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### 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.

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 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 Setting Variables) or pattern rules (see 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`. See 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 (see 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. See 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 acts like `include` in every way except that there is no error (not even a warning) if any of the filenames do not exist. 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.