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general.c

/*
 * libgit2 "general" example - shows basic libgit2 concepts
 *
 * Written by the libgit2 contributors
 *
 * To the extent possible under law, the author(s) have dedicated all copyright
 * and related and neighboring rights to this software to the public domain
 * worldwide. This software is distributed without any warranty.
 *
 * You should have received a copy of the CC0 Public Domain Dedication along
 * with this software. If not, see
 * <http://creativecommons.org/publicdomain/zero/1.0/>.
 */

libgit2 is a portable, pure C implementation of the Git core methods provided as a re-entrant linkable library with a solid API, allowing you to write native speed custom Git applications in any language which supports C bindings.

This file is an example of using that API in a real, compilable C file. As the API is updated, this file will be updated to demonstrate the new functionality.

If you're trying to write something in C using libgit2, you should also check out the generated API documentation. We try to link to the relevant sections of the API docs in each section in this file.

libgit2 (for the most part) only implements the core plumbing functions, not really the higher level porcelain stuff. For a primer on Git Internals that you will need to know to work with Git at this level, check out Chapter 9 of the Pro Git book.

Includes

Including the git2.h header will include all the other libgit2 headers that you need. It should be the only thing you need to include in order to compile properly and get all the libgit2 API.

#include <git2.h>
#include <stdio.h>

Almost all libgit2 functions return 0 on success or negative on error. This is not production quality error checking, but should be sufficient as an example.

static void check_error(int error_code, const char *action)
{
  const git_error *error = giterr_last();
  if (!error_code)
    return;

  printf("Error %d %s - %s\n", error_code, action,
       (error && error->message) ? error->message : "???");

  exit(1);
}

int main (int argc, char** argv)
{

Opening the Repository

There are a couple of methods for opening a repository, this being the simplest. There are also methods for specifying the index file and work tree locations, here we assume they are in the normal places.

(Try running this program against tests/resources/testrepo.git.)

  int error;
  const char *repo_path = (argc > 1) ? argv[1] : "/opt/libgit2-test/.git";
  git_repository *repo;

  error = git_repository_open(&repo, repo_path);
  check_error(error, "opening repository");

SHA-1 Value Conversions

For our first example, we will convert a 40 character hex value to the 20 byte raw SHA1 value.

  printf("*Hex to Raw*\n");
  char hex[] = "4a202b346bb0fb0db7eff3cffeb3c70babbd2045";

The git_oid is the structure that keeps the SHA value. We will use this throughout the example for storing the value of the current SHA key we're working with.

  git_oid oid;
  git_oid_fromstr(&oid, hex);

Once we've converted the string into the oid value, we can get the raw value of the SHA by accessing oid.id

Next we will convert the 20 byte raw SHA1 value to a human readable 40 char hex value.

  printf("\n*Raw to Hex*\n");
  char out[GIT_OID_HEXSZ+1];
  out[GIT_OID_HEXSZ] = '\0';

If you have a oid, you can easily get the hex value of the SHA as well.

  git_oid_fmt(out, &oid);
  printf("SHA hex string: %s\n", out);

Working with the Object Database

libgit2 provides direct access to the object database. The object database is where the actual objects are stored in Git. For working with raw objects, we'll need to get this structure from the repository.

  git_odb *odb;
  git_repository_odb(&odb, repo);

Raw Object Reading

  printf("\n*Raw Object Read*\n");
  git_odb_object *obj;
  git_otype otype;
  const unsigned char *data;
  const char *str_type;

We can read raw objects directly from the object database if we have the oid (SHA) of the object. This allows us to access objects without knowing thier type and inspect the raw bytes unparsed.

  error = git_odb_read(&obj, odb, &oid);
  check_error(error, "finding object in repository");

A raw object only has three properties - the type (commit, blob, tree or tag), the size of the raw data and the raw, unparsed data itself. For a commit or tag, that raw data is human readable plain ASCII text. For a blob it is just file contents, so it could be text or binary data. For a tree it is a special binary format, so it's unlikely to be hugely helpful as a raw object.

  data = (const unsigned char *)git_odb_object_data(obj);
  otype = git_odb_object_type(obj);

We provide methods to convert from the object type which is an enum, to a string representation of that value (and vice-versa).

  str_type = git_object_type2string(otype);
  printf("object length and type: %d, %s\n",
      (int)git_odb_object_size(obj),
      str_type);

For proper memory management, close the object when you are done with it or it will leak memory.

