emerge glibc
Calculating dependencies... done!

>>> Emerging (1 of 1) sys-libs/glibc-2.5 to /
 * glibc-libidn-2.5.tar.bz2 MD5 ;-) ...                                                                                                                [ ok ]
 * glibc-libidn-2.5.tar.bz2 RMD160 ;-) ...                                                                                                             [ ok ]
 * glibc-libidn-2.5.tar.bz2 SHA1 ;-) ...                                                                                                               [ ok ]
 * glibc-libidn-2.5.tar.bz2 SHA256 ;-) ...                                                                                                             [ ok ]
 * glibc-libidn-2.5.tar.bz2 size ;-) ...                                                                                                               [ ok ]
 * glibc-ports-2.5.tar.bz2 MD5 ;-) ...                                                                                                                 [ ok ]
 * glibc-ports-2.5.tar.bz2 RMD160 ;-) ...                                                                                                              [ ok ]
 * glibc-ports-2.5.tar.bz2 SHA1 ;-) ...                                                                                                                [ ok ]
 * glibc-ports-2.5.tar.bz2 SHA256 ;-) ...                                                                                                              [ ok ]
 * glibc-ports-2.5.tar.bz2 size ;-) ...                                                                                                                [ ok ]
 * glibc-2.5.tar.bz2 MD5 ;-) ...                                                                                                                       [ ok ]
 * glibc-2.5.tar.bz2 RMD160 ;-) ...                                                                                                                    [ ok ]
 * glibc-2.5.tar.bz2 SHA1 ;-) ...                                                                                                                      [ ok ]
 * glibc-2.5.tar.bz2 SHA256 ;-) ...                                                                                                                    [ ok ]
 * glibc-2.5.tar.bz2 size ;-) ...                                                                                                                      [ ok ]
 * glibc-linuxthreads-2.5.tar.bz2 MD5 ;-) ...                                                                                                          [ ok ]
 * glibc-linuxthreads-2.5.tar.bz2 RMD160 ;-) ...                                                                                                       [ ok ]
 * glibc-linuxthreads-2.5.tar.bz2 SHA1 ;-) ...                                                                                                         [ ok ]
 * glibc-linuxthreads-2.5.tar.bz2 SHA256 ;-) ...                                                                                                       [ ok ]
 * glibc-linuxthreads-2.5.tar.bz2 size ;-) ...                                                                                                         [ ok ]
 * glibc-2.5-patches-1.3.tar.bz2 MD5 ;-) ...                                                                                                           [ ok ]
 * glibc-2.5-patches-1.3.tar.bz2 RMD160 ;-) ...                                                                                                        [ ok ]
 * glibc-2.5-patches-1.3.tar.bz2 SHA1 ;-) ...                                                                                                          [ ok ]
 * glibc-2.5-patches-1.3.tar.bz2 SHA256 ;-) ...                                                                                                        [ ok ]
 * glibc-2.5-patches-1.3.tar.bz2 size ;-) ...                                                                                                          [ ok ]
 * checking ebuild checksums ;-) ...                                                                                                                   [ ok ]
 * checking auxfile checksums ;-) ...                                                                                                                  [ ok ]
 * checking miscfile checksums ;-) ...                                                                                                                 [ ok ]
 * checking glibc-2.5.tar.bz2 ;-) ...                                                                                                                  [ ok ]
 * checking glibc-ports-2.5.tar.bz2 ;-) ...                                                                                                            [ ok ]
 * checking glibc-libidn-2.5.tar.bz2 ;-) ...                                                                                                           [ ok ]
 * checking glibc-2.5-patches-1.3.tar.bz2 ;-) ...                                                                                                      [ ok ]
 * checking glibc-linuxthreads-2.5.tar.bz2 ;-) ...                                                                                                     [ ok ]
 * Sanity check to keep you from breaking your system:
 *  Downgrading glibc is not supported and a sure way to destruction

!!! ERROR: sys-libs/glibc-2.5 failed.
Call stack:
  ebuild.sh, line 1592:   Called dyn_setup
  ebuild.sh, line 666:   Called pkg_setup
  glibc-2.5.ebuild, line 1022:   Called die

!!! aborting to save your system
!!! If you need support, post the topmost build error, and the call stack if relevant.