The `iconv' Implementation in the GNU C library
-----------------------------------------------
After reading about the problems of `iconv' implementations in the
last section it is certainly good to note that the implementation in
the GNU C library has none of the problems mentioned above. What
follows is a step-by-step analysis of the points raised above. The
evaluation is based on the current state of the development (as of
January 1999). The development of the `iconv' functions is not
complete, but basic functionality has solidified.
The GNU C library's `iconv' implementation uses shared loadable
modules to implement the conversions. A very small number of
conversions are built into the library itself but these are only rather
trivial conversions.
All the benefits of loadable modules are available in the GNU C
library implementation. This is especially appealing since the
interface is well documented (see below), and it, therefore, is easy to
write new conversion modules. The drawback of using loadable objects
is not a problem in the GNU C library, at least on ELF systems. Since
the library is able to load shared objects even in statically linked
binaries, static linking need not be forbidden in case one wants to use
`iconv'.
The second mentioned problem is the number of supported conversions.
Currently, the GNU C library supports more than 150 character sets. The
way the implementation is designed the number of supported conversions
is greater than 22350 (150 times 149). If any conversion from or to a
character set is missing, it can be added easily.
Particularly impressive as it may be, this high number is due to the
fact that the GNU C library implementation of `iconv' does not have the
third problem mentioned above (i.e., whenever there is a conversion
from a character set A to B and from B to C it is always possible to
convert from A to C directly). If the `iconv_open' returns an error
and sets `errno' to `EINVAL', there is no known way, directly or
indirectly, to perform the wanted conversion.
Triangulation is achieved by providing for each character set a
conversion from and to UCS-4 encoded ISO 10646. Using ISO 10646 as an
intermediate representation it is possible to "triangulate" (i.e.,
convert with an intermediate representation).
There is no inherent requirement to provide a conversion to
ISO 10646 for a new character set, and it is also possible to provide
other conversions where neither source nor destination character set is
ISO 10646. The existing set of conversions is simply meant to cover all
conversions that might be of interest.
All currently available conversions use the triangulation method
above, making conversion run unnecessarily slow. If, for example,
somebody often needs the conversion from ISO-2022-JP to EUC-JP, a
quicker solution would involve direct conversion between the two
character sets, skipping the input to ISO 10646 first. The two
character sets of interest are much more similar to each other than to
ISO 10646.
In such a situation one easily can write a new conversion and
provide it as a better alternative. The GNU C library `iconv'
implementation would automatically use the module implementing the
conversion if it is specified to be more efficient.
Format of `gconv-modules' files
...............................
All information about the available conversions comes from a file
named `gconv-modules', which can be found in any of the directories
along the `GCONV_PATH'. The `gconv-modules' files are line-oriented
text files, where each of the lines has one of the following formats:
* If the first non-whitespace character is a `#' the line contains
only comments and is ignored.
* Lines starting with `alias' define an alias name for a character
set. Two more words are expected on the line. The first word
defines the alias name, and the second defines the original name
of the character set. The effect is that it is possible to use
the alias name in the FROMSET or TOSET parameters of `iconv_open'
and achieve the same result as when using the real character set
name.
This is quite important as a character set has often many different
names. There is normally an official name but this need not
correspond to the most popular name. Beside this many character
sets have special names that are somehow constructed. For
example, all character sets specified by the ISO have an alias of
the form `ISO-IR-NNN' where NNN is the registration number. This
allows programs that know about the registration number to
construct character set names and use them in `iconv_open' calls.
More on the available names and aliases follows below.
* Lines starting with `module' introduce an available conversion
module. These lines must contain three or four more words.
The first word specifies the source character set, the second word
the destination character set of conversion implemented in this
module, and the third word is the name of the loadable module.
