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Explain why we did not follow the standard.
Worth revisiting the proposal:
http://marc.info/?l=netbsd-tech-userlevel&m=110019726222841

.\" $NetBSD: iconv.3,v 1.15 2013/01/04 01:21:45 christos Exp $
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.Dd January 3, 2013
.Dt ICONV 3
.Os
.\" ----------------------------------------------------------------------
.Sh NAME
.Nm iconv_open ,
.Nm iconv_close ,
.Nm iconv
.Nd codeset conversion functions
.\" ----------------------------------------------------------------------
.Sh LIBRARY
.Lb libc
.\" ----------------------------------------------------------------------
.Sh SYNOPSIS
.In iconv.h
.Ft iconv_t
.Fn iconv_open "const char *dstname" "const char *srcname"
.Ft int
.Fn iconv_close "iconv_t cd"
.Ft size_t
.Fn iconv "iconv_t cd" "const char ** restrict src" "size_t * restrict srcleft" "char ** restrict dst" "size_t * restrict dstleft"
.\" ----------------------------------------------------------------------
.Sh DESCRIPTION
The
.Fn iconv_open
function opens a converter from the codeset
.Fa srcname
to the codeset
.Fa dstname
and returns its descriptor.
.Pp
The
.Fn iconv_close
function closes the specified converter
.Fa cd .
.Pp
The
.Fn iconv
function converts the string in the buffer
.Fa *src
of length
.Fa *srcleft
bytes and stores the converted string in the buffer
.Fa *dst
of size
.Fa *dstleft
bytes.
After calling
.Fn iconv ,
the values pointed to by
.Fa src ,
.Fa srcleft ,
.Fa dst ,
and
.Fa dstleft
are updated as follows:
.Bl -tag -width 01234567 -offset indent
.It Fa *src
Pointer to the byte just after the last character fetched.
.It Fa *srcleft
Number of remaining bytes in the source buffer.
.It Fa *dst
Pointer to the byte just after the last character stored.
.It Fa *dstleft
Number of remainder bytes in the destination buffer.
.El
.Pp
If the string pointed to by
.Fa *src
contains a byte sequence which is not a valid character in the source
codeset, the conversion stops just after the last successful conversion.
If the output buffer is too small to store the converted
character, the conversion also stops in the same way.
In these cases, the values pointed to by
.Fa src ,
.Fa srcleft ,
.Fa dst ,
and
.Fa dstleft
are updated to the state just after the last successful conversion.
.Pp
If the string pointed to by
.Fa *src
contains a character which is valid under the source codeset but
can not be converted to the destination codeset,
the character is replaced by an
.Dq invalid character
which depends on the destination codeset, e.g.,
.Sq \&? ,
and the conversion is continued.
.Fn iconv
returns the number of such
.Dq invalid conversions .
.Pp
If the source and/or destination codesets are stateful,
.Fn iconv
places these into their initial state.
There are two special cases of
.Fn iconv :
.Bl -enum -offset indent
.It
If both
.Fa dst
and
.Fa *dst
are
.No non- Ns Dv NULL ,
.Fn iconv
stores the shift sequence for the destination switching to the initial state
in the buffer pointed to by
.Fa *dst .
The buffer size is specified by the value pointed to by
.Fa dstleft
as above.
.Fn iconv
will fail if the buffer is too small to store the shift sequence.
.It
On the other hand,
.Fa dst
or
.Fa *dst
may be
.Dv NULL .
In this case, the shift sequence for the destination switching
to the initial state is discarded.
.El
.\" ----------------------------------------------------------------------
.Sh RETURN VALUES
Upon successful completion of
.Fn iconv_open ,
it returns a conversion descriptor.
Otherwise,
.Fn iconv_open
returns (iconv_t)\-1 and sets
.Va errno
to indicate the error.
.Pp
Upon successful completion of
.Fn iconv_close ,
it returns 0.
Otherwise,
.Fn iconv_close
returns \-1 and sets errno to indicate the error.
.Pp
Upon successful completion of
.Fn iconv ,
it returns the number of
.Dq invalid
conversions.
Otherwise,
.Fn iconv
returns (size_t)\-1 and sets errno to indicate the error.
.\" ----------------------------------------------------------------------
.Sh ERRORS
The
.Fn iconv_open
function may cause an error in the following cases:
.Bl -tag -width Er
.It Bq Er EINVAL
There is no converter specified by
.Fa srcname
and
.Fa dstname .
.It Bq Er ENOMEM
Memory is exhausted.
.El
.Pp
The
.Fn iconv_close
function may cause an error in the following case:
.Bl -tag -width Er
.It Bq Er EBADF
The conversion descriptor specified by
.Fa cd
is invalid.
.El
.Pp
The
.Fn iconv
function may cause an error in the following cases:
.Bl -tag -width Er
.It Bq Er E2BIG
The output buffer pointed to by
.Fa *dst
is too small to store the result string.
.It Bq Er EBADF
The conversion descriptor specified by
.Fa cd
is invalid.
.It Bq Er EILSEQ
The string pointed to by
.Fa *src
contains a byte sequence which does not describe a valid character of
the source codeset.
.It Bq Er EINVAL
The string pointed to by
.Fa *src
terminates with an incomplete character or shift sequence.
.El
.\" ----------------------------------------------------------------------
.Sh SEE ALSO
.Xr iconv 1
.\" ----------------------------------------------------------------------
.Sh STANDARDS
.Fn iconv_open ,
.Fn iconv_close ,
and
.Fn iconv
conform to
.St -p1003.1-2001 .
.Pp
Unfortunately due a historical mistake (documented in 
.Pa https://www5.opengroup.org/sophocles2/show_mail.tpl?&source=L&listname=austin-group-l&id=7404 ),
where the unix header page had the
second argument of
.Fn iconv
as
.Ft char **
and the manual page had the same as
.Ft const char ** ,
the definition of
.Ft iconv
has not been consistent across operating systems.
Given the committee's decicion to follow the header definition, we had to make
a decision between following the committee's change or keeping our existing
definition which is arguably more correct like others have (
.Pa http://www.gnu.org/savannah-checkouts/gnu/libiconv/documentation/libiconv-1.14/ ).
We chose to keep our definition and we believe that this is not a problem because
most packages are aware of this issue and handle it anyway during configuration.
.\" ----------------------------------------------------------------------
.Sh BUGS
If
.Fn iconv
is aborted due to the occurrence of some error,
the
.Dq invalid conversion
count mentioned above is unfortunately lost.