How to use string value as a logging file name

Question

I'm debugging a C program with GDB. I've got an array which the values are updated many times (a little bit less than 4.000). I want to write thoses value in a different file for each iterations.

I tried to set the log file name directly in a GDB file:

break test.c:95
  commands
  silent

  # Set the logging file
  set $_file=(char*)malloc(64)
  call snprintf($_file, 64, "~/path/to/logging/file_%04d.txt", i) # i is the for variable
  set logging file $_file
  call free($_file)

  set logging redirect on
  set pagination off
  set logging overwrite on
  set logging on

  set $_data=array

  set $_x=0
  while ($_x<2000)
    set $_y=0
    while ($_y<2000)
      printf "%f    ", $_data[$_x][$_y]
      set $_y++
    end
    printf "\n"
    set $_x++
  end

  # Log default setup
  set logging off
  set logging redirect off
  set logging overwrite off
  set pagination on

  continue
end

But gdb created a file $_file in my working directory instead of reading the value of the variable.

I then tried to put the file name in which I want log the output in a variable fileName in my code (also with snprintf). And the result is the same, a fileName file is created in my working directory.

Finaly I tried to put the debug code into a gdb function, and give the fileName as an argument of the function, but as previously, it does not interpret the string.

My question
With GDB, how can I use the value of a C-string as a logging file name?

Answer 1

You can use eval to insert variables value into gdb's commands. In your case, logging file name could be setted to $_file by command:

eval "set logging file %s", $_file

Also you can use dump, in case if you want to dump array to fileName by gdb builtin commands:

eval "dump value %s array", fileName

Note: dumped value is binary.