[Bug c/67266] New: Use of "cpp -P ..." collapses multiple blank lines

[Bug c/67266] New: Use of "cpp -P ..." collapses multiple blank lines

[mike at flyn dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67266

            Bug ID: 67266
           Summary: Use of "cpp -P ..." collapses multiple blank lines
           Product: gcc
           Version: 5.1.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: mike at flyn dot org
  Target Milestone: ---

I am using:

       cpp -C -P -nostdinc -std=c99 -Werror

to process something that is not C. Everything works, except that one side
effect of "-P" is that consecutive newlines are compressed into one.

For example,

$ cpp -P
x


x

outputs

x
x

I have not found documentation of this behavior. My hunch is that -P should not
behave this way because the man page states, "this might be useful when running
the preprocessor on something that is not C code." I would think that users
employing cpp to process non-C would expect newlines to be preserved.

I previously brought this up on the GCC mailing list. See
https://gcc.gnu.org/ml/gcc-help/2015-08/msg00069.html.

[Bug preprocessor/67266] Use of "cpp -P ..." collapses multiple blank lines

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67266

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
From
https://gcc.gnu.org/onlinedocs/gcc-5.2.0/cpp/Preprocessor-Output.html#Preprocessor-Output
:
The output from the C preprocessor looks much like the input, except that all
preprocessing directive lines have been replaced with blank lines and all
comments with spaces. Long runs of blank lines are discarded.



-P does not change that.

You might want to look into using -tranditional-cpp also because if you are
using preprocessor for other than C, # might be treated not like what you want
it to be treated

[Bug preprocessor/67266] Use of "cpp -P ..." collapses multiple blank lines

[mike at flyn dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67266

--- Comment #2 from W. Michael Petullo <mike at flyn dot org> ---
This does not seem to be the behavior of cpp without the -P flag:

$ cpp
x


x

outputs

# 1 "<stdin>"
# 1 "<built-in>"
# 1 "<command-line>"
# 1 "/usr/include/stdc-predef.h" 1 3 4
# 1 "<command-line>" 2
# 1 "<stdin>"
x


x

Here cpp does not collapse the newline.

[Bug preprocessor/67266] Use of "cpp -P ..." collapses multiple blank lines

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67266

Martin Sebor <msebor at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |msebor at gcc dot gnu.org

--- Comment #3 from Martin Sebor <msebor at gcc dot gnu.org> ---
The "long runs of blank lines are discarded" part refers to the constant 8
hardcoded in the maybe_print_line_1 function.  The constant's effect can be
seen in the following test case.  The first sequence of 7 newlines is retained
while the second one of 8 newlines is compressed.

$ cat -n a.txt && gcc -dU -E -xc a.txt | cat -n
     1  line 1
     2  
     3  
     4  
     5  
     6  
     7  
     8  line 8
     9  
    10  
    11  
    12  
    13  
    14  
    15  
    16  
    17  line 17
     1  # 1 "a.txt"
     2  # 1 "<built-in>"
     3  # 1 "<command-line>"
     4  # 1 "/usr/include/stdc-predef.h" 1 3 4
     5  #undef _STDC_PREDEF_H
     6  #define __GCC_IEC_559 2
     7  #define __GCC_IEC_559_COMPLEX 2
     8  # 1 "<command-line>" 2
     9  # 1 "a.txt"
    10  line 1
    11  
    12  
    13  
    14  
    15  
    16  
    17  line 8
    18  # 17 "a.txt"
    19  line 17