Chapter 4. Conditionals
A conditional is a directive that instructs the preprocessor to
select whether or not to include a chunk of code in the final token
stream passed to the compiler. Preprocessor conditionals can test
arithmetic expressions, or whether a name is defined as a macro, or both
simultaneously using the special defined operator.
A conditional in the C preprocessor resembles in some ways an if
statement in C, but it is important to understand the difference between
them. The condition in an if statement is tested during the
execution of your program. Its purpose is to allow your program to
behave differently from run to run, depending on the data it is
operating on. The condition in a preprocessing conditional directive is
tested when your program is compiled. Its purpose is to allow different
code to be included in the program depending on the situation at the
time of compilation.
However, the distinction is becoming less clear. Modern compilers often
do test if statements when a program is compiled, if their
conditions are known not to vary at run time, and eliminate code which
can never be executed. If you can count on your compiler to do this,
you may find that your program is more readable if you use if
statements with constant conditions (perhaps determined by macros). Of
course, you can only use this to exclude code, not type definitions or
other preprocessing directives, and you can only do it if the code
remains syntactically valid when it is not to be used.
GCC version 3 eliminates this kind of never-executed code even when
not optimizing. Older versions did it only when optimizing.
4.1. Conditional Uses
There are three general reasons to use a conditional.
A program may need to use different code depending on the machine or
operating system it is to run on. In some cases the code for one
operating system may be erroneous on another operating system; for
example, it might refer to data types or constants that do not exist on
the other system. When this happens, it is not enough to avoid
executing the invalid code. Its mere presence will cause the compiler
to reject the program. With a preprocessing conditional, the offending
code can be effectively excised from the program when it is not valid.
You may want to be able to compile the same source file into two
different programs. One version might make frequent time-consuming
consistency checks on its intermediate data, or print the values of
those data for debugging, and the other not.
A conditional whose condition is always false is one way to exclude code
from the program but keep it as a sort of comment for future reference.
Simple programs that do not need system-specific logic or complex
debugging hooks generally will not need to use preprocessing