Raw Object Writing

  printf("\n*Raw Object Write*\n");

You can also write raw object data to Git. This is pretty cool because it gives you direct access to the key/value properties of Git. Here we'll write a new blob object that just contains a simple string. Notice that we have to specify the object type as the git_otype enum.

  git_odb_write(&oid, odb, "test data", sizeof("test data") - 1, GIT_OBJ_BLOB);

Now that we've written the object, we can check out what SHA1 was generated when the object was written to our database.

  git_oid_fmt(out, &oid);
  printf("Written Object: %s\n", out);

Object Parsing

libgit2 has methods to parse every object type in Git so you don't have to work directly with the raw data. This is much faster and simpler than trying to deal with the raw data yourself.

Commit Parsing

Parsing commit objects is simple and gives you access to all the data in the commit - the author (name, email, datetime), committer (same), tree, message, encoding and parent(s).

  printf("\n*Commit Parsing*\n");

  git_commit *commit;
  git_oid_fromstr(&oid, "8496071c1b46c854b31185ea97743be6a8774479");

  error = git_commit_lookup(&commit, repo, &oid);
  check_error(error, "looking up commit");

  const git_signature *author, *cmtter;
  const char *message;
  time_t ctime;
  unsigned int parents, p;

Each of the properties of the commit object are accessible via methods, including commonly needed variations, such as git_commit_time which returns the author time and git_commit_message which gives you the commit message (as a NUL-terminated string).

  message  = git_commit_message(commit);
  author   = git_commit_author(commit);
  cmtter   = git_commit_committer(commit);
  ctime    = git_commit_time(commit);

The author and committer methods return [gitsignature] structures, which give you name, email and when, which is a `gittime` structure, giving you a timestamp and timezone offset.

  printf("Author: %s (%s)\n", author->name, author->email);

Commits can have zero or more parents. The first (root) commit will have no parents, most commits will have one (i.e. the commit it was based on) and merge commits will have two or more. Commits can technically have any number, though it's rare to have more than two.

  parents  = git_commit_parentcount(commit);
  for (p = 0;p < parents;p++) {
    git_commit *parent;
    git_commit_parent(&parent, commit, p);
    git_oid_fmt(out, git_commit_id(parent));
    printf("Parent: %s\n", out);
    git_commit_free(parent);
  }

Don't forget to close the object to prevent memory leaks. You will have to do this for all the objects you open and parse.

  git_commit_free(commit);

Writing Commits

libgit2 provides a couple of methods to create commit objects easily as well. There are four different create signatures, we'll just show one of them here. You can read about the other ones in the commit API docs.

  printf("\n*Commit Writing*\n");
  git_oid tree_id, parent_id, commit_id;
  git_tree *tree;
  git_commit *parent;

Creating signatures for an authoring identity and time is simple. You will need to do this to specify who created a commit and when. Default values for the name and email should be found in the user.name and user.email configuration options. See the config section of this example file to see how to access config values.

  git_signature_new((git_signature **)&author,
      "Scott Chacon", "schacon@gmail.com", 123456789, 60);
  git_signature_new((git_signature **)&cmtter,
      "Scott A Chacon", "scott@github.com", 987654321, 90);

Commit objects need a tree to point to and optionally one or more parents. Here we're creating oid objects to create the commit with, but you can also use

  git_oid_fromstr(&tree_id, "f60079018b664e4e79329a7ef9559c8d9e0378d1");
  git_tree_lookup(&tree, repo, &tree_id);
  git_oid_fromstr(&parent_id, "5b5b025afb0b4c913b4c338a42934a3863bf3644");
  git_commit_lookup(&parent, repo, &parent_id);

Here we actually create the commit object with a single call with all the values we need to create the commit. The SHA key is written to the commit_id variable here.

  git_commit_create_v(
    &commit_id, /* out id */
    repo,
    NULL, /* do not update the HEAD */
    author,
    cmtter,
    NULL, /* use default message encoding */
    "example commit",
    tree,
    1, parent);

