iconv.h文件里面注视说We would like to #include any system header file which could define
iconv_t, 1. in order to eliminate the risk that the user gets compilation
errors because some other system header file includes /usr/include/iconv.h
which defines iconv_t or declares iconv after this file, 2. when compiling
for LIBICONV_PLUG, we need the proper iconv_t type in order to produce
binary compatible code.
But gcc's #include_next is not portable. Thus, once libiconv's iconv.h
has been installed in /usr/local/include, there is no way any more to
include the original /usr/include/iconv.h. We simply have to get away
without it.
Ad 1. The risk that a system header file does
#include "iconv.h" or #include_next "iconv.h"
is small. They all do #include <iconv.h>.
Ad 2. The iconv_t type is a pointer type in all cases I have seen. (It
has to be a scalar type because (iconv_t)(-1) is a possible return value
from iconv_open().)