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3.3 Including Other Makefiles
=============================

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.  If FILENAMES is empty,
nothing is included and no error is printed.  

   Extra spaces are allowed and ignored at the beginning of the line,
but the first character must not be a tab (or the value of
`.RECIPEPREFIX')--if the line begins with a tab, it will be considered
a recipe 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.  *Note How to Use Variables:
Using 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 (Note: Setting Variables.)
or pattern rules (Note: Defining and Redefining Pattern Rules.
).

   Another such occasion is when you want to generate prerequisites from
source files automatically; the prerequisites can be put in a file that
is included by the main makefile.  This practice is generally cleaner
than that of somehow appending the prerequisites to the end of the main
makefile as has been traditionally done with other versions of `make'.
Note: Automatic Prerequisites.  

   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
`--include-dir' option are searched (Note: Summary of Options.
).  Then the following directories (if they exist) are
searched, in this order: `PREFIX/include' (normally `/usr/local/include'
(1)) `/usr/gnu/include', `/usr/local/include', `/usr/include'.

   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.  *Note How Makefiles Are Remade:
Remaking Makefiles.  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
or cannot be remade, with no error message, use the `-include'
directive instead of `include', like this:

     -include FILENAMES...

   This acts like `include' in every way except that there is no error
(not even a warning) if any of the FILENAMES (or any prerequisites of
any of the FILENAMES) do not exist or cannot be remade.

   For compatibility with some other `make' implementations, `sinclude'
is another name for `-include'.

   ---------- Footnotes ----------

   (1) GNU Make compiled for MS-DOS and MS-Windows behaves as if PREFIX
has been defined to be the root of the DJGPP tree hierarchy.