This is the closest one can get to the most recent C standard (the actual standard costs money) for free. It is the working document from just before the standard was adopted. http://www.open-std.org/jtc1/sc22/wg14/www/docs/n2454.pdf
and a good keyword reference https://www.c-programming-simple-steps.com/c-keywords.html
This is the standard for the C language (C11)
http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1570.pdf
This is the previous standard for the C language (C99)
http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1124.pdf
The wiki page on C
http://en.wikipedia.org/wiki/ANSI_C
The wiki page on gcc
http://en.wikipedia.org/wiki/GNU_Compiler_Collection
This is an absolutely wonderful reference from Dave Marshall...Thanks Dave!