Macro LEFT was expanded before the define macro The problem is that, within the second define, the In fact, quoting of the macro name is also recommended. (Unbalanced quote characters are more problematic LPAREN bracketed RPAREN # -> ( bracketed ) Unbalanced parentheses can be quoted to protect them: M4 also understands nested parentheses within Without a matching opening quote character (`), aĬlosing quote (') is simply echoed to the output.
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Opening comment delimiters can be protected by quotes: Thus, the previous line, if entered in full, wouldĪUTHOR is W. However, contrary to what you might expect, The contents of a comment are not examined by m4 In m4, the hash character # is the default openingĪ comment lasts up to and including the following M4 strips off these delimiters before outputting The default quote characters are the backtick (`) Quoting a string suppresses macro expansion. It's also possible to pass definitions on the command line Thus the following definition is equivalent to the No whitespace is allowed between a macro name and the If there were then that space would be echoed to the output. There is no space between the end of the macro and the dnl: You can suppress it using the “ delete to newline” If a blank line added to the output is a problem then
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However the newline at the end of the AUTHORĭefinition above would be echoed to the output. The define macro itself – including its twoĪrguments – expands to an empty string, that is, M4 comes with an initial set of built-in macros,Īfter this definition, the word “AUTHOR” is recognizedĪs a macro that expands to “W. If none is specified then it reads from stdin. Its arguments, if any, are the files it is to read (In fact, m4 is the source of some difficulties here becauseĪnd makes automated policy analysis difficult to apply.)īasics: Simple macros, whitespace, quoting, comments Security Enhanced Linux: SELinux policy files are.Scripts which make source code packages portable across different GNU Autoconf: m4 macros are used to produce “configure”.(/etc/mail/sendmail.cf) is generated using m4 from a templateįile that is much easier to read and edit (/etc/mail/sendmail.mc). sendmail: sendmail's rather cryptic configuration file.It is a far more powerful, general-purpose tool. While similar in principle to the better-known C preprocessor, This “preprocessor” or “macro processor” takes as inputĪn m4 template and sends this to the output,Īfter acting on any embedded directives, called macros.Īt its most basic, it can be used for simple embedded text The name “m4” also refers to the program which processes M4 can be called a “template language”, a “macro language” tips, e.g., macros to protect unbalanced quote characters.There is also some original material here: Is possible using just two or three flags and macros details on the myriad debugging flags: effective debugging.To observe in order to ensure compatibility) Macro (in practice, all you need to know are the restrictions the ways different versions of m4 handle the changequote.experimental features that may disappear in future versions.anything that might cause a problem if you weren't aware of itĮxamples of the kind of details omitted are:.
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