There are some problems with the C preprocessor definition in the C standard.
Following is a proposal to fix one area of those problems.
Does this seem like a good idea? Any suggestions on this proposal?
Any chance that clang would implement this before it becomes part of the C
standard (so that the C committee has feedback on how good/bad this idea is)?
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<title>N21xx C2X Proposal: Preprocessor line numbers</title>
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<p><br>
<!-- Who are the authors... -->
<b>Submitter:</b>Fred J. Tydeman<br>
<!-- What is the date of submission. yyyy-mm-dd -->
<b>Submission Date:</b>2016-09-??<br>
<b>Document:</b> WG14 N21xx<br></p>
<p>Problem being solved</p>
<p>With respect to preprocessor behaviour of line numbers and
__LINE__, there are differences between implementations, and
between releases of the same implementations.</p>
<p>For most implementations, it is not clear what conventions are
being used for line numbering, therefore, one cannot predict what
line number will be associated with a given preprocessor
token.</p>
<p>This makes if very difficult to write portable code that has
consistent behaviour.</p>
<p>This has shown up in (at least) DRs 173, 464, 483.</p>
<p>Solution</p>
<p>Add the following to 6.10.4 Line control:</p>
<blockquote>
<p>The presumed line number (footnote 1) associated with a
preprocessor token shall be the presumed line number of the
first character of the preprocessor token, except for the
preprocessor tokens in a macro replacement-list which get the
presumed line number of the macro invocation (footnote 2).</p>
<p>(footnote 1) The presumed line number is the same as the
physical line number unless changed by a #line directive.</p>
<p>(footnote 2) Required by the assert() macro.</p>
<p>The presumed line number associated with a macro invocation
shall be the presumed line number of the macro name of the
invocation.</p>
<p>The presumed line number of a preprocessing directive shall
be the presumed line number of the # preceding the directive
name.</p>
<p>A preprocessing directive of the form<br>
<code>#line __LINE__ "filename"</code><br>
shall change the presumed file name without changing the
presumed line number.</p>
</blockquote>
<p>Existing practice</p>
<p>Based upon my testing, each of the above requirements has been
implemented by at least one implementation, however, it is not
clear if there are any implementations that do them all.</p>
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