First... Look it up in your help file... In case you haven't discovered it yet, Pelles C comes with just about the best help file I've ever seen... Highlight the keyword and press F1.
Anway... memicmp() is not a standard C function. This means that in Pelles C it is prefixed with an underscore as in ... _memicmp() .... #include <string.h> to access it and check "Define Compatibility Names" in your Project Settings.