In the 'C' programming language, like most (all?) other High-Level Languages (HLLs), every 'identifier' must be defined before it can be used.

'Identifiers' are the names of program "objects"; eg,

A type name such as 'uint8_t' is an 'identifier';

A "variable" name is an 'identifier';

A function name is an 'identifier';

A label is an 'identifier';

etc, etc,...

So, if the compiler tells you that something is "undefined", then the solution is to provide the appropriate definition!

The first thing to check is the compiler Manual to see if the compiler itself provides a suitable definition.

The types supportes by Keil C51 are listed here:
http://www.keil.com/support/m...ctypes.htm

Note that uint8_t is not part of the original ANSI 'C' standard - it (and the other platform-independent type definitions) was an extension added by the 1999 revision of the standard - commonly known as "C99".
See, for example, http://www.netrino.com/Publications/Glossary/C.php

Keil C51 comforms to the earlier 1989/1990 revisions of the standard - commonly known as "C89" and "C90"
See: http://www.keil.com/support/docs/1893.htm
and: http://www.keil.com/product/isoansi.asp

Of course, you can easily build your own platform-independent type definitions using suitable typedefs and conditional compilation...

eg, see: http://www.8052.com/forum/read.phtml?id=123891

I use U8, S16, etc from force of habit, predating C99; as you're just starting, it would make sense for you to use the C99 names...


Update:
I couldn't find this earlier, but here's a thread with an extensive discussion on making a "portable" header file:
http://www.8052.com/forum/read.phtml?id=116161