Bryan O'Sullivan wrote:
What is the "it" that segfaults? The Haskell program shouldn't, at least.
The Haskell program. It does so rarely, however, and I'm unable to reproduce it with any consistency, only enough to notice something is wrong with what I've written. Upon closer examination my code also returns an error exit code for subprocesses that should've succeeded (pdflatex, but once in a blue moon also pstoedit) similarly rarely.
That said, your code for reading from child processes is wrong. You should read all of a child's output strictly (i.e. hGetContents alone will not do the trick) before you wait for its exit status. Your current scheme reverses this order, and there is hence no guarantee that it will read anything from the defunct child process. There's some possibility that this might be the cause of your segfault, which to me would suggest the presence of a bug in the runtime system.
Well, if there's a bug in the runtime system, I'll try to make sure I can reproduce it before filing a report. Meanwhile, you are absolutely right. How would you propose I properly perform that sort of interaction? It's often useful to use external programs to do things for you by throwing data at them and reading (or ignoring) the result until they die. Is there a Haskell idiom for this? Overall, I find Haskell to be very nice. I just keep getting bitten by laziness, at least until I get used to it. Sincerely, Rafal Kolanski. PS. The file I attached is the only real place where concurrency happens in my code. The rest of the uses are simple ... when rendering a slide using cairo: slideNaive = do ... paintLatex "\\textbf{datatype} ref = Ref int $\\mid$ Null" 50 0 ... which calls: paintLatex text x y = do svg <- liftIO $ updateSVG text paintSVG svg 3 x y updateSVG is defined in SVGLatex.hs which I attached earlier, and the rest: paintSVG :: FilePath -> Double -> Double -> Double -> Render () paintSVG file scale' x y = do save svg <- liftIO $ svgNewFromFile file let (w,h) = svgGetSize svg translate x y scale scale' scale' moveTo 0 0 svgRender svg restore which invokes functions defined in: import Graphics.Rendering.Cairo import Graphics.Rendering.Cairo.SVG