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1. Before building glibc, you must remove the following folders:

   /opt/crosstool/gcc-8.3.0-glibc-2.28/i586-linux-gnu/include
   /opt/crosstool/gcc-8.3.0-glibc-2.28/x86_64-linux-gnu/include
              

2. Update C:\Cygwin\opt\crosstool\src\build_gcc_cross.sh :

   #!/bin/bash
   export TARGET=i586-linux-gnu
   export PREFIX=/opt/crosstool/gcc-8.3.0-glibc-2.28/$TARGET
   export PATH=$PATH:$PREFIX/bin
   
   cd /opt/crosstool/src/build-glibc
   find . -delete
   ../glibc-2.28/configure --prefix=/usr \
       --with-headers=$PREFIX/usr/include \
       --enable-add-ons=nptl --enable-kernel=4.0.0 \
       --with-binutils=$PREFIX --build=x86_64-pc-cygwin \
       --host=$TARGET --enable-shared --disable-profile \
       --with-tls --with-__thread --without-gd --without-cvs \
       libc_cv_forced_unwind=yes libc_cv_c_cleanup=yes \
       libc_cv_ssp=no
   if [ "$?" -ne "0" ]; then
       echo "=== build script: failed to configure glibc ==="
       exit 1
   fi
   make
   if [ "$?" -ne "0" ]; then
       echo "=== build script: failed to make glibc ==="
       exit 1
   fi
   make install_root=$PREFIX install
   if [ "$?" -ne "0" ]; then
       echo "=== build script: failed to install glibc ==="
       exit 1
   fi
   
   echo "=== build script: OK ==="

3. Run C:\Cygwin\opt\crosstool\src\build_gcc_cross.cmd and wait until it completes.
4. If there are no errors, shutdown Cygwin VM and make snapshot "SNAP-6".
5. Replace TARGET=i586-linux-gnu with TARGET=x86_64-linux-gnu, then run C:\Cygwin\opt\crosstool\src\build_gcc_cross.cmd and wait until it completes.
6. If there are no errors, shutdown Cygwin VM and make snapshot "SNAP-7".

Notes:

• Glibc build will take a lot of time, 4-5 times more than binutils + gcc.
• "--enable-kernel" sets minimum version of Linux kernel for this glibc to run on. 4.0.0 should be good enough these days.
• "--prefix=/usr" is not dangerous and will not hurt your /usr in host system, unless you call "make install" in plain and wrong way. This is an old glibc joke.
• If you get the following error messages during build:
  In file included from ../sysdeps/unix/sysv/linux/i386/sysdep.h:27,
                   from <stdin>:1:
  ../sysdeps/i386/nptl/tls.h:24:11: fatal error: stdbool.h: No such file or directory
   # include <stdbool.h>
             ^~~~~~~~~~~
  compilation terminated.
  make[2]: *** [../Makerules:287: /opt/crosstool/src/build-glibc/tcb-offsets.h] Error 1
  make[2]: Leaving directory '/opt/crosstool/src/glibc-2.28/csu'
  make[1]: *** [Makefile:258: csu/subdir_lib] Error 2
  make[1]: Leaving directory '/opt/crosstool/src/glibc-2.28'
  make: *** [Makefile:9: all] Error 2
  
  then you forgot to delete the following folders before running the script:

  /opt/crosstool/gcc-8.3.0-glibc-2.28/i586-linux-gnu/include
  /opt/crosstool/gcc-8.3.0-glibc-2.28/x86_64-linux-gnu/include

• Also you may get error message about various missing syscalls (__-prefixed) if you forgot to replace .oS with .oZ in makefiles (Cygwin case-insensitive filesystem).

>> Read next section or buy already prepared cross-compiler (€10) to save your time.