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A.3 M4 Dependency

The macro processor m4⁸ must be installed wherever flex is installed. flex invokes ‘m4’, found by searching the directories in the PATH environment variable. Any code you place in section 1 or in the actions will be sent through m4. Please follow these rules to protect your code from unwanted m4 processing.

• Do not use symbols that begin with, ‘m4_’, such as, ‘m4_define’, or ‘m4_include’, since those are reserved for m4 macro names. If for some reason you need m4_ as a prefix, use a preprocessor #define to get your symbol past m4 unmangled.
• Do not use the strings ‘[[’ or ‘]]’ anywhere in your code. The former is not valid in C, except within comments and strings, but the latter is valid in code such as x[y[z]]. The solution is simple. To get the literal string "]]", use "]""]". To get the array notation x[y[z]], use x[y[z] ]. Flex will attempt to detect these sequences in user code, and escape them. However, it’s best to avoid this complexity where possible, by removing such sequences from your code.

m4 is only required at the time you run flex. The generated scanner is ordinary C or C++, and does not require m4.