The three ways to #include
#include in C is used to include other files, be it
header files or source files (useful for example when generating
code when you have small sections of code that are architecture
dependent, or as an alternative to templates found in C++).
To include a file named “file.h”, you either write
#include "file.h" or #include <file.h>.
Unfortunately it's rather common the see the wrong one used.
#include <file.h> is used to include an installed
header file, such as one in /usr/include, whereas
#include "file.h" is used for files within the code
base, these are name relative to the file it is included from
(not the file it is included to), but the compiler also lets
the user specify additional directories to search from. Although
extremely uncommon, absolute file paths may also be used.
But there is a third way to use #include, namely
to specify the file to include dynamically using a macro.
That is, #include FILE can be used to include whichever
filename the macro FILE expands to. This can be used
to deal with platform and implementation differences; letting
the user specify at compile-time which file should be used.
FILE must (ultimately) expand to a string either a
regular double-quote string, to the #include-specific
angle-bracket string. Unfortunately, string concatenation is
not supported, although you will probably never need that.