In the introduction of this chapter it was said that certain character
sets use a stateful encoding. That is, the encoded values depend
in some way on the previous bytes in the text.
Since the conversion functions allow converting a text in more than one
step we must have a way to pass this information from one call of the
functions to another.
— Data type: mbstate_t
A variable of type mbstate_t can contain all the information
about the shift state needed from one call to a conversion
function to another.
mbstate_t is defined in wchar.h. It was introduced in
Amendment 1 to ISO C90.
To use objects of type mbstate_t the programmer has to define such
objects (normally as local variables on the stack) and pass a pointer to
the object to the conversion functions. This way the conversion function
can update the object if the current multibyte character set is stateful.
There is no specific function or initializer to put the state object in
any specific state. The rules are that the object should always
represent the initial state before the first use, and this is achieved by
clearing the whole variable with code such as follows:
memset (&state, '\0', sizeof (state));
/* from now on state can be used. */
When using the conversion functions to generate output it is often
necessary to test whether the current state corresponds to the initial
state. This is necessary, for example, to decide whether to emit
escape sequences to set the state to the initial state at certain
sequence points. Communication protocols often require this.
— Function: int mbsinit (const mbstate_t *ps)
The mbsinit function determines whether the state object pointed
to by ps is in the initial state. If ps is a null pointer or
the object is in the initial state the return value is nonzero. Otherwise
it is zero.
mbsinit was introduced in Amendment 1 to ISO C90 and is
declared in wchar.h.
The code to emit the escape sequence to get back to the initial state is
interesting. The wcsrtombs function can be used to determine the
necessary output code (see Converting Strings). Please note that on
GNU systems it is not necessary to perform this extra action for the
conversion from multibyte text to wide character text since the wide
character encoding is not stateful. But there is nothing mentioned in
any standard that prohibits making wchar_t using a stateful
Published under the terms of the GNU General Public License