[cfe-dev] C preprocessor and __LINE__

Fred J. Tydeman via cfe-dev cfe-dev at lists.llvm.org
Wed Sep 21 22:06:24 PDT 2016

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)?

<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">

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  "HTML Tidy for OS/2 (vers 1 September 2005), see www.w3.org">

  <title>N21xx C2X Proposal: Preprocessor line numbers</title>

  <!-- 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

  <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>Add the following to 6.10.4 Line control:</p>

    <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

    <p>The presumed line number of a preprocessing directive shall
    be the presumed line number of the # preceding the directive

    <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>

  <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>

Fred J. Tydeman        Tydeman Consulting
tydeman at tybor.com      Testing, numerics, programming
+1 (775) 287-5904      Vice-chair of PL22.11 (ANSI "C")
Sample C99+FPCE tests: http://www.tybor.com
Savers sleep well, investors eat well, spenders work forever.

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