[externl]

Add back the -k option of mcpp 2.7.2, so that a token in the preprocessed output keeps the column it has in the source file. The source spans in zeroc-ice/ice#7007 need this to be exact.

The option was removed in 42be792 (April 2015), which cut this fork down to what Ice used at the time. The code added here is the original 2.7.2 code, from the import in e6e749e. Homebrew and the Linux distributions package the 2.7.2 release from SourceForge, Homebrew with a few of our patches, so their mcpp still accepts -k.

• Accept -k again, and list it in the usage text.
• Write spaces and tabs out as they are in the input when -k is given, and replace each comment with spaces of the same length (parse_line and read_a_comment in support.c).
• Give a comment no padding when the line buffer is already full, where the 2.7.2 code computes a negative padding length and writes past the buffer. Apart from this line, and a check for a preprocessing mode this fork no longer has, the code is the 2.7.2 original.
• Leave the output unchanged when -k isn't given.

[Copilot]

🟡 Changes recommended

The -k comment-padding path can write past its allocation, while multiline comments and non-invoked function-like macros still shift token columns.

3 open issues

• Prevent comment padding from overflowing a full buffer · New
• Retain whitespace when a macro name is not invoked · New
• Preserve token columns after multiline comments · New

What changed in this PR

Adds the upstream mcpp -k option to preserve horizontal whitespace, supporting accurate source-column spans in the Slice compiler.

Changes:

• Registers and documents -k.
• Adds whitespace-preservation state and updates it during directive processing.
• Preserves spaces and tabs and pads block comments based on their length.

File	Description
system.c	Parses -k and adds usage text.
support.c	Preserves horizontal whitespace and calculates comment padding.
mcpp_main.c	Initializes and enables whitespace-preservation mode.
internal.H	Declares the new option and shared state.
directive.c	Updates preservation state after directives.

_{Review effort: Balanced}

[pepone]

🤖 Claude (Opus 5.5):

• With -k -M, a function-like macro call that spans lines and has tokens after it sets src_col, but only the output path in mcpp_main clears it, so every later line skips directive(). With the input below, mcpp -k -M prints dep2.o: dep2.ice without dep.h, while -M alone lists it. Please clear src_col next to wrong_line = FALSE in the no_output branch (mcpp_main.c:511). I checked that this fixes -k -M and leaves the -k and -C -k output unchanged.

  #define F(x) x
  F(
  1) z
  #include "dep.h"

[externl]

Fixed in 0133d03: src_col is cleared in the no_output branch, and -k -M lists dep.h for your input. Homebrew's mcpp has the same bug, so it comes with the 2.7.2 code.

🤖 Addressed by Claude Code

[externl]

Reverted in ef1e5eb; my earlier comment said this was fixed. -k here has to behave as it does in the mcpp 2.7.2 that Homebrew and the Linux distributions package, so that it is the same on every platform, and that mcpp has this -k -M behavior too.

[InsertCreativityHere]

I approve of this PR because it brings our fork closer to what the other MCPPs do.
And I would rather be faithful to the original, than go around fixing minor bugs.

But, -k is not sufficient to the goal of preserving column counts, at least not with the logic of this fork. I didn't check everything, but at the very least, MCPP expands/moves macros and comments in ways that -k can't handle.

So even with this PR, we won't always have accurate column counts in our Slice files.

[pepone]

🤖 Claude (Opus 5.5): Agreed: Ice links the Homebrew mcpp on macOS and the distribution's libmcpp on Linux, so fixing this only here wouldn't help those platforms. The Ice change that adds -k should then pass it only when preprocessing, not together with -M for --depend.

[pepone]

🤖 Claude (Opus 5.5): LGTM.

[externl]

Agreed, -k won't give exact columns everywhere: a macro expansion or a block comment that spans lines still moves what follows it. So the language server will treat the span's columns as a hint and match each name against the editor's text, falling back to the whole line when the text doesn't match. With -k that hint should be exact for nearly all real Slice files.