The filename is constructed by appending the usual shared object
suffix (normally `.so') and this file is then supposed to be found
in the same directory the `gconv-modules' file is in. The last
word on the line, which is optional, is a numeric value
representing the cost of the conversion. If this word is missing,
a cost of 1 is assumed. The numeric value itself does not matter
that much; what counts are the relative values of the sums of
costs for all possible conversion paths. Below is a more precise
description of the use of the cost value.
Returning to the example above where one has written a module to
directly convert from ISO-2022-JP to EUC-JP and back. All that has to
be done is to put the new module, let its name be ISO2022JP-EUCJP.so,
in a directory and add a file `gconv-modules' with the following
content in the same directory:
module ISO-2022-JP// EUC-JP// ISO2022JP-EUCJP 1
module EUC-JP// ISO-2022-JP// ISO2022JP-EUCJP 1
To see why this is sufficient, it is necessary to understand how the
conversion used by `iconv' (and described in the descriptor) is
selected. The approach to this problem is quite simple.
At the first call of the `iconv_open' function the program reads all
available `gconv-modules' files and builds up two tables: one
containing all the known aliases and another that contains the
information about the conversions and which shared object implements
them.
Finding the conversion path in `iconv'
......................................
The set of available conversions form a directed graph with weighted
edges. The weights on the edges are the costs specified in the
`gconv-modules' files. The `iconv_open' function uses an algorithm
suitable for search for the best path in such a graph and so constructs
a list of conversions that must be performed in succession to get the
transformation from the source to the destination character set.
Explaining why the above `gconv-modules' files allows the `iconv'
implementation to resolve the specific ISO-2022-JP to EUC-JP conversion
module instead of the conversion coming with the library itself is
straightforward. Since the latter conversion takes two steps (from
ISO-2022-JP to ISO 10646 and then from ISO 10646 to EUC-JP), the cost
is 1+1 = 2. The above `gconv-modules' file, however, specifies that
the new conversion modules can perform this conversion with only the
cost of 1.
A mysterious item about the `gconv-modules' file above (and also the
file coming with the GNU C library) are the names of the character sets
specified in the `module' lines. Why do almost all the names end in
`//'? And this is not all: the names can actually be regular
expressions. At this point in time this mystery should not be
revealed, unless you have the relevant spell-casting materials: ashes
from an original DOS 6.2 boot disk burnt in effigy, a crucifix blessed
by St. Emacs, assorted herbal roots from Central America, sand from
Cebu, etc. Sorry! *The part of the implementation where this is used
is not yet finished. For now please simply follow the existing
examples. It'll become clearer once it is. -drepper*
A last remark about the `gconv-modules' is about the names not
ending with `//'. A character set named `INTERNAL' is often mentioned.
From the discussion above and the chosen name it should have become
clear that this is the name for the representation used in the
intermediate step of the triangulation. We have said that this is UCS-4
but actually that is not quite right. The UCS-4 specification also
includes the specification of the byte ordering used. Since a UCS-4
value consists of four bytes, a stored value is effected by byte
ordering. The internal representation is _not_ the same as UCS-4 in
case the byte ordering of the processor (or at least the running
process) is not the same as the one required for UCS-4. This is done
for performance reasons as one does not want to perform unnecessary
byte-swapping operations if one is not interested in actually seeing
the result in UCS-4. To avoid trouble with endianess, the internal
representation consistently is named `INTERNAL' even on big-endian
systems where the representations are identical.
`iconv' module data structures
..............................
So far this section has described how modules are located and
considered to be used. What remains to be described is the interface
of the modules so that one can write new ones. This section describes
the interface as it is in use in January 1999. The interface will
change a bit in the future but, with luck, only in an upwardly
compatible way.
The definitions necessary to write new modules are publicly available
in the non-standard header `gconv.h'. The following text, therefore,
describes the definitions from this header file. First, however, it is
necessary to get an overview.
From the perspective of the user of `iconv' the interface is quite
simple: the `iconv_open' function returns a handle that can be used in
calls to `iconv', and finally the handle is freed with a call to
`iconv_close'. The problem is that the handle has to be able to
represent the possibly long sequences of conversion steps and also the
state of each conversion since the handle is all that is passed to the
`iconv' function. Therefore, the data structures are really the
elements necessary to understanding the implementation.
