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Conditionals and Macros
Conditionals are useful in connection with macros or assertions,
because those are the only ways that an expression's value can vary
from one compilation to another. A `#if' directive whose expression
uses no macros or assertions is equivalent to `#if 1' or `#if 0'; you
might as well determine which one, by computing the value of the
expression yourself, and then simplify the program.
For example, here is a conditional that tests the expression
`BUFSIZE == 1020', where `BUFSIZE' must be a macro.
#if BUFSIZE == 1020
printf ("Large buffers!\n");
#endif /* BUFSIZE is large */
(Programmers often wish they could test the size of a variable or
data type in `#if', but this does not work. The preprocessor does not
understand `sizeof', or typedef names, or even the type keywords such
The special operator `defined' is used in `#if' expressions to test
whether a certain name is defined as a macro. Either `defined NAME' or
`defined (NAME)' is an expression whose value is 1 if NAME is defined
as macro at the current point in the program, and 0 otherwise. For the
`defined' operator it makes no difference what the definition of the
macro is; all that matters is whether there is a definition. Thus, for
#if defined (vax) || defined (ns16000)
would succeed if either of the names `vax' and `ns16000' is defined as
a macro. You can test the same condition using assertions (
Assertions.), like this:
#if #cpu (vax) || #cpu (ns16000)
If a macro is defined and later undefined with `#undef', subsequent
use of the `defined' operator returns 0, because the name is no longer
defined. If the macro is defined again with another `#define',
`defined' will recommence returning 1.
Conditionals that test whether just one name is defined are very
common, so there are two special short conditional directives for this
is equivalent to `#if defined (NAME)'.
is equivalent to `#if ! defined (NAME)'.
Macro definitions can vary between compilations for several reasons.
* Some macros are predefined on each kind of machine. For example,
on a Vax, the name `vax' is a predefined macro. On other
machines, it would not be defined.
* Many more macros are defined by system header files. Different
systems and machines define different macros, or give them
different values. It is useful to test these macros with
conditionals to avoid using a system feature on a machine where it
is not implemented.
* Macros are a common way of allowing users to customize a program
for different machines or applications. For example, the macro
`BUFSIZE' might be defined in a configuration file for your
program that is included as a header file in each source file. You
would use `BUFSIZE' in a preprocessing conditional in order to
generate different code depending on the chosen configuration.
* Macros can be defined or undefined with `-D' and `-U' command
options when you compile the program. You can arrange to compile
the same source file into two different programs by choosing a
macro name to specify which program you want, writing conditionals
to test whether or how this macro is defined, and then controlling
the state of the macro with compiler command options.
Assertions are usually predefined, but can be defined with
preprocessor directives or command-line options.
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