A local variable is a variable that is declared inside a function
definition. A global variable is a variable that is defined outside of any
function definition.
C isolates parameters and local variables, enabling you to reuse their
names. Accordingly, the values of a function's parameters and local
variables are not available after that function has returned. Also, when
one function calls another, the values of the parameters and local
variables in the calling function are not available during the execution of
the called function.
You can obtain a value for a global variable everywhere, except in
functions in which there is a parameter or local variable that happens to
have the same name. You can assign a value to a global variable anywhere
that you can obtain a value for that variable.
You can obtain a value for a global constant everywhere, except in
functions in which there is a parameter or local variable that happens to
have the same name.
Macro substitutions take place before the main work of compilation
begins.
If you want to define a macro substitution,
then instantiate the following pattern:
