变更概要 Appendix C - Summary of Changes

Since the publication of the first edition of this book, the definition of the C language has undergone changes. Almost all were extensions of the original language, and were carefully designed to remain compatible with existing practice; some repaired ambiguities in the original description; and some represent modifications that change existing practice. Many of the new facilities were announced in the documents accompanying compilers available from AT&T, and have subsequently been adopted by other suppliers of C compilers. More recently, the ANSI committee standardizing the language incorporated most of the changes, and also introduced other significant modifications. Their report was in part participated by some commercial compilers even before issuance of the formal C standard.

This Appendix summarizes the differences between the language defined by the first edition of this book, and that expected to be defined by the final standard. It treats only the language itself, not its environment and library; although these are an important part of the standard, there is little to compare with, because the first edition did not attempt to prescribe an environment or library.

• Preprocessing is more carefully defined in the Standard than in the first edition, and is extended: it is explicitly token based; there are new operators for concatenation of tokens (##), and creation of strings (#); there are new control lines like #elif and #pragma; redeclaration of macros by the same token sequence is explicitly permitted; parameters inside strings are no longer replaced. Splicing of lines by \ is permitted everywhere, not just in strings and macro definitions. See Par.A.12.
• The minimum significance of all internal identifiers increased to 31 characters; the smallest mandated significance of identifiers with external linkage remains 6 monocase letters. (Many implementations provide more.)
• Trigraph sequences introduced by ?? allow representation of characters lacking in some character sets. Escapes for #\^^
  
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