> For the complete documentation index, see [llms.txt](https://theiotlearninginitiative.gitbook.io/linuxkernel/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://theiotlearninginitiative.gitbook.io/linuxkernel/build-system.md).

# Build System

Please read the "Kbuild: the Linux Kernel Build System" carefully, you will understand how this system works [Kbuild Linux Kernel Build System](http://www.linuxjournal.com/content/kbuild-linux-kernel-build-system)

Do you want to do another Linux Kernel Build System exercise by writing a Hello World Kernel Module? then keep reading...

## Linux Kernel Build System, Hello World Module

Make a "helloworld" directory under drivers

```bash
    user@workstation:~/linux$ mkdir drivers/helloworld
```

Create helloworld.c file under our helloworld directory and add the C code below, this is a simple Hello World Kernel Module

```bash
    user@workstation:~/linux$$ nano drivers/helloworld/helloworld.c
```

```c
#include <linux/init.h>
#include <linux/kernel.h>
#include <linux/module.h>

static int module_init_function(void)clear

{
    printk(KERN_INFO "Module? Hello!\n");
    return 0;
}

static void module_exit_function(void)
{
    printk(KERN_INFO "Module? Bye!\n");
}

MODULE_LICENSE("GPL");
MODULE_AUTHOR("xe1gyq");
MODULE_DESCRIPTION("My First Linux Kernel Module");

module_init(module_init_function);
module_exit(module_exit_function);
```

## Linux Kernel Build System, Hello World Kconfig

Create the Kconfig file under helloworld directory and add the code below, make sure indentation is correct

```bash
    user@workstation:~/linux$ nano drivers/helloworld/Kconfig

    menu "Hello Module Kernel Support"

    config HELLO_WORLD
            tristate "Hello Module Driver"
            depends on X86
            help
              Select this option to run a Hello World Module!
    endmenu
```

## Linux Kernel Build System, Hello World Makefile

Create the Makefile under helloworld directory and add the code below

```bash
    user@workstation:~/linux$ nano drivers/helloworld/Makefile
    obj-$(CONFIG_HELLO_WORLD)               += helloworld.o
```

## Linux Kernel Build System, Device Drivers Kconfig seeing Hello World Directory

Modify Kconfig under drivers directory and add the line with helloworld

```bash
    user@workstation:~/linux$ nano drivers/Kconfig
    menu "Device Drivers"
    source "drivers/helloworld/Kconfig"
    source "drivers/amba/Kconfig"
```

## Linux Kernel Build System, Device Drivers Makefile compiling Hello World Directory

Modify Makefile under drivers directory and add the line with **CONFIG\_HELLO\_WORLD**

```bash
user@workstation:~/linux$ nano drivers/Makefile
    ...
    # Rewritten to use lists instead of if-statements.
    #
    obj-$(CONFIG_HELLO_WORLD) += helloworld/
    obj-y += irqchip/
    ...
```

## Linux Kernel Build System, Hello World Menuconfig

We are ready to view our Hello World Module under menuconfig

```bash
    user@workstation:~/linux$ make menuconfig
```

Go to its location under

```bash
    -> Device Drivers
      -> Hello Module Kernel Support
```

Understand the menu options seen below including their fast paths (one letter invocation)

```bash
    <Select>    < Exit >    < Help >    < Save >    < Load >
```

Get help for the Hello Module Kernel Support using Help function, you should see this

```bash
    CONFIG_HELLO_WORLD:
    Select this option to run a Hello World Module!
    Symbol: HELLO_WORLD [=n]
    Type : tristate
    Prompt: Hello Module Driver
       Location:
         -> Device Drivers
           -> Hello Module Kernel Support
       Defined at drivers/helloworld/Kconfig:3
       Depends on: X86 [=y]
```

Understand about the following options from Kconfig by googling or looking at other Kconfigs

* default
* tristate
* Depends on

Take a look at the default building state for our Hello World Module and modify Kconfig so you can have it built as default

```bash
     Symbol: HELLO_WORLD [=n]
```

## Linux Kernel Build System, Hello World Compilation

Now compile your Hello World Module both as module and built-in into the Kernel image making sure you boot your system twice to confirm your changes using dmesg command

As Module (M)

```bash
    user@workstation:~/linux$ make
      CHK     include/config/kernel.release
      CHK     include/generated/uapi/linux/version.h
      CHK     include/generated/utsrelease.h
      CALL    scripts/checksyscalls.sh
      CHK     include/generated/compile.h
      LD      drivers/helloworld/built-in.o
      CC [M]  drivers/helloworld/helloworld.o
    Kernel: arch/x86/boot/bzImage is ready  (#2)
      Building modules, stage 2.
      MODPOST 2255 modules
      CC      drivers/helloworld/helloworld.mod.o
      LD [M]  drivers/helloworld/helloworld.ko
    root@workstation:~/linux# make modules_install
    root@workstation:~/linux# make install
    root@workstation:~/linux# shutdown -r now
    <reboot>
    root@workstation:~/linux# modprobe helloworld
    root@workstation:~/linux# dmesg
```

Built-In (\*)

```bash
    user@workstation:~/linux$ make
```

```bash
    scripts/kconfig/conf --silentoldconfig Kconfig
      CHK     include/config/kernel.release                    
      CHK     include/generated/uapi/linux/version.h                    
      CHK     include/generated/utsrelease.h            
      CALL    scripts/checksyscalls.sh                    
      CHK     include/generated/compile.h                    
      CC      drivers/helloworld/helloworld.o                    
      LD      drivers/helloworld/built-in.o                    
      LD      drivers/built-in.o                    
      LINK    vmlinux                    
      LD      vmlinux.o                    
      MODPOST vmlinux.o                    
      GEN     .version                    
      CHK     include/generated/compile.h                    
      UPD     include/generated/compile.h
      CC      init/version.o                    
      LD      init/built-in.o                    
      KSYM    .tmp_kallsyms1.o                    
      KSYM    .tmp_kallsyms2.o                    
      LD      vmlinux
      SORTEX  vmlinux
      SYSMAP  System.map
      VOFFSET arch/x86/boot/voffset.h
      OBJCOPY arch/x86/boot/compressed/vmlinux.bin
      GZIP    arch/x86/boot/compressed/vmlinux.bin.gz
      MKPIGGY arch/x86/boot/compressed/piggy.S
      AS      arch/x86/boot/compressed/piggy.o
      LD      arch/x86/boot/compressed/vmlinux
      ZOFFSET arch/x86/boot/zoffset.h
      AS      arch/x86/boot/header.o
      CC      arch/x86/boot/version.o
      LD      arch/x86/boot/setup.elf
      OBJCOPY arch/x86/boot/setup.bin
      OBJCOPY arch/x86/boot/vmlinux.bin
      BUILD   arch/x86/boot/bzImage
    Setup is 17516 bytes (padded to 17920 bytes).
    System is 4046 kB
    CRC 113cf27d
    Kernel: arch/x86/boot/bzImage is ready  (#3)
      Building modules, stage 2.
      MODPOST 2254 modules
    root@workstation:~/linux# make modules_install
    root@workstation:~/linux# make install
    user@workstation:~/linux# shutdown -r now
    <reboot>
    user@workstation:~$ dmesg
```


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