14.11.2.
The #define directive
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 | 14.11. The Preprocessor |  |
14.11.2.
The #define directive
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#define directive
The #define directive has two versions. If the
directive is followed by only one string as in the example
#define MAEMO_SYSTEMthe preprocessor simply annotates internally that the symbol is “defined”. In Section 14.11.3 a directive is described that can check for those symbols that are defined.
The second version of this directive is when it is followed
by two strings. In this case, from this point on, the preprocessor replaces
all appearances of the first string by the second. The following example
defines the symbol MAXIMUM_LOAD to be replaced by the value
1500.
#define MAXIMUM_LOAD 1500The effect of this directive is
identical to the use of the -D
option in the compiler. In fact, when the compiler is run with the option
-Dname=value, this definition is the same as if the
preprocessor would find the line #define name value.
With the #define directive you must take into
account the following:
This directive is usually at the top of the source
files, or if needed in several of them, in a file with extension
“.h” that is included in other
files.
To differentiate in the code the regular symbols in a
program from those defined with the #define directive and
that will be replaced by their equivalent, the latter are typically
written with all capital letters (this is a convention, the preprocessor
does not perform any check).
Replacing a symbol by its value is done in all the text in a file. This includes the also the preprocessor directives. In the following example
#define ONE 1
#define ANOTHER_ONE ONE
int main(argc, char *argv[])
{
printf("%d\n", ANOTHER_ONE);
}the program prints the number one. This means that the
second #define directive is processes as #define
ANOTHER_ONE 1 after replacing ONE by the definition
of the previous line.
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| 14.11. The Preprocessor |  | 14.11.3.
The #ifdef, #else and
#endif directives
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