Re: writing a jump table

[Nicolas Bock <nicolasbock@gmail.com>]

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Using a larger switch statement in C, I looked at the code that gcc
produces and changed a few things in my assembly function. It now looks
like below. This function works now, it actually jumps to label_02. When
I try to put that function into a project that uses libtool to build a
dynamic library however, I get the following linking error:

/usr/lib/gcc/x86_64-pc-linux-gnu/4.4.5/../../../../x86_64-pc-linux-gnu/bin/ld:
./.libs/libspamm_kernel.a(jump_table.o): relocation R_X86_64_32S against
`.rodata' can not be used when making a shared object; recompile with -fPIC
./.libs/libspamm_kernel.a(jump_table.o): could not read symbols: Bad value

What does this error mean?

Thanks,

nick





jump_table.S:

  .text
  .global jump_table
  .type jump_table, @function

jump_table:
  # Push stack pointer so we can make room for local storage.
  push %rax

  mov $0x02, %rax # Move index into rax; 2 is supposed to end up at
label_02.
  jmp *table(, %rax, 8) # Jump into the table.

  .section .rodata
  .align 8
table:
  .quad label_00
  .quad label_01
  .quad label_02

  .text
label_00:
  jmp done

label_01:
  jmp done

label_02:
  jmp done

done:
  pop %rax
  ret

  .size jump_table, .-jump_table


On 03/15/11 18:25, Brian Raiter wrote:
>> I am trying to write a jump table, but unfortunately with limited
>> success. When I compile the code and disassemble it, the offset of
>> "table" is 0, which I guess means that something didn't work out.
> 
> Actually, table appears immediately following your indirect jump
> instruction, so I would assume that the offset would be zero. Did you
> actually try this code to verify that it doesn't do what you expect?
> 
> b
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