Q:

Is there something like the official C documentation?

I'm looking for the documentation of all the syntax and built-in functions in C, but I can't find any site online which seems like an ultimate official source of standard C knowledge.
Apart from the famous book by Kernighan and Ritchie, isn't there any online C specification? Maybe there is, and I don't know how to find it. Or maybe the problem is that I don't exactly know what I'm looking for.

A:

You can obtain a PDF copy of the C99 standard (ISO/IEC 9899:1999) from ANSI (and other fine standards organizations) for your private use for a modest fee - I believe it was 18 USD when I bought mine.  Having that available is invaluable to me.  But if you find a copy in public, then it is contraband.
You can find final committee drafts and current working documents of the C (C0x) Standard Committee at the JTC1, SC22, WG14 web site at the Open Standards Organization.  (That's Joint Technical Committee 1, Sub-Committee 22, Working Group 14, I believe).
There is also a hardback book 'The C Standard: Incorporating Technical Corrigendum 1' (also including the Rationale) available for a slightly less modest 85 USD or thereabouts.
One of the best books about C is 'C: A Reference Manual' in its fifth edition.  However, you asked for 'ultimate official source of standard C knowledge', and the only 'official' such source is the C standard, plus its corrigenda.
See also the material at http://www.coding-guidelines.com/ and http://c0x.coding-guidelines.com/.

A:

The official C99 standard is available for purchase from your national standards body; however, in practice everyone reads the draft with post-1999 corrections/amendments included, which you can download for free:
http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1256.pdf

A:

There is this online website devdocs
They have multiple API documentations which are well organized. It is free and open-source.
For c doc click this link http://devdocs.io/c/

