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debugginggdbcpu-registerswatchpoint
user174916
user174916

Reputation: 97

How to monitor resources needed to set watchpoints in gdb?

On x86 GDB uses some special hardware resources (debug registers?) to set watchpoints. In some situations, when there is not enough of that resources, GDB will set the watchpoint, but it won't work. Is there any way to programmatically monitor the availability of this resources on Linux? Maybe some info in procfs, or something. I need this info to choose machine in pool for debugging.

From GDB Internals: "Since they depend on hardware resources, hardware breakpoints may be limited in number; when the user asks for more, gdb will start trying to set software breakpoints. (On some architectures, notably the 32-bit x86 platforms, gdb cannot always know whether there's enough hardware resources to insert all the hardware breakpoints and watchpoints. On those platforms, gdb prints an error message only when the program being debugged is continued.)"

"Too many different watchpoints requested. (On some architectures, this situation is impossible to detect until the debugged program is resumed.) Note that x86 debug registers are used both for hardware breakpoints and for watchpoints, so setting too many hardware breakpoints might cause watchpoint insertion to fail."

"The 32-bit Intel x86 processors feature special debug registers designed to facilitate debugging. gdb provides a generic library of functions that x86-based ports can use to implement support for watchpoints and hardware-assisted breakpoints."

Upvotes: 2

Views: 448

Answers (1)

Employed Russian
Employed Russian

Reputation: 213526

I need this info to choose machine in pool for debugging.

No, you don't. The x86 debug registers (there are 4) are per-process resource, not per-machine resource [1]. You can have up to 4 hardware watchpoints for every process you are debugging. If someone else is debugging on the same machine, you are not going to interfere with each other.

[1] More precisely, the registers are multiplexed by the kernel: in the same way as e.g. the EAX register. Every process on the system and the kernel itself uses EAX, there is only a single EAX register on (single-core) CPU, yet it all works fine through the magic of time-slicing.

Upvotes: 1

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