Watch a memory range in gdb?

Question

I am debugging a program in gdb and I want the program to stop when the memory region 0x08049000 to 0x0804a000 is accessed. When I try to set memory breakpoints manually, gd

Answer 1

If you use GDB 7.4 together with Valgrind 3.7.0, then you have unlimited "emulated" hardware watchpoints.

Start your program under Valgrind, giving the arguments --vgdb=full --vgdb-error=0 then use GDB to connect to it (target remote | vgdb). Then you can e.g. watch or awatch or rwatch a memory range by doing rwatch (char[100]) *0x5180040

See the Valgrind user manual on gdb integration for more details

Answer 2

The feature that detects when a memory address has changed is called a hardware breakpoint, and it's actually a feature of the CPU — a register inside the memory controller that detects when a specific address is accessed, and triggers a debugger break interrupt. Unfortunately the x86 architecture only has four such registers and that's why you're limited in the number of memory watch breakpoints you can set.

That's why you need to use something like valgrind; if you want to watch an entire region, you have to do it by using software that simulates the memory access patterns. I don't know if valgrind actually supports watching entire memory ranges, though. You may have to patch it yourself. Modify VALGRIND_MAKE_MEM_NOACCESS() to throw a breakpoint but then allow the program to continue, maybe.