QA C enforces the C programming language as defined in the ISO/IEC 9899:1990 document along with 2 Technical Corrigenda and Normative Addendum 1 which addresses issues of internationalization. The same standard was originally published by the ANSI committee as X3.159-1989.
These documents are in many respects identical. QA C references follow the ISO numbering system in which the first section number is 3 greater than the corresponding section in the ANSI document. So, for example, Section 6.1.2.4 (Storage Durations of Objects ) in the ISO document is numbered 3.1.2.4 in the ANSI document.
The ANSI standard, in addition to the language specification, also contains a very helpful appendix which provides a rationale behind some of the decisions made by the standardisation committee.