Now we can take a look at the commit SHA we've generated.

  git_oid_fmt(out, &commit_id);
  printf("New Commit: %s\n", out);

Tag Parsing

You can parse and create tags with the tag management API, which functions very similarly to the commit lookup, parsing and creation methods, since the objects themselves are very similar.

  printf("\n*Tag Parsing*\n");
  git_tag *tag;
  const char *tmessage, *tname;
  git_otype ttype;

We create an oid for the tag object if we know the SHA and look it up the same way that we would a commit (or any other object).

  git_oid_fromstr(&oid, "b25fa35b38051e4ae45d4222e795f9df2e43f1d1");

  error = git_tag_lookup(&tag, repo, &oid);
  check_error(error, "looking up tag");

Now that we have the tag object, we can extract the information it generally contains: the target (usually a commit object), the type of the target object (usually 'commit'), the name ('v1.0'), the tagger (a git_signature - name, email, timestamp), and the tag message.

  git_tag_target((git_object **)&commit, tag);
  tname = git_tag_name(tag);		// "test"
  ttype = git_tag_target_type(tag);	// GIT_OBJ_COMMIT (otype enum)
  tmessage = git_tag_message(tag);	// "tag message\n"
  printf("Tag Message: %s\n", tmessage);

  git_commit_free(commit);

Tree Parsing

Tree parsing is a bit different than the other objects, in that we have a subtype which is the tree entry. This is not an actual object type in Git, but a useful structure for parsing and traversing tree entries.

  printf("\n*Tree Parsing*\n");

  const git_tree_entry *entry;
  git_object *objt;

Create the oid and lookup the tree object just like the other objects.

  git_oid_fromstr(&oid, "2a741c18ac5ff082a7caaec6e74db3075a1906b5");
  git_tree_lookup(&tree, repo, &oid);

Getting the count of entries in the tree so you can iterate over them if you want to.

  size_t cnt = git_tree_entrycount(tree); // 3
  printf("tree entries: %d\n", (int)cnt);

  entry = git_tree_entry_byindex(tree, 0);
  printf("Entry name: %s\n", git_tree_entry_name(entry)); // "hello.c"

You can also access tree entries by name if you know the name of the entry you're looking for.

  entry = git_tree_entry_byname(tree, "README");
  git_tree_entry_name(entry); // "hello.c"

Once you have the entry object, you can access the content or subtree (or commit, in the case of submodules) that it points to. You can also get the mode if you want.

  git_tree_entry_to_object(&objt, repo, entry); // blob

Remember to close the looked-up object once you are done using it

Blob Parsing

The last object type is the simplest and requires the least parsing help. Blobs are just file contents and can contain anything, there is no structure to it. The main advantage to using the simple blob api is that when you're creating blobs you don't have to calculate the size of the content. There is also a helper for reading a file from disk and writing it to the db and getting the oid back so you don't have to do all those steps yourself.

  printf("\n*Blob Parsing*\n");
  git_blob *blob;

  git_oid_fromstr(&oid, "1385f264afb75a56a5bec74243be9b367ba4ca08");
  git_blob_lookup(&blob, repo, &oid);

You can access a buffer with the raw contents of the blob directly. Note that this buffer may not be contain ASCII data for certain blobs (e.g. binary files): do not consider the buffer a NULL-terminated string, and use the git_blob_rawsize attribute to find out its exact size in bytes

  printf("Blob Size: %ld\n", (long)git_blob_rawsize(blob)); // 8
  git_blob_rawcontent(blob); // "content"

Revwalking

The libgit2 revision walking api provides methods to traverse the directed graph created by the parent pointers of the commit objects. Since all commits point back to the commit that came directly before them, you can walk this parentage as a graph and find all the commits that were ancestors of (reachable from) a given starting point. This can allow you to create git log type functionality.

  printf("\n*Revwalking*\n");
  git_revwalk *walk;
  git_commit *wcommit;

  git_oid_fromstr(&oid, "5b5b025afb0b4c913b4c338a42934a3863bf3644");