We need two different kinds of data structures. The first describes
the conversion and the second describes the state etc. There are
really two type definitions like this in `gconv.h'.
- Data type: struct __gconv_step
This data structure describes one conversion a module can perform.
For each function in a loaded module with conversion functions
there is exactly one object of this type. This object is shared
by all users of the conversion (i.e., this object does not contain
any information corresponding to an actual conversion; it only
describes the conversion itself).
`struct __gconv_loaded_object *__shlib_handle'
`const char *__modname'
`int __counter'
All these elements of the structure are used internally in
the C library to coordinate loading and unloading the shared.
One must not expect any of the other elements to be
available or initialized.
`const char *__from_name'
`const char *__to_name'
`__from_name' and `__to_name' contain the names of the source
and destination character sets. They can be used to identify
the actual conversion to be carried out since one module
might implement conversions for more than one character set
and/or direction.
`gconv_fct __fct'
`gconv_init_fct __init_fct'
`gconv_end_fct __end_fct'
These elements contain pointers to the functions in the
loadable module. The interface will be explained below.
`int __min_needed_from'
`int __max_needed_from'
`int __min_needed_to'
`int __max_needed_to;'
These values have to be supplied in the init function of the
module. The `__min_needed_from' value specifies how many
bytes a character of the source character set at least needs.
The `__max_needed_from' specifies the maximum value that
also includes possible shift sequences.
The `__min_needed_to' and `__max_needed_to' values serve the
same purpose as `__min_needed_from' and `__max_needed_from'
but this time for the destination character set.
It is crucial that these values be accurate since otherwise
the conversion functions will have problems or not work at
all.
`int __stateful'
This element must also be initialized by the init function.
`int __stateful' is nonzero if the source character set is
stateful. Otherwise it is zero.
`void *__data'
This element can be used freely by the conversion functions
in the module. `void *__data' can be used to communicate
extra information from one call to another. `void *__data'
need not be initialized if not needed at all. If `void
*__data' element is assigned a pointer to dynamically
allocated memory (presumably in the init function) it has to
be made sure that the end function deallocates the memory.
Otherwise the application will leak memory.
It is important to be aware that this data structure is
shared by all users of this specification conversion and
therefore the `__data' element must not contain data specific
to one specific use of the conversion function.
- Data type: struct __gconv_step_data
This is the data structure that contains the information specific
to each use of the conversion functions.
`char *__outbuf'
`char *__outbufend'
These elements specify the output buffer for the conversion
step. The `__outbuf' element points to the beginning of the
buffer, and `__outbufend' points to the byte following the
last byte in the buffer. The conversion function must not
assume anything about the size of the buffer but it can be
safely assumed the there is room for at least one complete
character in the output buffer.
Once the conversion is finished, if the conversion is the
last step, the `__outbuf' element must be modified to point
after the last byte written into the buffer to signal how
much output is available. If this conversion step is not the
last one, the element must not be modified. The
`__outbufend' element must not be modified.
`int __is_last'
This element is nonzero if this conversion step is the last
one. This information is necessary for the recursion. See
the description of the conversion function internals below.
This element must never be modified.
`int __invocation_counter'
The conversion function can use this element to see how many
calls of the conversion function already happened. Some
character sets require a certain prolog when generating
output, and by comparing this value with zero, one can find
out whether it is the first call and whether, therefore, the
prolog should be emitted. This element must never be
modified.
`int __internal_use'
This element is another one rarely used but needed in certain
situations. It is assigned a nonzero value in case the
conversion functions are used to implement `mbsrtowcs' et.al.
(i.e., the function is not used directly through the `iconv'
interface).
