What parameters this DLL function expects? If it expects strings they have to be null-terminated strings. Try

strBaseDir="C:\test" + CHR(0)
strCommand="SIGN" + CHR(0)

>*this works
>DECLARE integer FSL_Command IN rapidsign string strBaseDir, string strCommand
>MR=FSL_command( "C:\TEST", "SIGN")
>
>*this does not work
>DECLARE integer FSL_Command IN rapidsign string@ strBaseDir, string@ strCommand
>strBaseDir="C:\test"
>strCommand="SIGN"
>MR=FSL_command(@strBaseDir, @strCommand)
>
>any explanation ?
>
>*****************************************************************************
> the DLL specifications are :
> * The functions have a PASCAL calling convertion (same as win APIs).
> * The functions provided by this DLL are *NOT* reentrant.
> * Many of these functions are blocking.
> * The complex data types (structures) expected must have a member alignment of 1 byte (ie no padding).