n_chars characters of
chars into the contents of the buffer, at position
n_chars is negative, then characters from chars will be inserted until a null-terminator is found. If
n_chars are out of bounds, or the maximum buffer text length is exceeded, then they are coerced to sane values.
Note that the position and length are in characters, not in bytes.
the position at which to insert text.
the text to insert into the buffer.
the length of the text in characters, or -1
The number of characters actually inserted.