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8. File inclusion

m4 allows you to include named files at any point in the input.

8.1 Including named files  
8.2 Searching for include files  

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8.1 Including named files

There are two builtin macros in m4 for including files:


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.

error-->30.include:2: m4: Cannot open no-such-file: No such file or directory

Assume in the following that the file `incl.m4' contains the lines:
Include file start
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 file start
=>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
=>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.

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8.2 Searching for include files

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 6.3 Controlling debugging output) can help isolate the problem.

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