This sometimes makes a difference as it is expected that the
`iconv' functions are used to translate entire texts while the
`mbsrtowcs' functions are normally used only to convert single
strings and might be used multiple times to convert entire
texts.
But in this situation we would have problem complying with
some rules of the character set specification. Some
character sets require a prolog, which must appear exactly
once for an entire text. If a number of `mbsrtowcs' calls
are used to convert the text, only the first call must add
the prolog. However, because there is no communication
between the different calls of `mbsrtowcs', the conversion
functions have no possibility to find this out. The
situation is different for sequences of `iconv' calls since
the handle allows access to the needed information.
The `int __internal_use' element is mostly used together with
`__invocation_counter' as follows:
if (!data->__internal_use
&& data->__invocation_counter == 0)
/* Emit prolog. */
...
This element must never be modified.
`mbstate_t *__statep'
The `__statep' element points to an object of type `mbstate_t'
(Note:Keeping the state). The conversion of a stateful
character set must use the object pointed to by `__statep' to
store information about the conversion state. The `__statep'
element itself must never be modified.
`mbstate_t __state'
This element must _never_ be used directly. It is only part
of this structure to have the needed space allocated.
`iconv' module interfaces
.........................
With the knowledge about the data structures we now can describe the
conversion function itself. To understand the interface a bit of
knowledge is necessary about the functionality in the C library that
loads the objects with the conversions.
It is often the case that one conversion is used more than once
(i.e., there are several `iconv_open' calls for the same set of
character sets during one program run). The `mbsrtowcs' et.al.
functions in the GNU C library also use the `iconv' functionality, which
increases the number of uses of the same functions even more.
Because of this multiple use of conversions, the modules do not get
loaded exclusively for one conversion. Instead a module once loaded can
be used by an arbitrary number of `iconv' or `mbsrtowcs' calls at the
same time. The splitting of the information between conversion-
function-specific information and conversion data makes this possible.
The last section showed the two data structures used to do this.
This is of course also reflected in the interface and semantics of
the functions that the modules must provide. There are three functions
that must have the following names:
`gconv_init'
The `gconv_init' function initializes the conversion function
specific data structure. This very same object is shared by all
conversions that use this conversion and, therefore, no state
information about the conversion itself must be stored in here.
If a module implements more than one conversion, the `gconv_init'
function will be called multiple times.
`gconv_end'
The `gconv_end' function is responsible for freeing all resources
allocated by the `gconv_init' function. If there is nothing to do,
this function can be missing. Special care must be taken if the
module implements more than one conversion and the `gconv_init'
function does not allocate the same resources for all conversions.
`gconv'
This is the actual conversion function. It is called to convert
one block of text. It gets passed the conversion step information
initialized by `gconv_init' and the conversion data, specific to
this use of the conversion functions.
There are three data types defined for the three module interface
functions and these define the interface.
- Data type: int (*__gconv_init_fct) (struct __gconv_step *)
This specifies the interface of the initialization function of the
module. It is called exactly once for each conversion the module
implements.
As explained in the description of the `struct __gconv_step' data
structure above the initialization function has to initialize
parts of it.
`__min_needed_from'
`__max_needed_from'
`__min_needed_to'
`__max_needed_to'
These elements must be initialized to the exact numbers of
the minimum and maximum number of bytes used by one character
in the source and destination character sets, respectively.
If the characters all have the same size, the minimum and
maximum values are the same.
`__stateful'
This element must be initialized to an nonzero value if the
source character set is stateful. Otherwise it must be zero.
If the initialization function needs to communicate some
information to the conversion function, this communication can
happen using the `__data' element of the `__gconv_step' structure.
But since this data is shared by all the conversions, it must not
be modified by the conversion function. The example below shows
how this can be used.
