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Flex can generate three kinds of lexers:

• Interactive

• Non-interactive ("batch")

• Sometimes interactive (the default).

Non-interactive lexers are the most efficient; they read input a buffer at a time. That's great for parsing files, but it's annoying for parsing console input, particularly when the intent is to respond to each line typed. Interactive lexers avoid this problem by reading input one character at a time, so that a token is reported as soon as possible. But that can be inefficient for input with large tokens.

The default lexer tests to see if its input is a console and uses that information to choose between interactive and non-interactive buffer handling. The only problem with this strategy is that the standard C library does not provide any way to tell if an input stream is a console. Unix(-like) systems do have a function which answers that question, but it doesn't work with standard C FILE* streams; it requires the underlying "file number". The fileno function extracts the underlying file number from a FILE*, but it is obviously not a standard C function.

So if you have a default lexer, it needs to call fileno. However, fileno is not declared in any standard header unless you #define a feature-test macro. But you have to define these macros right at the beginning of the C program, which is hard to do when the C program is generated automatically. The easiest solution is to add the macro definition on the compiler command line using the -D option: