The include directive tells make to suspend reading the
current makefile and read one or more other makefiles before continuing.
The directive is a line in the makefile that looks like this:
include filenames...
filenames can contain shell file name patterns.
Extra spaces are allowed and ignored at the beginning of the line, but
a tab is not allowed. (If the line begins with a tab, it will be
considered a command line.) Whitespace is required between
include and the file names, and between file names; extra
whitespace is ignored there and at the end of the directive. A
comment starting with `#' is allowed at the end of the line. If
the file names contain any variable or function references, they are
expanded. See section How to Use Variables.
For example, if you have three `.mk' files, `a.mk',
`b.mk', and `c.mk', and $(bar) expands to
bish bash, then the following expression
include foo *.mk $(bar)
is equivalent to
include foo a.mk b.mk c.mk bish bash
When make processes an include directive, it suspends
reading of the containing makefile and reads from each listed file in
turn. When that is finished, make resumes reading the
makefile in which the directive appears.
One occasion for using include directives is when several programs,
handled by individual makefiles in various directories, need to use a
common set of variable definitions
(see section Setting Variables) or pattern rules
(see section Defining and Redefining Pattern Rules).
Another such occasion is when you want to generate dependencies from
source files automatically; the dependencies can be put in a file that
is included by the main makefile. This practice is generally cleaner
than that of somehow appending the dependencies to the end of the main
makefile as has been traditionally done with other versions of
make. See section Generating Dependencies Automatically.
If the specified name does not start with a slash, and the file is not found in the current directory, several other directories are searched. First, any directories you have specified with the `-I' or with the `--include-dir' option are searched (see section Summary of Options). Then the following directories (if they exist) are searched, in this order:
If an included makefile cannot be found in any of these directories, a
warning message is generated, but it is not an immediately fatal error;
processing of the makefile containing the include continues.
Once it has finished reading makefiles, make will try to remake
any that are out of date or don't exist.
See section How Makefiles Are Remade.
Only after it has tried to find a way to remake a makefile and failed,
will make diagnose the missing makefile as a fatal error.
If you want make to simply ignore a makefile which does not exist
and cannot be remade, with no error message, use the -include
directive instead of include, like this:
-include filenames...
This is acts like include in every way except that there is no
error (not even a warning) if any of the filenames do not exist.