#define MIN_NEEDED_FROM 1
#define MAX_NEEDED_FROM 4
#define MIN_NEEDED_TO 4
#define MAX_NEEDED_TO 4
int
gconv_init (struct __gconv_step *step)
{
/* Determine which direction. */
struct iso2022jp_data *new_data;
enum direction dir = illegal_dir;
enum variant var = illegal_var;
int result;
if (__strcasecmp (step->__from_name, "ISO-2022-JP//") == 0)
{
dir = from_iso2022jp;
var = iso2022jp;
}
else if (__strcasecmp (step->__to_name, "ISO-2022-JP//") == 0)
{
dir = to_iso2022jp;
var = iso2022jp;
}
else if (__strcasecmp (step->__from_name, "ISO-2022-JP-2//") == 0)
{
dir = from_iso2022jp;
var = iso2022jp2;
}
else if (__strcasecmp (step->__to_name, "ISO-2022-JP-2//") == 0)
{
dir = to_iso2022jp;
var = iso2022jp2;
}
result = __GCONV_NOCONV;
if (dir != illegal_dir)
{
new_data = (struct iso2022jp_data *)
malloc (sizeof (struct iso2022jp_data));
result = __GCONV_NOMEM;
if (new_data != NULL)
{
new_data->dir = dir;
new_data->var = var;
step->__data = new_data;
if (dir == from_iso2022jp)
{
step->__min_needed_from = MIN_NEEDED_FROM;
step->__max_needed_from = MAX_NEEDED_FROM;
step->__min_needed_to = MIN_NEEDED_TO;
step->__max_needed_to = MAX_NEEDED_TO;
}
else
{
step->__min_needed_from = MIN_NEEDED_TO;
step->__max_needed_from = MAX_NEEDED_TO;
step->__min_needed_to = MIN_NEEDED_FROM;
step->__max_needed_to = MAX_NEEDED_FROM + 2;
}
/* Yes, this is a stateful encoding. */
step->__stateful = 1;
result = __GCONV_OK;
}
}
return result;
}
The function first checks which conversion is wanted. The module
from which this function is taken implements four different
conversions; which one is selected can be determined by comparing
the names. The comparison should always be done without paying
attention to the case.
Next, a data structure, which contains the necessary information
about which conversion is selected, is allocated. The data
structure `struct iso2022jp_data' is locally defined since,
outside the module, this data is not used at all. Please note
that if all four conversions this modules supports are requested
there are four data blocks.
One interesting thing is the initialization of the `__min_' and
`__max_' elements of the step data object. A single ISO-2022-JP
character can consist of one to four bytes. Therefore the
`MIN_NEEDED_FROM' and `MAX_NEEDED_FROM' macros are defined this
way. The output is always the `INTERNAL' character set (aka
UCS-4) and therefore each character consists of exactly four
bytes. For the conversion from `INTERNAL' to ISO-2022-JP we have
to take into account that escape sequences might be necessary to
switch the character sets. Therefore the `__max_needed_to'
element for this direction gets assigned `MAX_NEEDED_FROM + 2'.
This takes into account the two bytes needed for the escape
sequences to single the switching. The asymmetry in the maximum
values for the two directions can be explained easily: when
reading ISO-2022-JP text, escape sequences can be handled alone
(i.e., it is not necessary to process a real character since the
effect of the escape sequence can be recorded in the state
information). The situation is different for the other direction.
Since it is in general not known which character comes next, one
cannot emit escape sequences to change the state in advance. This
means the escape sequences that have to be emitted together with
the next character. Therefore one needs more room than only for
the character itself.
The possible return values of the initialization function are:
`__GCONV_OK'
The initialization succeeded
`__GCONV_NOCONV'
The requested conversion is not supported in the module.
This can happen if the `gconv-modules' file has errors.
`__GCONV_NOMEM'
Memory required to store additional information could not be
allocated.
The function called before the module is unloaded is significantly
easier. It often has nothing at all to do; in which case it can be left
out completely.