To use the revwalker, create a new walker, tell it how you want to sort the output and then push one or more starting points onto the walker. If you want to emulate the output of git log you would push the SHA of the commit that HEAD points to into the walker and then start traversing them. You can also 'hide' commits that you want to stop at or not see any of their ancestors. So if you want to emulate git log branch1..branch2, you would push the oid of branch2 and hide the oid of branch1.

  git_revwalk_new(&walk, repo);
  git_revwalk_sorting(walk, GIT_SORT_TOPOLOGICAL | GIT_SORT_REVERSE);
  git_revwalk_push(walk, &oid);

  const git_signature *cauth;
  const char *cmsg;

Now that we have the starting point pushed onto the walker, we start asking for ancestors. It will return them in the sorting order we asked for as commit oids. We can then lookup and parse the commited pointed at by the returned OID; note that this operation is specially fast since the raw contents of the commit object will be cached in memory

  while ((git_revwalk_next(&oid, walk)) == 0) {
    error = git_commit_lookup(&wcommit, repo, &oid);
  check_error(error, "looking up commit during revwalk");

    cmsg  = git_commit_message(wcommit);
    cauth = git_commit_author(wcommit);
    printf("%s (%s)\n", cmsg, cauth->email);

    git_commit_free(wcommit);
  }

Like the other objects, be sure to free the revwalker when you're done to prevent memory leaks. Also, make sure that the repository being walked it not deallocated while the walk is in progress, or it will result in undefined behavior

Index File Manipulation

The index file API allows you to read, traverse, update and write the Git index file (sometimes thought of as the staging area).

  printf("\n*Index Walking*\n");

  git_index *index;
  unsigned int i, ecount;

You can either open the index from the standard location in an open repository, as we're doing here, or you can open and manipulate any index file with git_index_open_bare(). The index for the repository will be located and loaded from disk.

  git_repository_index(&index, repo);

For each entry in the index, you can get a bunch of information including the SHA (oid), path and mode which map to the tree objects that are written out. It also has filesystem properties to help determine what to inspect for changes (ctime, mtime, dev, ino, uid, gid, filesize and flags) All these properties are exported publicly in the `gitindex_entry` struct

  ecount = git_index_entrycount(index);
  for (i = 0; i < ecount; ++i) {
    const git_index_entry *e = git_index_get_byindex(index, i);

    printf("path: %s\n", e->path);
    printf("mtime: %d\n", (int)e->mtime.seconds);
    printf("fs: %d\n", (int)e->file_size);
  }

  git_index_free(index);

References

The reference API allows you to list, resolve, create and update references such as branches, tags and remote references (everything in the .git/refs directory).

  printf("\n*Reference Listing*\n");

Here we will implement something like git for-each-ref simply listing out all available references and the object SHA they resolve to.

  git_strarray ref_list;
  git_reference_list(&ref_list, repo);

  const char *refname;
  git_reference *ref;

Now that we have the list of reference names, we can lookup each ref one at a time and resolve them to the SHA, then print both values out.

  for (i = 0; i < ref_list.count; ++i) {
    refname = ref_list.strings[i];
    git_reference_lookup(&ref, repo, refname);

    switch (git_reference_type(ref)) {
    case GIT_REF_OID:
      git_oid_fmt(out, git_reference_target(ref));
      printf("%s [%s]\n", refname, out);
      break;

    case GIT_REF_SYMBOLIC:
      printf("%s => %s\n", refname, git_reference_symbolic_target(ref));
      break;
    default:
      fprintf(stderr, "Unexpected reference type\n");
      exit(1);
    }
  }

  git_strarray_free(&ref_list);

Config Files

The config API allows you to list and updatee config values in any of the accessible config file locations (system, global, local).

  printf("\n*Config Listing*\n");

  const char *email;
  int32_t j;

  git_config *cfg;

Open a config object so we can read global values from it.

  char config_path[256];
  sprintf(config_path, "%s/config", repo_path);
  check_error(git_config_open_ondisk(&cfg, config_path), "opening config");

  git_config_get_int32(&j, cfg, "help.autocorrect");
  printf("Autocorrect: %d\n", j);

  git_config_get_string(&email, cfg, "user.email");
  printf("Email: %s\n", email);

Finally, when you're done with the repository, you can free it as well.

  git_repository_free(repo);

  return 0;
}