m4 allows you to include named files at any point in the input.
There are two builtin macros in m4 for including files:
include(filename) sinclude(filename)
both of which cause the file named filename to be read by
m4. When the end of the file is reached, input is resumed from
the previous input file.
The expansion of include and sinclude is therefore the
contents of filename.
It is an error for an included file not to exist. If you do
not want error messages about non-existent files, sinclude can
be used to include a file, if it exists, expanding to nothing if it
does not.
include(`no-such-file') => error-->30.include:2: m4: Cannot open no-such-file: No such file or directory sinclude(`no-such-file') =>
Assume in the following that the file `incl.m4' contains the lines:
Include file start foo Include file end
Normally file inclusion is used to insert the contents of a file
into the input stream. The contents of the file will be read by
m4 and macro calls in the file will be expanded:
define(`foo', `FOO') => include(`incl.m4') =>Include file start =>FOO =>Include file end =>
The fact that include and sinclude expand to the contents
of the file can be used to define macros that operate on entire files.
Here is an example, which defines `bar' to expand to the contents
of `incl.m4':
define(`bar', include(`incl.m4')) => This is `bar': >>>bar<<< =>This is bar: >>>Include file start =>foo =>Include file end =><<<
This use of include is not trivial, though, as files can contain
quotes, commas and parentheses, which can interfere with the way the
m4 parser works.
The builtin macros include and sinclude are recognized
only when given arguments.
GNU m4 allows included files to be found in other directories
than the current working directory.
If a file is not found in the current working directory, and the file name is not absolute, the file will be looked for in a specified search path. First, the directories specified with the `-I' option will be searched, in the order found on the command line. Second, if the `M4PATH' environment variable is set, it is expected to contain a colon-separated list of directories, which will be searched in order.
If the automatic search for include-files causes trouble, the `p' debug flag (see section Controlling debugging output) can help isolate the problem.