N 1842: Clarifying the Behavior of the #line Directive

Submitter: David Keaton (suggested by Max Woodbury)
Submission Date: 2014-06-27
Source: WG14
Reference Document: N/A
Version: 1.0
Date: June 2014
Subject: Clarifying the Behavior of the #line Directive



In a distributed development environment, the exact file name passed to the compiler or preprocessor may vary from site to site. It is therefore desirable to be able to set the file name as seen by __FILE__ and elsewhere to a uniform value. The mechanism to do this is the '#line <num> "<string>"' form of the '#line' preprocessor directive. It is also necessary that such a directive leave the line numbering sequence unchanged. Further, it is desirable that edits that change the location of the directive in the source module should not require modification to the directive and that comments embedded in the directive likewise do not have to be accounted for.

Searches of the online literature show that a directive of the form '#line __LINE__ "string"' is expected to have this property.

Despite this, at least one compiler/preprocessor does not allow this.

Technical argument:

The value substituted for the predefined macro '__LINE__' is specified in as the presumed line number of the current source line. The presumed line number is initially (6.10.4p2) the number of newline characters (or their equivalent) seen in phase 1 of the translation process, plus 1, at the time of substitution. (Note that this is not the same as the time of tokenization, which is where the failing compilers make their mistake.) The mechanism for transferring this count between phase 1 and phase 4, where macro substitution takes place, is not specified, but may be presumed to exist and be reliable. (If it were not, the __LINE__ predefined macro would be useless.) That makes the question 'when does the substitution take place?'

Macro substitution in directives is a separate issue from macro expansion in code. It does not always take place. If and when it occurs depends on the directive and the details of its form. That means the entire directive has to be 'in hand' in order to be evaluated, and that means, in turn, that the newline that terminates the directive has to have been seen. The standard goes to some length to specify the various directive forms and all include the terminating newline in their specification.

Therefore, when a substitution is made for '__LINE__', its value should be the line count following the end of the directive, which is the same as the line number of next line in the source module. This is precisely the value that produces the desired property of the '#line __LINE__ "string"' directive.

Correction requested:

While there is no need to change the standard's normative text, a note that '#line __LINE__ "string"' and similar directives leaves line numbering unchanged would both be educational and make misinterpretations more difficult.

Suggested Technical Corrigendum

Append the following to footnote 177 in

#line __LINE__ "newfilename" changes the presumed file name without changing the presumed line number.