- Data type: void (*__gconv_end_fct) (struct gconv_step *)
The task of this function is to free all resources allocated in the
initialization function. Therefore only the `__data' element of
the object pointed to by the argument is of interest. Continuing
the example from the initialization function, the finalization
function looks like this:
void
gconv_end (struct __gconv_step *data)
{
free (data->__data);
}
The most important function is the conversion function itself, which
can get quite complicated for complex character sets. But since this
is not of interest here, we will only describe a possible skeleton for
the conversion function.
- Data type: int (*__gconv_fct) (struct __gconv_step *, struct
__gconv_step_data *, const char **, const char *, size_t *,
int)
The conversion function can be called for two basic reason: to
convert text or to reset the state. From the description of the
`iconv' function it can be seen why the flushing mode is
necessary. What mode is selected is determined by the sixth
argument, an integer. This argument being nonzero means that
flushing is selected.
Common to both modes is where the output buffer can be found. The
information about this buffer is stored in the conversion step
data. A pointer to this information is passed as the second
argument to this function. The description of the `struct
__gconv_step_data' structure has more information on the
conversion step data.
What has to be done for flushing depends on the source character
set. If the source character set is not stateful, nothing has to
be done. Otherwise the function has to emit a byte sequence to
bring the state object into the initial state. Once this all
happened the other conversion modules in the chain of conversions
have to get the same chance. Whether another step follows can be
determined from the `__is_last' element of the step data structure
to which the first parameter points.
The more interesting mode is when actual text has to be converted.
The first step in this case is to convert as much text as
possible from the input buffer and store the result in the output
buffer. The start of the input buffer is determined by the third
argument, which is a pointer to a pointer variable referencing the
beginning of the buffer. The fourth argument is a pointer to the
byte right after the last byte in the buffer.
The conversion has to be performed according to the current state
if the character set is stateful. The state is stored in an
object pointed to by the `__statep' element of the step data
(second argument). Once either the input buffer is empty or the
output buffer is full the conversion stops. At this point, the
pointer variable referenced by the third parameter must point to
the byte following the last processed byte (i.e., if all of the
input is consumed, this pointer and the fourth parameter have the
same value).
What now happens depends on whether this step is the last one. If
it is the last step, the only thing that has to be done is to
update the `__outbuf' element of the step data structure to point
after the last written byte. This update gives the caller the
information on how much text is available in the output buffer.
In addition, the variable pointed to by the fifth parameter, which
is of type `size_t', must be incremented by the number of
characters (_not bytes_) that were converted in a non-reversible
way. Then, the function can return.
In case the step is not the last one, the later conversion
functions have to get a chance to do their work. Therefore, the
appropriate conversion function has to be called. The information
about the functions is stored in the conversion data structures,
passed as the first parameter. This information and the step data
are stored in arrays, so the next element in both cases can be
found by simple pointer arithmetic:
int
gconv (struct __gconv_step *step, struct __gconv_step_data *data,
const char **inbuf, const char *inbufend, size_t *written,
int do_flush)
{
struct __gconv_step *next_step = step + 1;
struct __gconv_step_data *next_data = data + 1;
...
The `next_step' pointer references the next step information and
`next_data' the next data record. The call of the next function
therefore will look similar to this:
next_step->__fct (next_step, next_data, &outerr, outbuf,
written, 0)
But this is not yet all. Once the function call returns the
conversion function might have some more to do. If the return
value of the function is `__GCONV_EMPTY_INPUT', more room is
available in the output buffer. Unless the input buffer is empty
the conversion, functions start all over again and process the
rest of the input buffer. If the return value is not
`__GCONV_EMPTY_INPUT', something went wrong and we have to recover
from this.
A requirement for the conversion function is that the input buffer
pointer (the third argument) always point to the last character
that was put in converted form into the output buffer. This is
trivially true after the conversion performed in the current step,
but if the conversion functions deeper downstream stop
prematurely, not all characters from the output buffer are
consumed and, therefore, the input buffer pointers must be backed
off to the right position.
