How to code "git commit" in libgit2? - libgit2

Question

I have searched Google and Stackoverflow for an answer to the question of how to code the equivalent of
git commit -a -m "message"
in libgit2 (https://libgit2.github.com) and C or C++.
But I cannot find a ready and working answer to this question.
I am using libgit2-0.21.
Below is code that initializes a git repository, adds two files to it, and stages the two files so they are ready to be committed.
My question is how to code "git commit -a -m "msg" in libgit2?
#include <sys/stat.h>
#include <string>
#include <fstream>
#include <iostream>
#include <git2.h>
using namespace std;
int main (int argc, char** argv)
{
git_threads_init ();
// Create repository directory.
string directory = "repository";
mkdir (directory.c_str(), 0777);
// Initialize the repository: git init.
git_repository *repo = NULL;
int result = git_repository_init (&repo, directory.c_str(), false);
if (result != 0) cerr << giterr_last ()->message << endl;
// Store two files in the repository directory.
ofstream file;
file.open ("repository/file1", ios::binary | ios::trunc);
file << "Contents of file one";
file.close ();
file.open ("repository/file2", ios::binary | ios::trunc);
file << "Contents of file two";
file.close ();
// Run the equivalent of "git add ."
// Get the git index.
git_index * index = NULL;
result = git_repository_index (&index, repo);
if (result != 0) cerr << giterr_last ()->message << endl;
// Add all files to the git index.
result = git_index_add_all (index, NULL, 0, NULL, NULL);
if (result != 0) cerr << giterr_last ()->message << endl;
// Write the index to disk.
result = git_index_write (index);
if (result != 0) cerr << giterr_last ()->message << endl;
// Run the equivalent of "git commit -a -m "commit message".
// How to do that through libgit2?
// Run "git status" to see the result.
system ("cd repository; git status");
// Free resources.
git_index_free (index);
git_repository_free (repo);
git_threads_shutdown ();
return 0;
}
The code can be compiled as follows:
g++ -Wall -I/opt/local/include -L/opt/local/lib -lgit2 -o test git.cpp
Below is the output of running the compiled binary:
On branch master
Initial commit
Changes to be committed:
(use "git rm --cached <file>..." to unstage)
new file: file1
new file: file2

Answer 1

Once the index is updated
create a tree from it through git_index_write_tree()
create a commit that references this tree through git_commit_create_v()
See this end to end test which performs the equivalent of the following
$ echo "test" > test.txt
$ git add .
$ git commit -m "Initial commit"