The iconv-based codec (gh-152997) fails to build on illumos/Solaris #154048

Building CPython main on illumos (tested on OpenIndiana Hipster 2026.04, gcc 10) fails to compile Objects/unicodeobject.c:

../Objects/unicodeobject.c:8282:32: error: passing argument 2 of 'iconv' from incompatible pointer type [-Wincompatible-pointer-types]
../Objects/unicodeobject.c:8455:48: error: passing argument 2 of 'iconv' from incompatible pointer type [-Wincompatible-pointer-types]

This is a core object file, so the whole build stops here.

The iconv-based codec added in gh-152997 (GH-153001) calls iconv() with a char ** input buffer:

char *inptr = (char *)in;
...
size_t ret = iconv(cd, &inptr, &inleft, &outptr, &outleft);

POSIX declares iconv()'s second argument as char **, but some systems declare it as const char ** — notably illumos/Solaris (and old GNU libiconv). There, passing char ** is an incompatible-pointer error. This is the long-standing "iconv inbuf const-ness varies by platform" portability wart.

Linked PRs

• gh-154048: Fix building the iconv codec on illumos/Solaris #154049