Print all C comments to a separate text file

Question

I would like to print all my C comments to a separate text file.

• Using awk, sed, grep, or bash
• output all multi-line C comments between /* ... */ (inclusive)
• output lines with // comments
• Optional: print line numbers

I tried these solutions but it did not work on Ubuntu

• Parsing C Style Multiline Comment
• https://stackoverflow.com/questions/57723357/how-can-i-grep-matching-a-comment-string

The purpose of my request is to be able to quickly use the source code comments as a starting point for good documentation. I do not like the extra clutter and proprietary embedded commands (i.e. Doxygen) of dedicated documentation programs. For example, properly commenting each source code function and removing superficial one line comments will be a great time saver, and provide a nearly complete reference. This will also encourage better source code comments.

Answer 1

There's been quite a few answers using shell-magic already, but I think it can be done a lot easier by using the tools you probably already have. Namely, gcc.

diff -u <(gcc -fpreprocessed -dD -E main.c) main.c | grep '^+' | cut -c 2-

How it works?

1. gcc -fpreprocessed -dD -E main.c Removes all comments from a file and puts it on stdout

2. diff -u <(...) main.c Takes the input from stdout and compares it with the original

3. grep '^+' Filters on all lines starting with a +. In other words: filter on what was previously a determined a comment

4. cut -c 2- Remove the + symbols from the output

No need for super complex regex, perl or awk stuff while also covering all edge cases that the other answers might have missed.

Answer 2

It's not as trivial as it may seem if you take into account things like: puts("string with /*") bearing in mind that "s can occur in ch = '"'.

Or line continuations:

printf("...");    /\
* yes, this is a comment */
/\
/ and this as well

Or trigraphs.

To cover those, we can adapt this answer to the opposite question to make it print rather than remove the comments:

perl -0777 -pe '
  s{
    (?<comment>
      # /* ... */ C comments
      / (?<lc> # line continuation
          (?<bs> # backslash in its regular or trigraph form
            \\ | \?\?/
          )
          (?: \n | \r\n?) # handling LF, CR and CRLF line delimiters
        )* \* .*? \* (?&lc)* /
      | / (?&lc)* / (?:(?&lc) | [^\r\n])* # // C++/C99 comments
    ) |
       "(?:(?&bs)(?&lc)*.|.)*?" # "strings" literals
       | '\''(?&lc)*(?:(?&bs)(?&lc)*(?:\?\?.|.))?(?:\?\?.|.)*?'\'' # (w)char literals
       | \?\?'\'' # trigraph form of ^
       | .[^'\''"/?]* # anything else
  }{$+{comment} eq "" ? "" : "$+{comment}\n"}exsg'

Which on the contrived examples from the other question which cover most of the corner cases:

#include <stdio.h>
int main()
{
  printf("%d %s %s %c%c%c%c%c %s %s %d\n",
  1-/* comment */-1,
  /\
* comment */
  "/* not a comment */",
  /* multiline
  comment */
  // comment
  /\
/ comment
  // multiline\
comment
  "// not a comment",
  '"' /* comment */ , '"',
  '\'','"'/* comment */,
  '\
\
"', /* comment */
  "\\
" /* not a comment */ ",
  "??/" /* not a comment */ ",
  '??''+'"' /* "comment" */);
  return 0;
}

Gives:

/* comment */
/\
* comment */
/* multiline
  comment */
// comment
/\
/ comment
// multiline\
comment
/* comment */
/* comment */
/* comment */
/* "comment" */

To get the line numbers, as we're running in slurp mode where the subject is the whole input as opposed to processing the input one line at a time, it's a bit more tricky. We could do it though using the (?{code}) regexp operator to increment a counter each time a line delimiter (CR, LF or CRLF in C) is found:

perl -0777 -pe '
  s{
    (?<comment>(?{$l=$n+1})
      /
      (?<lc>  # line continuation
        (?<bs> # backslash in its regular or trigraph form
          \\ | \?\?/
        ) (?<nl>(?:\n|\r\n?) (?{$n++})) # handling LF, CR and CRLF line delimiters
      )*
      (?:
        \* (?: (?&nl) | .)*? \* (?&lc)* / # /* ... */ C comments
        | / (?:(?&lc) | [^\r\n])*         # // C++/C99 comments
      )
    ) |
       "(?:(?&bs)(?&lc)*.|.)*?" # "strings" literals
       | '\''(?&lc)*(?:(?&bs)(?&lc)*(?:\?\?.|.))?(?:\?\?.|.)*?'\'' # (w)char literals
       | \?\?'\'' # trigraph form of ^
       | (?&nl)
       | .[^'\''"/?\r\n]* # anything else
  }{$+{comment} eq "" ? "" : sprintf("%5d %s\n", $l, $+{comment})}exsg'

Which on that same sample gives:

    5 /* comment */
    6 /\
* comment */
    9 /* multiline
  comment */
   11 // comment
   12 /\
/ comment
   14 // multiline\
comment
   17 /* comment */
   18 /* comment */
   21 /* comment */
   26 /* "comment" */

Answer 3

It can be done in awk as follows:

#!/bin/awk

# Handles case where both /* and */ are on the same line
{ line_printed = 0; }

# Find the beginning of a multiline comment
/^[[:space:]]*\/\*/ {
    multiline = 1;

    # Remove leading spaces
    sub(/^[[:space:]]+/,"");
    printf "[%d] %s\n", NR, $0;
    line_printed = 1;
}

# Find the end of a multiline comment
/\*\/[[:space:]]*$/ {
    multiline = 0;
    if (line_printed == 0)
        printf "%s", $0;

    print "\n"
    next;
}

# The content between /* and */
{
    if ( multiline == 1 && line_printed == 0 )
    {
        print $0;
        next
    }
}

# A single line comment
/^[[:space:]]*\/\// {
    # Remove leading spaces
    sub(/^[[:space:]]+/,"");
    printf "[%d] %s\n\n", NR, $0;
}

Save this script as foo.awk (or any other name; the extension is optional) and then run with awk -f foo.awk input.c. The script will print all comments (separated by an extra newline) and will add the line number before every comment.

Answer 4

Well definitely not the fanciest one, nor the most recommended, because it has some flaws. But I think it looks really cool:

awk '/\/\//; /\/\*.*\*\//; /\*\//; /^\/\*/ { a=1 } /\*\// { a=0 } a && $0 != "/*" { print }'

Answer 5

Update 2/15/24 - While learning to use Raylib, I came across a C "parser" included in their software suite that appears to do exactly what I needed, see: https://github.com/raysan5/raylib It has the added benefit of finding and nicely formatting all strucs, defines, functions, call backs, etc.

typical example

Function 232: DrawRectangleRounded() (4 input parameters)
  Name: DrawRectangleRounded
  Return type: void
  Description: Draw rectangle with rounded edges
  Param[1]: rec (type: Rectangle)
  Param[2]: roundness (type: float)
  Param[3]: segments (type: int)
  Param[4]: color (type: Color)