通过在寄存器中传递函数地址，从asm文件中调用C函数

Question

I have a function named system_handler which is defined in a func.c . I need to call system_handler from another assembly program by passing the address of the function to a register and call the register.

So far I have written this:

       .extern system_handler   ; Is defined in func.c
       mov system_handler, %eax
       call %eax   ; This call is making run time error in emulator 
                   ; fatal: Trying to execute code outside RAM or ROM at 0x8b1cec83

While assembling the asm file I am getting a warning:

Warning: indirect call without '*'

compiler and assembler used: i586-gnu-{gcc/as} , Using AT&T format in asm file.

Answer 1

In AT&T syntax you should use mov $system_handler, %eax to load the address, what you did was loading the first dword of your function from memory.

To fix the warning, you should of course write call *%eax , which is another peculiarity of AT&T.

If you are not familiar with AT&T syntax, you can switch both gcc and gas to intel syntax.