How to create a patch ignoring indentation differences in the code?

Question

I'm trying to create patch a file using diff tool.But facing an issues.The way I am doing is below.

I've created one Directory named a and put original file in to it.

a/original_file.c

Now I have created other Directory named b and put same file with modified content in to it.

b/original_file.c

Now content of b/original_file.c file I have copied from internet and put it into some text editor.

After giving command: diff -Naur a b > patch_file.patch, the file patch_file.patch is generated and it has some unwanted changes (its related to indentation).

For example:

return mg_nw (MG_READY_NOY, &rmsg, seqnr,
-                 sizeof (struct mg_rdy_notify));
+                  sizeof (struct mg_rdy_notify)); 

Now you can see there are changed related to indentation where sizeof (struct mg_rdy_notify)) is replaced by same sizeof (struct mg_rdy_notify)) but one basis of indentation which is what we don't want.

Answer 1

diff has more than one option related to whitespace. However, one is less useful for patches. The manual page gives an obscure hint, referring to both GNU:

   -B, --ignore-blank-lines
          ignore changes where lines are all blank
   -b, --ignore-space-change
          ignore changes in the amount of white space
   -w, --ignore-all-space
          ignore all white space

and FreeBSD

   -b     Ignore changes in amount of white space.
   -B     Ignore changes that just insert or delete blank lines.
   -w     Ignore white space when comparing lines.

Usually one uses -b, because that is less likely to overlook significant changes. If you have changed only indentation, then both -b and -w give the same result. On the other hand, if you inserted spaces where there were none, or deleted existing spaces (leaving none), that could be a change in your program. Here is an example:

$ diff foo.c foo2.c
4c4
<     setlocale(LC_ALL, "");
---
>     setlocale(LC_ALL, " ");
6,7c6,7
<     printw("\U0001F0A1");
<     getch();
---
>     printw ("\U0001F0A1");
>     getch();  /* comment */
$ diff -b foo.c foo2.c
4c4
<     setlocale(LC_ALL, "");
---
>     setlocale(LC_ALL, " ");
6,7c6,7
<     printw("\U0001F0A1");
<     getch();
---
>     printw ("\U0001F0A1");
>     getch();  /* comment */
$ diff -w foo.c foo2.c
7c7
<     getch();
---
>     getch();  /* comment */

In this case, the -w option allows you to ignore the change to the setlocale parameter (perhaps not what was intended).

POSIX diff, by the way, has only the -b option.

For patch, POSIX documents the -l option:

-l
(The letter ell.) Cause any sequence of <blank> characters in the difference script to match any sequence of <blank> characters in the input file. Other characters shall be matched exactly.

Answer 2

diff has an option to ignore whitespace changes (-w), same for patch (-l). In general, it's a bad idea to ignore whitespace though, so you should reserve its use for special cases, when someone's editor did something horrible...