Q: Where can I get a BNF or YACC grammar for C?
A: The definitive grammar is of course the one in the ANSI standard; see question 11.2. Another grammar by Jim Roskind is available at ftp.eskimo.com in home/scs/roskind_grammar.Z. A fleshed-out, working instance of the ANSI C90 grammar (due to Jeff Lee) is on ftp.uu.net (see question 18.16) in usenet/net.sources/ansi.c.grammar.Z (including a companion lexer). [footnote] The FSF's GNU C compiler contains a grammar, as does the appendix to K&R2.
The comp.compilers archives contain more information about grammars; see question 18.3.
K&R1 Sec. A18 pp. 214-219
K&R2 Sec. A13 pp. 234-239
ISO Sec. B.2
H&S pp. 423-435 Appendix B