INTROL-C COMPILER REFERENCE MANUAL

#undef: The #undef directive causes the named identifier to be
"undefined". Thus any subsequent #ifdef and #ifndef directives which
reference the identifier will operate as if it was never defined. It
has the form
#undef
where is the name of the identifier that is to be
undefined.
DATA CONVENTIONS
All user defined identifiers have two attributes, (1) storage class
and (2) type, which are described below.
STORAGE CLASS
An identifier's storage class indicates the location, scope and
lifetime of the storage associated with the identifier. There are
four different storage classes: auto, extern, static, and register.
auto: Automatic variables are local to the block or function in
which they are defined. They exist only while the block or function
in which they were defined is executing. Their contents are
discarded upon exit from the block. Variables in a function which
are not explicitly defined as having a specific storage class are
assumed to be automatic (ie auto) variables.
extern: External variables exist for the entire execution of the
program and retain their values throughout the execution of the
program. An external variable may be referenced by any function in
the program file in which it was defined. Also, separately compiled
program files which declare external variables of the same name
refer to the same variable, thus allowing communication between
separately compiled program files.
In Introl-C there is little distinction made between an external
"definition" and an external "declaration". It is possible to link
several files together in which an external variable has been
declared but never defined; the linker will simply define the
variable to fit the declarations. It is also permitted to link files
in which an external variable has been defined more than once; the
linker will simply treat the extra definitions as if they were
declarations. The linker will issue a warning if an external
variable has multiple incompatible definitions in a group of files
to be linked. An external variable may be initialized only once
among all the program files-to be linked together.
register: The idea behind the register storage class is that it may
be desirable to have a frequently used variable stored in a high
speed register. The register storage class is a hint to the compiler
that it should, if possible, place this variable in a high speed
register. In the case of Introl-C, the compiler makes most of these
kinds of decisions on its own. Specifying a variable as being of
C.6.6