Issue 0017.21: Is translation phase 4 defined unambiguously?

This issue has been automatically converted from the original issue lists and some formatting may not have been preserved.

Authors: Derek M. Jones, WG14
Date: 1992-12-10
Reference document: X3J11/90-056
Submitted against: C90
Status: Closed
Converted from: dr.htm, dr_017.html

Self references in translation phase 4

The following queries arise because of the imprecise way in which phase 4 interacts with itself. While processing a token within phase 4 it is sometime necessary to get the following tokens from the input, i.e. reading the arguments to a function-like macro. But when getting these tokens it is not clear how many phases operate on them:

1. Do the following tokens only get processed by phases 1-3?
2. Do the following tokens get processed by phases 1-4?

When an identifier declared as a function-like macro is encountered, how hard should an implementation try to locate the opening/closing parentheses?

In:

#define lparen (
 #define f_m(a) a
 f_m lparen "abc" )

should the object-like macro be expanded while searching for an opening parenthesis? Or does the lack of a readily available left parenthesis indicate that the macro should not be expanded?

Subclause 6.8.3, on page 89, lines 34-35 says “... followed by a ( as the next preprocessing token ...” This sentence does not help because in translation phase 4 all tokens are preprocessing tokens. They don't get converted to “real” tokens until phase 7. Thus it cannot be argued that lparen is not correct in this situation, because its result is a preprocessing token.

In:

#define i(x) 3
 #define a i(yz
 #define b )
 a b )   /* goes to 3) or 3 */

does b get expanded to complete the call i(yz, or does the parenthesis to its right get used?

Comment from WG14 on 1997-09-23:

Response

Concerning the first example:

#define lparen (
 #define f_m(a) a
 f_m lparen "abc" )

According to subclause 5.1.1.2 Translation phases, page 5, lines 25-39, the translation phases 1-3 do not cause macros to be expanded. Phase 4 does expand. To apply subclause 6.8.3 Macro replacement page 89, lines 34-35 to the example: Since lparen is not ( in “f_m lparen "abc" ),” this construct is not recognized as a function-like macro invocation. Therefore the example expands to

f_m("abc")

The same principle applies to the second example:

#define i(x) 3
 #define a i(yz
 #define b )
 a b )       /* expands via the following stages: */

 i(yz b )    /* ) delimits the argument list before b is expanded  */
 i([yz ) ])
 3

This is how we interpret subclause 6.8.3, page 89, lines 36-38: The sequence of preprocessing tokens is terminated by the right-parenthesis preprocessing token.