how to get git log -p to show changes in merge commits

Question

I have a branch with a function definition missing but the function is still being used causing a crash. The function definition must have been deleted during a merge. I have do

Answer 1

This leads me to believe that 'git log -p' doesn't show merge changes? How can I get this functionality?

You are correct: by default, git log -p shows you the merge commit, but does not even attempt to show a diff for it.

As odradek wrote in a comment, adding the -c option will make git log show combined diffs. You may also use --cc (note two dashes for --cc, vs one dash for -c) which also shows a combined diff, or -m, which effectively splits the merge for diff purposes, and shows one diff per parent, against that parent.

These same three options can be used with git show. For whatever reason, though, git show defaults to --cc while git log defaults to nothing at all.

The difference between the three options is clear only with some merges, and it's a bit tricky to show them. We can, however, say one thing pretty clearly: any combined diff shows only files that differ from all parents. That is, both git show -c and git show --cc trim what's shown to try to be helpful. (The --cc form may trim more than -c, depending on what there is that can be shown. I do not have a handy example of this, though.)

For example, consider commit 3e5c63943d35be1804d302c0393affc4916c3dc3 in the Git repository for Git. This is a merge (with parents c13c783... and 20690b2..., and if we run two separate git diff commands, we can see that, as compared to its first parent, only two files change:

$ git diff --name-status 3e5c639^1 3e5c639
M       builtin/remote.c
M       t/t5505-remote.sh

but as compared to its second parent, many files (including those same two) change:

$ git diff --name-status 3e5c639^2 3e5c639 | expand
M       .gitignore
M       .mailmap
M       Documentation/Makefile
A       Documentation/RelNotes/2.12.0.txt
M       Documentation/SubmittingPatches
A       Documentation/asciidoctor-extensions.rb
M       Documentation/blame-options.txt
M       Documentation/cat-texi.perl
M       Documentation/config.txt
M       Documentation/diff-config.txt
[snipped here - but the same two files do appear in the 339-entry list]

If I run git show --cc on this, I get no diff listing at all; if I run git show -c on it, I get a diff listing for builtin/remote.c and for t/t5505-remote.sh.

If I run git show -m on this, I get instead two separate git diff listings. Neither is a "combined diff". The first starts with:

commit 3e5c63943d35be1804d302c0393affc4916c3dc3 (from
 c13c783c9d3d7d3eff937b7bf3642d2a7fe32644)

and shows just the first two files. The second starts with:

commit 3e5c63943d35be1804d302c0393affc4916c3dc3 (from
 20690b213975ad3e9b1bc851f434d818bd2d1de9)

and shows all 339 files.

Using -m is the Really Big Hammer, but you are left with a lot of sorting to do through a lot of pieces. Using --cc or -c is usually sufficient.

(Something else that may help, when looking for this sort of change with git log -p, is to add --full-history to make sure that git log follows both branches down each merge. This is only required if you are doing history simplification by adding -- options.)