problems linking/running printf

[andyfozzy]

I would like to use printf(), or something similar, as a basic debugging tool, but I keep running into problems either building or running. If I follow the printf example in the documentation, like so:

@EXTERNAL
FUNCTION printf : DINT
VAR_INPUT 
    format: STRING;
    args: ...;
END_VAR
END_FUNCTION

FUNCTION main : DINT
	printf('hello, world %d !$N', 1);
END_FUNCTION

it builds and links fine, but I get a segfault when I run it. The same happens for the hello_world.st if I remove {ref} from puts:

@EXTERNAL
FUNCTION puts : DINT
VAR_INPUT
    str: STRING;
END_VAR
END_FUNCTION

On the other hand if I try to use {ref} in printf:

@EXTERNAL
FUNCTION printf : DINT
VAR_INPUT {ref}
    format: STRING;
    args: ...;
END_VAR
END_FUNCTION

FUNCTION main : DINT
	printf('hello, world %d !$N', 1);
END_FUNCTION

I get this error during building:

Error: SyntaxError { message: "Unknown type: VOID", range: SourceRange { range: 0..0 }, err_no: type__unknown_type }

I am not sure if I should be using a reference or not, but I suspect I should, since the printf() c function takes a pointer as its first argument.

Any information about how to get this working would be welcome.

Thanks,
Andrew

[ghaith]

Hello Andrew,

This should definitely not be happening :)
As you already said the VAR_INPUT block requires the {ref} pragma for print.
I will look into this today to see if it broke with some changes but my best guess at the moment is to tell the linker that you need libc (I'm not sure if this happens automatically)
You can do this by using -lc in the command line, or adding a system library entry to the plc.json with no include files and that has the name libc

EDIT: I just noticed that the segfault is only when no {ref} is used. addit the libc won't help there, I will take a look later today.

[andyfozzy]

After some playing around, I found a way to make it work. I put the string reference in one VAR_INPUT block and the variadic arguments in separate block, like so:

{external} 
FUNCTION printf : DINT
VAR_INPUT {ref}
    format: STRING;
END_VAR
VAR_INPUT
    args: ...;
END_VAR
END_FUNCTION

FUNCTION main : DINT
VAR
    temp: DINT := 2;
END_VAR
    printf('numbers: %d %d $N ', 1, temp);
END_FUNCTION

[ghaith]

Ah yes, that makes sense, the variadic arguments cannot be references.
I will add an issue to correct the documentation with this.

On second thought, there is a WIP commit that will make variadics support references, #568. This however, will not work for the printf arguments as these arguments themselves are not pointers.