How to set GDB read watchpoint in registers?

Question

I'm trying to set a read watch point in registers. By reading the user manual. I understood how to set watchpoint in registers

watch [-l|-location] expr [thread thread-id] [mask maskvalue]

Set a watchpoint for an expression. GDB will break when the expression expr is written into by the program and its value changes.

For example: watch $r1. It means GDB will break when the register r1 is written into by the program and its value changes. It works!

But when I use: rwatch $r1, the gdb's reply is "Expression cannot be implemented with read/access watchpoint."

So I want to know how can I set a read watch point in registers. or are There any ways to break when the register is read by the program?

Many Thanks!

Answer 1

So I want to know how can I set a read watch point in registers

I don't believe GDB provides any facility to do so.

When you set a watchpoint on memory location, GDB can implement it very efficiently on platforms that provide special debug registers (e.g. x86_64). The program then runs at full speed, until the location is accessed.

On platforms that don't provide debug registers, GDB has to single-step the program, and compare the value at location with a value it previously recorded. This is very slow (but sometimes useful), and can only work for write watchpoints for obvious reason.

Since registers don't have a "location", when you set a watchpoint on register, GDB performs the same "single step and compare with previous value" dance. And the read watchpoint can't work at all.

What you ask for could be implemented in GDB: single step and examine every instruction -- does it access the named register or not?

But the need for this is very rare, and the implementation complexity is quite high, so I don't think anybody would be willing to actually implement this.