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-rw-r--r--Documentation/user-manual.txt219
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diff --git a/Documentation/user-manual.txt b/Documentation/user-manual.txt
index dd1578d..9e61798 100644
--- a/Documentation/user-manual.txt
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@@ -3160,6 +3160,225 @@ confusing and scary messages, but it won't actually do anything bad. In
contrast, running "git prune" while somebody is actively changing the
repository is a *BAD* idea).
+[[birdview-on-the-source-code]]
+A birdview on Git's source code
+-----------------------------
+
+While Git's source code is quite elegant, it is not always easy for
+new developers to find their way through it. A good idea is to look
+at the contents of the initial commit:
+_e83c5163316f89bfbde7d9ab23ca2e25604af290_ (also known as _v0.99~954_).
+
+Tip: you can see what files are in there with
+
+----------------------------------------------------
+$ git show e83c5163316f89bfbde7d9ab23ca2e25604af290:
+----------------------------------------------------
+
+and look at those files with something like
+
+-----------------------------------------------------------
+$ git show e83c5163316f89bfbde7d9ab23ca2e25604af290:cache.h
+-----------------------------------------------------------
+
+Be sure to read the README in that revision _after_ you are familiar with
+the terminology (<<glossary>>), since the terminology has changed a little
+since then. For example, we call the things "commits" now, which are
+described in that README as "changesets".
+
+Actually a lot of the structure as it is now can be explained by that
+initial commit.
+
+For example, we do not call it "cache" any more, but "index", however, the
+file is still called `cache.h`. Remark: Not much reason to change it now,
+especially since there is no good single name for it anyway, because it is
+basically _the_ header file which is included by _all_ of Git's C sources.
+
+If you grasp the ideas in that initial commit (it is really small and you
+can get into it really fast, and it will help you recognize things in the
+much larger code base we have now), you should go on skimming `cache.h`,
+`object.h` and `commit.h` in the current version.
+
+In the early days, Git (in the tradition of UNIX) was a bunch of programs
+which were extremely simple, and which you used in scripts, piping the
+output of one into another. This turned out to be good for initial
+development, since it was easier to test new things. However, recently
+many of these parts have become builtins, and some of the core has been
+"libified", i.e. put into libgit.a for performance, portability reasons,
+and to avoid code duplication.
+
+By now, you know what the index is (and find the corresponding data
+structures in `cache.h`), and that there are just a couple of object types
+(blobs, trees, commits and tags) which inherit their common structure from
+`struct object`, which is their first member (and thus, you can cast e.g.
+`(struct object *)commit` to achieve the _same_ as `&commit->object`, i.e.
+get at the object name and flags).
+
+Now is a good point to take a break to let this information sink in.
+
+Next step: get familiar with the object naming. Read <<naming-commits>>.
+There are quite a few ways to name an object (and not only revisions!).
+All of these are handled in `sha1_name.c`. Just have a quick look at
+the function `get_sha1()`. A lot of the special handling is done by
+functions like `get_sha1_basic()` or the likes.
+
+This is just to get you into the groove for the most libified part of Git:
+the revision walker.
+
+Basically, the initial version of `git log` was a shell script:
+
+----------------------------------------------------------------
+$ git-rev-list --pretty $(git-rev-parse --default HEAD "$@") | \
+ LESS=-S ${PAGER:-less}
+----------------------------------------------------------------
+
+What does this mean?
+
+`git-rev-list` is the original version of the revision walker, which
+_always_ printed a list of revisions to stdout. It is still functional,
+and needs to, since most new Git programs start out as scripts using
+`git-rev-list`.
+
+`git-rev-parse` is not as important any more; it was only used to filter out
+options that were relevant for the different plumbing commands that were
+called by the script.
+
+Most of what `git-rev-list` did is contained in `revision.c` and
+`revision.h`. It wraps the options in a struct named `rev_info`, which
+controls how and what revisions are walked, and more.
+
+The original job of `git-rev-parse` is now taken by the function
+`setup_revisions()`, which parses the revisions and the common command line
+options for the revision walker. This information is stored in the struct
+`rev_info` for later consumption. You can do your own command line option
+parsing after calling `setup_revisions()`. After that, you have to call
+`prepare_revision_walk()` for initialization, and then you can get the
+commits one by one with the function `get_revision()`.
+
+If you are interested in more details of the revision walking process,
+just have a look at the first implementation of `cmd_log()`; call
+`git-show v1.3.0~155^2~4` and scroll down to that function (note that you
+no longer need to call `setup_pager()` directly).
