How to commit to a git repository using libgit2?

Since there is no copy-paste example for creating a commit without using files on disk with libgit2 as far as I can tell I thought I should add one.

Don't forget that libgit2 is in full development at this time (March 2013) so have a look at official documentation and source code, as new features are added daily:

libgit2 API
headers are very well commented - here's an example
extensive tests may be a source of inspiration
there are some official examples, general.c is a good place to start
inspiration may be found in LibGit2Sharp - here are some tests

bool addGitCommit ( 
  git_repository * repo, git_signature * sign, 
  const char * content, int content_sz,
  const char * message )
{
  int rc;              /* return code for git_ functions */
  git_oid oid_blob;    /* the SHA1 for our blob in the tree */
  git_oid oid_tree;    /* the SHA1 for our tree in the commit */
  git_oid oid_commit;  /* the SHA1 for our initial commit */
  git_blob * blob;     /* our blob in the tree */
  git_tree * tree_cmt; /* our tree in the commit */
  git_treebuilder * tree_bld;  /* tree builder */
  bool b = false;

  /* create a blob from our buffer */
  rc = git_blob_create_frombuffer( 
        &oid_blob,
        repo, 
        content, 
        content_sz );
  if ( rc == 0 ) { /* blob created */
    rc = git_blob_lookup( &blob, repo, &oid_blob );
    if ( rc == 0 ) { /* blob created and found */
      rc = git_treebuilder_create( &tree_bld, NULL );
      if ( rc == 0 ) { /* a new tree builder created */
        rc = git_treebuilder_insert( 
              NULL, 
              tree_bld, 
              "name-of-the-file.txt", 
              &oid_blob, 
              GIT_FILEMODE_BLOB );
        if ( rc == 0 ) { /* blob inserted in tree */
          rc = git_treebuilder_write( 
                &oid_tree, 
                repo, 
                tree_bld );
          if ( rc == 0 ) { /* the tree was written to the database */
            rc = git_tree_lookup(
                  &tree_cmt, repo, &oid_tree );
            if ( rc == 0 ) { /*we've got the tree pointer */  
              rc = git_commit_create(
                    &oid_commit, repo, "HEAD",
                    sign, sign, /* same author and commiter */
                    NULL, /* default UTF-8 encoding */
                    message,
                    tree_cmt, 0, NULL );
              if ( rc == 0 ) {
                b = true;
              }
              git_tree_free( tree_cmt );
            }
          }
        }
        git_treebuilder_free( tree_bld );
      }
      git_blob_free( blob );
    }
  }
  return b;
}

The repository comes from git_repository_init() or git_repository_open(). The signature comes from git_signature_now() or git_signature_new().

The function updates the HEAD for current branch.

If you do a git status after the function executes you will notice that the file name-of-the-file.txt appears as being deleted. That is because the function does not create an actual file, only an entry in the git database.

Also, note the last arguments of git_commit_create(). 0 and NULL means that this is the first (root) commit. For all other there should be at least a parent commit specified, maybe obtained using git_commit_lookup().

I'm just learning these things. Please improve this answer if you know better.