ahh i figured what my problem was: I #included the <web/sproc.h>-file and not the <web/web.h>. For some Reason the compiler had problems linking all that stuff...
I got the problem, that In this code "pipe" is NEVER invalid, as long as I have pgnuplot as starting command. Even if there appears a problem with starting pgnuplot for example, if I use invalid options and similar stuff.
Second Problem is, that for some reason I can not use the Kill()-Command. I am just not able to finish gnuplot when I started it...