How to commit from the given files? - libgit2

Question

I need to use libgit2 to implement the "git commit -F ..." command.
The code below will commit 1.txt and 2.txt:
git_libgit2_init();
git_repository* pRepository;
git_index* pIndex;
git_oid oidTree, oidCommitted;
git_tree* pTree;
git_signature* pSignature;
git_repository_init(&pRepository, "C:\\Temp", false);
git_repository_index(&pIndex, pRepository);
git_index_add_bypath(pIndex, "1.txt");
git_index_add_bypath(pIndex, "2.txt");
git_index_write(pIndex);
git_index_write_tree(&oidTree, pIndex);
git_tree_lookup(&pTree, pRepository, &oidTree);
git_signature_now(&pSignature, "My name", "My email");
git_commit_create(&oidCommitted, pRepository, "refs/heads/master",
pSignature, pSignature, NULL, "Initial commit with 1.txt", pTree, 0, NULL);
git_signature_free(pSignature);
git_tree_free(pTree);
git_index_free(pIndex);
git_repository_free(pRepository);
git_libgit2_shutdown();
How to change my code to implement:
git add 1.txt 2.txt
git commit 1.txt -m "Initial commit with 1.txt"

Answer 1

You want to come up with a tree object to pass to git_commit_create without modifying the repository's index. There are several ways to do that. The easiest is to create your own in-memory index and use that instead of the repository's index. Essentially, do something like this:
...
git_repository_init(&repo, ...); // same as before
git_index_new(&index); // create in-memory index
git_index_read_tree(index, headTree); // initialize to the current HEAD
git_index_add_by_path(index, "1.txt"); // update the nominated file(s)
git_index_write_tree_to(&oid, index, repo); // write the tree into the repo
git_tree_lookup(&tree, repo, &oid); // same as before
...

Answer 2

Method One:
Either modify the repository's index to reflect what you want to commit: call git_index_add to put the data for 2.txt back in the index, then remove it again after creating the tree for the commit.
Method Two:
Duplicate the repository's index as a temporary, in-memory index and mutate that one, adding the 1.txt back in. Then create the tree from that index instead of the repository's index. git_index_add_by_path will fail when operating on an in-memory index. It can’t find that path, it has no concept of where to look for it.
Git_index_add_by_path works by reading the file on disk to create a git_index_entry. It’s a convenience function.
Instead, you can use git_index_add with the index entry data.