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UEFI App Bare Bones: Difference between revisions
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== Prerequisites ==
Developers will need a [[GCC Cross-Compiler]] or Clang targeting the '''x86_64-w64-mingw32''' target (for [[PE]] output), and the [https://sourceforge.net/projects/gnu-efi/ '''gnu-efi'''] package (for UEFI headers). Most Linux distros provide cross-compilers for this target, so it's usually not necessary to build it yourself. This example does not link against [[GNU-EFI]] or follow its build process; only the headers are used.
To build the EFI filesystem image, developers can use [[MTools]] or [https://github.com/jncronin/mkgpt '''mkgpt'''] to create a hard disk image. To build a CD image, '''xorriso''' (in [[mkisofs]] emulation mode) will be needed. To run under an emulator, it is best to use '''qemu-system-x86_64''' coupled with the [http://tianocore.sourceforge.net/wiki/OVMF '''x64 OVMF firmware'''].
Under an apt-based system (e.g. Debian/Ubuntu), developers can run: <
To install mkgpt you can run these commands:<
cd mkgpt
automake --add-missing
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./configure
make
sudo make install</
== Testing the emulator ==
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Now is a good time to check the emulator is working successfully with the OVMF firmware.
<
== Preparing the files ==
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Next, create a file with the following:
<
#include <efilib.h>
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return Status;
}</
=== gnu-efi/lib/data.c ===
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To build, use the cross-compiler:
<
# compile: (flags before -o become CFLAGS in the Makefile)
x86_64-w64-mingw32-gcc -ffreestanding -Ipath/to/gnu-efi/inc -Ipath/to/gnu-efi/inc/x86_64 -Ipath/to/gnu-efi/inc/protocol -c -o hello.o hello.c
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# link: (flags before -o become LDFLAGS in the Makefile)
x86_64-w64-mingw32-gcc -nostdlib -Wl,-dll -shared -Wl,--subsystem,10 -e efi_main -o BOOTX64.EFI hello.o data.o
</syntaxhighlight>
Note here that '--subsystem 10' specifies an EFI application for ld.
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The build sequence under LLVM/clang is essentially the same, although there is the advantage of having ''all'' targets installed by default:
<
CFLAGS='-target x86_64-unknown-windows
-ffreestanding
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clang $CFLAGS -c -o data.o path/to/gnu-efi/lib/data.c
clang $LDFLAGS -o BOOTX64.EFI hello.o data.o
</syntaxhighlight>
Passing '--target x86_64-unknown-windows' to clang tells it to compile for x86_64 "Windows". This is quite not the same as 64-bit UEFI PE yet, but as before the "freestanding" part makes it a good kernel image. An example of this toolchain is found in the [https://github.com/c-util/c-efi c-efi] project.
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Next, create a FAT filesystem image.
<
dd if=/dev/zero of=fat.img bs=1k count=1440
mformat -i fat.img -f 1440 ::
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mmd -i fat.img ::/EFI/BOOT
mcopy -i fat.img BOOTX64.EFI ::/EFI/BOOT
</syntaxhighlight>
=== Running as a USB stick image ===
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The FAT image can either be written directly to a USB stick and used in in a UEFI machine, or it can be run directly in QEMU:
<
=== Creating and running the HD image ===
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The HD image is a disk image in the [[GPT]] format, with the FAT image specially identified as a 'EFI System Partition'.
<
qemu-system-x86_64 -L OVMF_dir/ -pflash OVMF.fd -hda hdimage.bin</
=== Creating and running the CD image ===
The ISO image is a standard ISO9660 image which contains the FAT image as a file. A special El Torito option (-e) then points EFI aware systems to this image to be loaded. The CD image can either be burned to a CD and ran in a UEFI machine, or run directly in QEMU:
<
cp fat.img iso
xorriso -as mkisofs -R -f -e fat.img -no-emul-boot -o cdimage.iso iso
qemu-system-x86_64 -L OVMF_dir/ -pflash OVMF.fd -cdrom cdimage.iso</
== What to do next? ==
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* [[UEFI ISO Bare Bones]]
* [[GNU-EFI]]
* [[POSIX-UEFI]]
[[Category:Bare bones tutorials]]
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