+
+Nowadays, `git log` is a builtin, which means that it is _contained_ in the
+command `git`. The source side of a builtin is
+
+- a function called `cmd_<bla>`, typically defined in `builtin-<bla>.c`,
+ and declared in `builtin.h`,
+
+- an entry in the `commands[]` array in `git.c`, and
+
+- an entry in `BUILTIN_OBJECTS` in the `Makefile`.
+
+Sometimes, more than one builtin is contained in one source file. For
+example, `cmd_whatchanged()` and `cmd_log()` both reside in `builtin-log.c`,
+since they share quite a bit of code. In that case, the commands which are
+_not_ named like the `.c` file in which they live have to be listed in
+`BUILT_INS` in the `Makefile`.
+
+`git log` looks more complicated in C than it does in the original script,
+but that allows for a much greater flexibility and performance.
+
+Here again it is a good point to take a pause.
+
+Lesson three is: study the code. Really, it is the best way to learn about
+the organization of Git (after you know the basic concepts).
+
+So, think about something which you are interested in, say, "how can I
+access a blob just knowing the object name of it?". The first step is to
+find a Git command with which you can do it. In this example, it is either
+`git show` or `git cat-file`.
+
+For the sake of clarity, let's stay with `git cat-file`, because it
+
+- is plumbing, and
+
+- was around even in the initial commit (it literally went only through
+ some 20 revisions as `cat-file.c`, was renamed to `builtin-cat-file.c`
+ when made a builtin, and then saw less than 10 versions).
+
+So, look into `builtin-cat-file.c`, search for `cmd_cat_file()` and look what
+it does.
+
+------------------------------------------------------------------
+ git_config(git_default_config);
+ if (argc != 3)
+ usage("git-cat-file [-t|-s|-e|-p|<type>] <sha1>");
+ if (get_sha1(argv[2], sha1))
+ die("Not a valid object name %s", argv[2]);
+------------------------------------------------------------------
+
+Let's skip over the obvious details; the only really interesting part
+here is the call to `get_sha1()`. It tries to interpret `argv[2]` as an
+object name, and if it refers to an object which is present in the current
+repository, it writes the resulting SHA-1 into the variable `sha1`.
+
+Two things are interesting here:
+
+- `get_sha1()` returns 0 on _success_. This might surprise some new
+ Git hackers, but there is a long tradition in UNIX to return different
+ negative numbers in case of different errors -- and 0 on success.
+
+- the variable `sha1` in the function signature of `get_sha1()` is `unsigned
+ char *`, but is actually expected to be a pointer to `unsigned
+ char[20]`. This variable will contain the 160-bit SHA-1 of the given
+ commit. Note that whenever a SHA-1 is passed as "unsigned char *", it
+ is the binary representation, as opposed to the ASCII representation in
+ hex characters, which is passed as "char *".
+
+You will see both of these things throughout the code.
+
+Now, for the meat:
+
+-----------------------------------------------------------------------------
+ case 0:
+ buf = read_object_with_reference(sha1, argv[1], &size, NULL);
+-----------------------------------------------------------------------------
+
+This is how you read a blob (actually, not only a blob, but any type of
+object). To know how the function `read_object_with_reference()` actually
+works, find the source code for it (something like `git grep
+read_object_with | grep ":[a-z]"` in the git repository), and read
+the source.
+
+To find out how the result can be used, just read on in `cmd_cat_file()`:
+
+-----------------------------------
+ write_or_die(1, buf, size);
+-----------------------------------
+
+Sometimes, you do not know where to look for a feature. In many such cases,
+it helps to search through the output of `git log`, and then `git show` the
+corresponding commit.
+
+Example: If you know that there was some test case for `git bundle`, but
+do not remember where it was (yes, you _could_ `git grep bundle t/`, but that
+does not illustrate the point!):
+
+------------------------
+$ git log --no-merges t/
+------------------------
+
+In the pager (`less`), just search for "bundle", go a few lines back,
+and see that it is in commit 18449ab0... Now just copy this object name,
+and paste it into the command line
+
+-------------------
+$ git show 18449ab0
+-------------------
+
+Voila.
+
+Another example: Find out what to do in order to make some script a
+builtin:
+
+-------------------------------------------------
+$ git log --no-merges --diff-filter=A builtin-*.c
+-------------------------------------------------
+
+You see, Git is actually the best tool to find out about the source of Git
+itself!
+
[[glossary]]
include::glossary.txt[]