上下文保存–​​如何读取C中的段寄存器和指令指针？

Question

Hell'o. I want to create a program which saves register set – process context. When I try to read CS register, my compiler and IDE tell me, that register "CS" doesn't exist.

    register int *cs asm ("cs");

Am I doing it right? Saving process context means saving every register – especially Code Segment (and other segments as well) – those registers are required to restore process image and start executing where last execution stopped.

Answer 1

Saving process context means saving every register

Of course you can save cs and eip . However ...

• ... these two registers contain the location (address) of the instruction currently being executed.

  This means that these registers will always contain the address of the code that saves the context registers.

  Why do you want to store this information when these registers always contain the same value?

• ... what do you think what happens when you restore these registers?

  Because these two registers contain the address of the instruction currently being executed, the only way to "restore" these two registers is to jump to the instruction that saved the values of the register.

  You'll run into an endless loop...

  Is this really what you want to do?

I want to create a program which saves register set – process context.

 register int *cs asm ("cs"); 

Now it's getting very scary:

If you really want to write some code that saves the CPU context (eg for manually implementing a try - catch mechanism in "low-level" C), you should write the complete function in assembler.

Don't try to write this using inline assembly within C functions!

Otherwise you don't know how the "compiled" C code will interact with the assembler code; it's highly probable that your program won't work.