Character conversion between the multibyte representation and the wide character representation uses conversion state, of type mbstate_t. Conversion of a string uses a finite-state machine; when it is interrupted after the complete conversion of a number of characters, it may need to save a state for processing the remaining characters. Such a conversion state is needed for the sake of encodings such as ISO-2022 and UTF-7.
The initial state is the state at the beginning of conversion of a string. There are two kinds of state: The one used by multibyte to wide character conversion functions, such as mbsrtowcs, and the one used by wide character to multibyte conversion functions, such as wcsrtombs, but they both fit in a mbstate_t, and they both have the same representation for an initial state.
For 8-bit encodings, all states are equivalent to the initial state. For multibyte encodings like UTF-8, EUC-*, BIG5 or SJIS, the wide character to multibyte conversion functions never produce non-initial states, but the multibyte to wide character conversion functions like mbrtowc do produce non-initial states when interrupted in the middle of a character.