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
Summary
Context:
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 6.10.8.1p1 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 6.10.8.1p1:
#line __LINE__ "newfilename" changes the presumed
file name without changing the presumed line number.