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a.How to create a json file in ubuntu and what is the extension used to save json file
JSON is a standard for passing objects as plain, readable text. Similar to XML. The file extension can be json but doesn't have to be since it's just text (depends on how you're using it and whether the software you're using has some associate between a .json extension and a parser).
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b.does any header file or package has to be included
Not to make a JSON file, but you likely need something to parse things in/out of JSON.
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c.how do i use json to get data from MySQL using C programming
Not sure there's a direct between JSON and MySQL. MySQL has it's own calls. There may be software that interfaces the two but there's no direct link between them.
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d.standard procedure in compilation of json
Again, JSON is just a text format for exchanging object data. Just like you can't compile XML, you can't compile JSON.