Flex, version 2.5. A fast scanner generator by Paxson Vern PDF

By Paxson Vern

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Extra resources for Flex, version 2.5. A fast scanner generator

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You can tell whether ex generated a 7-bit or an 8-bit scanner by inspecting the ag summary in the `-v' output as described above. Note that if you use `-Cfe' or `-CFe' (those table compression options, but also using equivalence classes as discussed see below), ex still defaults to generating an 8-bit scanner, since usually with these compression options full 8-bit tables are not much more expensive than 7-bit tables. , one which can recognize 8-bit characters. This ag is only needed for scanners generated using `-Cf' or `-CF', as otherwise ex defaults to generating an 8-bit scanner anyway.

Another nal note regarding performance: as mentioned above in the section How the Input is Matched, dynamically resizing yytext to accommodate huge tokens is a slow process because it presently requires that the (huge) token be rescanned from the beginning. Thus if performance is vital, you should attempt to match "large" quantities of text but not "huge" quantities, where the cuto between the two is at about 8K characters/token. 19 Generating C++ scanners provides two di erent ways to generate scanners for use with C++.

The flex restriction that `input()' cannot be rede ned is in accordance with the POSIX speci cation, which simply does not specify any way of controlling the scanner's input other than by making an initial assignment to yyin. - The `unput()' routine is not rede nable. This restriction is in accordance with POSIX. - flex scanners are not as reentrant as lex scanners. In particular, if you have an interactive scanner and an interrupt handler which long-jumps out of the scanner, and the scanner is subsequently called again, you may get the following message: fatal flex scanner internal error--end of buffer missed To reenter the scanner, rst use yyrestart( yyin ) Note that this call will throw away any bu ered input usually this isn't a problem with an interactive scanner.

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Flex, version 2.5. A fast scanner generator by Paxson Vern


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