass the actual string.

For example, when generating C code for :

eml.ceval('MyCExternalFunction', arg1,['MyString' char(0)]);

I get :

MyCExternalFunction (arg1, tmp)

where tmp is a pointer to "MyString"

I would like to have :

MyCExternalFunction (arg1, "MyString")

Answer 1

[MathWorks Support Team]

When using strings in Embedded MATLAB, there is one first thing that is important to know :

'MyString' in MATLAB and "MyString" in C are not equivalent. In C, strings are null-terminated, whereas in MATLAB we treat them as character arrays (without null-termination). Therefore, it is not correct to pass

'MyString' to a C function and assume it is treated as a "MyString" in C.

Thus, the correct code for

eml.ceval('MyCExternalFunction', arg1, 'MyString');

should be:

eml.ceval('MyCExternalFunction', arg1, ['MyString' 0]);

Then, in order to pass a constant C string to a custom C function, one can use the EML.OPAQUE instruction. For example, EML.CEVAL call becomes :

eml.ceval('MyCExternalFunction', arg1, eml.opaque('const char *', '"MyString"'));

and that will just "glue-in" the "MyString" constant in the generated code :

MyCExternalFunction(arg1, "MyString");

Note : refer to related solution 1-7K299R for Null-terminated C strings.

[cui,xingxing]

what about R2022b now? pass a string scalar to matlab coder to generate C code may does't work!

for example :

mystring = "C:/temp/1.jpg";
mychar_string = char(mystring);
coder.ceval('MyExternalFunction',coder.rref(mychar_string))

where 'MyExternalFunction' declaration is :

void MyExternalFunction(const char* mychar_string);

However, matlab variable `mychar_string` pass to C is `C:/temp/1.jpg<*>`， one extra * garbled character at the end. how to fix it?

in generate C code