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#REDIRECT [[Bare Bones]] |
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{{Rating|2}} |
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:''This tutorial sets up only the most basic bootable C++ kernel. For more information on '''C++ kernels''' see [[C PlusPlus|C++]].'' |
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{{Redirect page}} |
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If you have a [[Tutorial:C kernel|GRUB-booting C kernel]] (please read and follow through that document first!), extending it to C++ isn't difficult. All that is needed are a couple of lines to cater for C++ specifics. |
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==linker.ld== |
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Using C++ adds two more sections of interest to your kernel binaries: The constructors and destructors of static objects. You have to modify the linker script to include them. |
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<pre> |
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.data ALIGN (0x1000) : { |
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start_ctors = .; |
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*(.ctor*) |
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end_ctors = .; |
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start_dtors = .; |
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*(.dtor*) |
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end_dtors = .; |
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*(.data) |
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} |
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</pre> |
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==loader.s== |
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The static constructors have to be called before you enter the main kernel function, and the destructors have to be called after that function returns. This is done by modifying loader.s as follows: |
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===NASM=== |
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<pre> |
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extern start_ctors, end_ctors, start_dtors, end_dtors |
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loader: |
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mov esp, stack+STACKSIZE ; set up the stack |
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push eax ; Multiboot magic number |
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push ebx ; Multiboot info structure |
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static_ctors_loop: |
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mov ebx, start_ctors |
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jmp .test |
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.body: |
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call [ebx] |
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add ebx,4 |
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.test: |
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cmp ebx, end_ctors |
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jb .body |
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call _main ; call kernel proper |
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static_dtors_loop: |
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mov ebx, start_dtors |
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jmp .test |
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.body: |
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call [ebx] |
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add ebx,4 |
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.test: |
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cmp ebx, end_dtors |
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jb .body |
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hlt ; halt machine should kernel return |
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</pre> |
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===GAS=== |
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<pre> |
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loader: |
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mov $(stack + STACKSIZE), %esp # set up the stack |
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push %eax # Multiboot magic number |
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push %ebx # Multiboot data structure |
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# calling static constructors |
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mov $start_ctors, %ebx |
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jmp 2f |
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1: |
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call *(%ebx) |
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add $4, %ebx |
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2: |
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cmp $end_ctors, %ebx |
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jb 1b |
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call _main # call kernel proper |
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# calling static destructors |
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mov $start_dtors, %ebx |
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jmp 4f |
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3: |
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call *(%ebx) |
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add $4, %ebx |
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4: |
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cmp $end_dtors, %ebx |
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jb 3b |
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hlt # halt machine should kernel return |
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</pre> |
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===C/C++=== |
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<pre> |
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typedef unsigned long vintp; //- integer type to store a pointer. |
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extern start_ctors, end_ctors, start_dtors, end_dtors; |
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void loader(void) { |
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//- call all the static constructors in the list. |
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for(vintp * call = &start_ctors; call < &end_ctors; call++) { |
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((void (*)(void))*call)(); |
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} |
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//- call kernel proper |
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main(); |
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//- call all the static destructors in the list. |
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for(vintp * call = &start_dtors; call < &end_dtors; call++) { |
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((void (*)(void))*call)(); |
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} |
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} |
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</pre> |
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==main.cpp== |
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Now, all that is needed is to declare C style linkage for the kernel entry function, so that its name will not get mangled to C++ linkage style: |
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<pre> |
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extern "C" void _main(struct multiboot_data* mbd, unsigned int magic); |
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void _main( struct multiboot_data* mbd, unsigned int magic ) |
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{ |
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// write your kernel here |
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} |
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</pre> |
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==Notes== |
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In this code "loader" is the entry point to the binary. Make sure that your linker script has something like |
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<pre> |
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ENTRY(loader) |
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</pre> |
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==Questions== |
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;Does anyone know if those .ctors and .dtors are specific to some compiler or if there's an ABI stating them ? |
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:It's defined in the ELF ABI for System V platforms, but it's used by most unices. The concept of using a constructor/destructor list for bootup is not so much specified by any C++ ABI, but it is used in pretty much all implementations (not checked, but probably all). |
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;Does anyone know how to control the order of the static ctor's ? |
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:It is assumed that the compiler puts them into the correct order. However, it is probably best to keep interdependencies at an absolute minimum. |
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==See Also== |
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===Articles=== |
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*[[C PlusPlus|C++]] |
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*[[Bare bones]] |
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[[Category:Bare bones tutorials|C PlusPlus bare bones]] |
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Latest revision as of 09:33, 22 June 2024
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