Correcting the input buffers is easy to do if the input and output
character sets have a fixed width for all characters. In this
situation we can compute how many characters are left in the
output buffer and, therefore, can correct the input buffer pointer
appropriately with a similar computation. Things are getting
tricky if either character set has characters represented with
variable length byte sequences, and it gets even more complicated
if the conversion has to take care of the state. In these cases
the conversion has to be performed once again, from the known
state before the initial conversion (i.e., if necessary the state
of the conversion has to be reset and the conversion loop has to be
executed again). The difference now is that it is known how much
input must be created, and the conversion can stop before
converting the first unused character. Once this is done the
input buffer pointers must be updated again and the function can
return.
One final thing should be mentioned. If it is necessary for the
conversion to know whether it is the first invocation (in case a
prolog has to be emitted), the conversion function should
increment the `__invocation_counter' element of the step data
structure just before returning to the caller. See the
description of the `struct __gconv_step_data' structure above for
more information on how this can be used.
The return value must be one of the following values:
`__GCONV_EMPTY_INPUT'
All input was consumed and there is room left in the output
buffer.
`__GCONV_FULL_OUTPUT'
No more room in the output buffer. In case this is not the
last step this value is propagated down from the call of the
next conversion function in the chain.
`__GCONV_INCOMPLETE_INPUT'
The input buffer is not entirely empty since it contains an
incomplete character sequence.
The following example provides a framework for a conversion
function. In case a new conversion has to be written the holes in
this implementation have to be filled and that is it.
int
gconv (struct __gconv_step *step, struct __gconv_step_data *data,
const char **inbuf, const char *inbufend, size_t *written,
int do_flush)
{
struct __gconv_step *next_step = step + 1;
struct __gconv_step_data *next_data = data + 1;
gconv_fct fct = next_step->__fct;
int status;
/* If the function is called with no input this means we have
to reset to the initial state. The possibly partly
converted input is dropped. */
if (do_flush)
{
status = __GCONV_OK;
/* Possible emit a byte sequence which put the state object
into the initial state. */
/* Call the steps down the chain if there are any but only
if we successfully emitted the escape sequence. */
if (status == __GCONV_OK && ! data->__is_last)
status = fct (next_step, next_data, NULL, NULL,
written, 1);
}
else
{
/* We preserve the initial values of the pointer variables. */
const char *inptr = *inbuf;
char *outbuf = data->__outbuf;
char *outend = data->__outbufend;
char *outptr;
do
{
/* Remember the start value for this round. */
inptr = *inbuf;
/* The outbuf buffer is empty. */
outptr = outbuf;
/* For stateful encodings the state must be safe here. */
/* Run the conversion loop. `status' is set
appropriately afterwards. */
/* If this is the last step, leave the loop. There is
nothing we can do. */
if (data->__is_last)
{
/* Store information about how many bytes are
available. */
data->__outbuf = outbuf;
/* If any non-reversible conversions were performed,
add the number to `*written'. */
break;
}
/* Write out all output that was produced. */
if (outbuf > outptr)
{
const char *outerr = data->__outbuf;
int result;
result = fct (next_step, next_data, &outerr,
outbuf, written, 0);
if (result != __GCONV_EMPTY_INPUT)
{
if (outerr != outbuf)
{
/* Reset the input buffer pointer. We
document here the complex case. */
size_t nstatus;
/* Reload the pointers. */
*inbuf = inptr;
outbuf = outptr;
/* Possibly reset the state. */
/* Redo the conversion, but this time
the end of the output buffer is at
`outerr'. */
}
/* Change the status. */
status = result;
}
else
/* All the output is consumed, we can make
another run if everything was ok. */
if (status == __GCONV_FULL_OUTPUT)
status = __GCONV_OK;
}
}
while (status == __GCONV_OK);
/* We finished one use of this step. */
++data->__invocation_counter;
}
return status;
}
This information should be sufficient to write new modules. Anybody
doing so should also take a look at the available source code in the GNU
C library sources. It contains many examples of working and optimized
modules.