Sometimes it is necessary to select one of several different header files to be included into your program. They might specify configuration parameters to be used on different sorts of operating systems, for instance. You could do this with a series of conditionals,
#if SYSTEM_1
# include "system_1.h"
#elif SYSTEM_2
# include "system_2.h"
#elif SYSTEM_3
...
#endif
That rapidly becomes tedious. Instead, the preprocessor offers the ability to use a macro for the header name. This is called a computed include. Instead of writing a header name as the direct argument of #include, you simply put a macro name there instead:
#define SYSTEM_H "system_1.h"
...
#include SYSTEM_H
SYSTEM_H will be expanded, and the preprocessor will look for
system_1.h as if the #include had been written that way
originally. SYSTEM_H could be defined by your Makefile with a
-D option.
You must be careful when you define the macro. #define saves tokens, not text. The preprocessor has no way of knowing that the macro will be used as the argument of #include, so it generates ordinary tokens, not a header name. This is unlikely to cause problems if you use double-quote includes, which are close enough to string constants. If you use angle brackets, however, you may have trouble.
The syntax of a computed include is actually a bit more general than the above. If the first non-whitespace character after #include is not " or <, then the entire line is macro-expanded like running text would be.
If the line expands to a single string constant, the contents of that string constant are the file to be included. CPP does not re-examine the string for embedded quotes, but neither does it process backslash escapes in the string. Therefore
#define HEADER "a\"b"
#include HEADER
looks for a file named a\"b. CPP searches for the file according to the rules for double-quoted includes.
If the line expands to a token stream beginning with a < token and including a > token, then the tokens between the < and the first > are combined to form the filename to be included. Any whitespace between tokens is reduced to a single space; then any space after the initial < is retained, but a trailing space before the closing > is ignored. CPP searches for the file according to the rules for angle-bracket includes.
In either case, if there are any tokens on the line after the file name, an error occurs and the directive is not processed. It is also an error if the result of expansion does not match either of the two expected forms.
These rules are implementation-defined behavior according to the C standard. To minimize the risk of different compilers interpreting your computed includes differently, we recommend you use only a single object-like macro which expands to a string constant. This will also minimize confusion for people reading your program.