add already tracked file to index - libgit2

Question

in git it would be
1. create file a.txt
2. commit file a.txt
3. a.txt is tracked
I can do that in libgit2 easily.
However, If I modify a.txt and wanted to add it to index I will do
git add a.txt
Unfortunately I cannot emulate that with libgit2. I have tried virtually everything the internet offered yet nothing worked. So I feel I miss something basic here. Note that I can add new files that aren't tracked to index with no problem, only I cannot find the way to add them.
Here is the code that I have.
void add_file(char *file)
{
git_index *index;
int error;
const git_index_entry *entry;
git_index_entry new_entry;
error = git_repository_index(&index, m_repo);
entry = git_index_get_bypath(index,file, 0);
if(entry)
{
memcpy(&new_entry, entry, sizeof(git_index_entry));
new_entry.path = file;
new_entry.mode = GIT_FILEMODE_BLOB;
error = git_index_add(index, &entry);
}
else
error = git_index_add_bypath(index, file);
error = git_index_write(index);
git_index_free(index);
}
EDIT:
After Ed's post I update my code but it is still adding only new files that aren't tracked.
git_index *index;
git_oid tree;
int error;
error = git_repository_index(&index, m_repo);
error = git_index_add_bypath(index, file);
error = git_index_write(index);
error = git_index_write_tree(&tree, index);
git_index_free(index);

Answer 1

Your git_index_get_bypath gives you the index entry as it exists currently. You're then memcpying the entry that is in the index and git_index_adding it right back. You're not actually changing the entry at all.
(That is, unless the index entry is currently executable. In which case you are removing the execute bit by setting the mode to GIT_FILEMODE_BLOB.)
You probably don't want to use git_index_add here, as that actually edits the low-level contents of the index directly. You would need to add the file to the object database and then update the index with the resultant OID.
If the file is changed on disk, then just run git_index_add_bypath and let libgit2 update the index with the contents as they exist on disk.