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Lex and Yacc

Posted on 1998-10-25
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How do I get Lex and Yacc to read files for input?
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Question by:laeuchli
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Expert Comment

by:rwilson032697
ID: 1253817
Lex is a parser - you feed it a description of a grammar it is to read (such as C or C++) and it can parse that grammar.

Yacc (or Yet Another Compiler Compiler) uses Lex to parse a grammar which is fed back as a stream of tokens - yacc's purpose is to allow a 'compiler' to produce executables...

So you have to describe the grammar to lex, then use yacc (or yuour own program) to make use of the parsed input.

Start by having a really good read of the man pages for lex and yacc.

The GNU/Linux versions of these are Flex and Bison.

Cheers,

Raymond.
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by:laeuchli
ID: 1253818
I know how to program lex/yacc, I want to know how to feed it text files. Up to now I have been typeing in the info by hand.

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elfie earned 200 total points
ID: 1253819
If you are using one input file, you can use redirection:
eg. prog < input_file.

If you want to change your program, you can modify the function 'main'. Inside 'main' and fopen is done.
A normal way of working is parse the option of the program (eg add a -f option) and then pass the file name to the fopen call.
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by:laeuchli
ID: 1253820
I don't quite understand please explain.

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by:elfie
ID: 1253821
When you are programming lex/yacc and you compile it, in some source there must be a main function. Normally inside this main function the input and output are opened.
If you can't find it , try doing a 'grep' in the source files.

So if you want to change the source files, first look for this function, then change it. When you are about to change it, a 'clean' solution is use argument parsing (eg. getopt) to parse (argc, argv).

need more info? explain what you don't understand, and we'll try to answer.
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by:laeuchli
ID: 1253822
I found the main but the part where the files are opened is not there.What do I do?
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Expert Comment

by:elfie
ID: 1253823
can you do a grep for 'fopen' in the source files?

or

See which functions are called in the main functions and check the first few of them for an fopen call.

If no fopen can be found, check for the usage for a stdin filedescriptor.
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by:laeuchli
ID: 1253824
ohh in the source files. I will look.Thanks.



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