Freeing memory allocated with newCString

Question

As library docs say CString created with newCString must be freed with free function. I have been expecting that when CString is created it would take some memory and when it is released with free memory usage would go down, but it didn't! Here is example code:

module Main where

import Foreign
import Foreign.C.String
import System.IO

wait = do
  putStr "Press enter" >> hFlush stdout
  _ <- getLine
  return ()

main = do
  let s = concat $ replicate 1000000 ['0'..'9']
  cs <- newCString s
  cs `seq` wait   -- (1)

  free cs
  wait   -- (2)

When program stopped at (1), htop program showed that memory usage is somewhere around 410M - this is OK. I press enter and the program stops at line (2), but memory usage is still 410M despite cs has been freed!

How is this possible? Similar program written in C behaves as it should. What am I missing here?

Answer 1

The issue is that free just indicates to the garbage collector that it can now collect the string. That doesn't actually force the garbage collector to run though -- it just indicates that the CString is now garbage. It is still up to the GC to decide when to run, based on heap pressure heuristics.

You can force a major collection by calling performGC straight after the call to free, which immediately reduces the memory to 5M or so.

E.g. this program:

import Foreign
import Foreign.C.String
import System.IO
import System.Mem

wait = do
  putStr "Press enter" >> hFlush stdout
  _ <- getLine
  return ()

main = do
  let s = concat $ replicate 1000000 ['0'..'9']
  cs <- newCString s
  cs `seq` wait   -- (1)

  free cs
  performGC
  wait   -- (2)

Behaves as expected, with the following memory profile - the first red dot is the call to performGC, immediately deallocating the string. The program then hovers around 5M until terminated.