A quick verification...  If I cut&paste your code below, I get:
"error:  Unable to find opcode entry for 'fprints' with matching argument types:
Found: (null) fprints SSS"

OTOH, the "%s" in fprints is essentially superfluous here.  If I just use
" fprints(Sfilename, Sname)" It works fine!  A definite  bug, I'd say!

	-- Pete --


On Tue, Mar 30, 2021 at 11:36:37PM +0200, Jeanette C. wrote:
> Hey hey,
> I have problems printing an Svariable (string) to a file. Consider these 
> instruments:
> 
> instr Print
>   Sname chnget "MyString"
>   Sfilename = strcat(Sname, ".txt")
>   fprints(Sfilename,"%s", Sname)
>   turnoff
> endin
> 
> instr Caller
>   Sname = "Output"
>   chnset(Sname, "MyString")
>   schedule("Print", 0, .1)
>   turnoff
> endin
> 
> Of course, in the real code the string Sname is generated dynamically. The 
> output is only one control character (<ASCII 32). Strange, because 
> Sfilename is properly created, so Sname should be valid and assigned when 
> the fprints opcode runs.
> 
> Best wishes,
> 
> Jeanette
> 
> -- 
>  * Website: http://juliencoder.de - for summer is a state of sound
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>  * GitHub: https://github.com/jeanette-c
> 
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