Dumping memory in gdb - how to choose the file name at run time

Question

Hi,

I'm running a program that does processing on a file. I want to be able to supply the program with several files, and by attaching to it with gdb, I want to get a memory dump at a certain point in the code for each of the files. I want the dump for each file to go to a file with the same filename as the input file (maybe after formatting it a little, say adding a suffix)

So suppose I have a function called HereIsTheFileName(char* filename), and another function called DumpThisMemoryRegion(void* startAddr, void* endAddr), I want to do something like the following:

To get the file name to an environment variable:

• break HereIsTheFileName
• commands 1
• set $filename = malloc(strlen(filename) + 1)
• call memcpy($filename, filename, strlen(filename) + 1)
• end

Then to dump the memory to the filename I saved earlier:

• break DumpThisMemoryRegion
• commands 2
• append binary memory "%s.memory"%$filename startAddr endAddr
• end

(I would even settle for the filename as it is, without formatting, if that turns out to be the difficult part)

However, I couldn't get gdb to accept anything except an exlicit file name for the append/dump commands. when I ran "append binary memory $filename ..." I got the output in the file "/workdir/$filename". Is there any way to make gdb choose the file name at runtime?

Thanks!

Answer 1

I don't know how to make append accept a runtime filename, but you can always cheat a bit by writing the whole thing to a file and then sourcing that file, using logging.

By putting this in your ~/.gdbinit

define reallyappend
  printf "using gdbtmp.log to dump memory to file %s\n", $arg0
  set logging file gdbtmp.log
  set logging overwrite on
  set logging redirect on
  set logging on
  printf "append binary memory %s 0x%x 0x%x", $arg0, $arg1, $arg2
  set logging off
  set logging redirect off
  set logging overwrite off
  source gdbtmp.log
end

you can use the function reallyappend instead, for example with

(gdb) set $filename = "somethingruntimegenerated"
(gdb) reallyappend $filename startAddr endAddr

I don't know if logging works ok in an "commands" environment, but you can give it a shot at least.

Answer 2

Yeah, you can't use a variable here for the filename argument. The best suggestion I can offer is to write a script that will set all the breakpoints and set up the "append" commands, and use text editing or awk and sed to set up the filenames in the script.