When I click on Run button, a warning appears at the startup screen : "Taken too large, exceeds YYLMAX"... Anybody knows about that?

probably some string or maybe variable/fucnection etc name in your code is too long

I don't think so ... My variable,function and string names are short

Here They Are

1.s_sifre (function)
2.c_sifre (function)
3.s_alfabe(function)
4.c_alfabe(function)
5.sayi (variable)
6.a (variable)
7.deger (variable)
8.karakter (char*)
9.yazi (STRING*)
10.sifrelendi (STRING*)
11.temp (STRING*)
12.cozulen (STRING*)
13.cozulecek (STRING*)
14.deger2 (variable)
15.sifre (variable)

Try removing or commenting one by one only those pieces of code which use STRING* or char* and then test..if it crashes then you'll got your code that crahses.

Is anything assigned to Taken as a var char etc. If so then try to reduce it's size.
Just a guees...

Good luck.