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10761 :: cannot open: /dev/stderr

• stbalbach (orginal) [2023-12-13T04:01:26+01:00] view original

  This occasionally comes up

  .nim(855)              writeFile
  Error: unhandled exception: cannot open: /dev/stderr [IOError]
  Run

  The only proc I have that uses writeFile

  #
  #  >
  #
  # Write 'text\n' to 'filename', overwrite previous content. Close on finish.
  #  example:
  #    "Hello" & " world" >* "/tmp/test.txt"
  #    "Hello" >* "/dev/stderr"
  #
  proc `>*`*(text, filename: string): bool {.discardable.} =
    var text = text & "\n"
    try:
      writeFile(filename, text)
    except ValueError as e:
      echo e.msg
  Run

  It's called by this proc

  # Send message to stderr
  #
  #  stdErr("Ending OK")
  #
  #
  proc stdErr*(s: string) =
    
    s >* "/dev/stderr"
  Run

  What's happening it can't open stderr? I'm running an old version of nim 1.4 and don't want to upgrade only for this. Is there another recommend method for writing to stderr? Thanks!

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• jtv (orginal) [2023-12-13T04:11:32+01:00] view original

  What's wrong with stdout.write(s)?

  I'd totally forgotten about symlinks in /dev. You can always just write to fd #2 :)

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• stbalbach (orginal) [2023-12-13T06:22:32+01:00] view original

  stderr.write(s) and writeFile("/dev/stderr", s) appear to use the same code, c_fputs(), which is a call to a C library function fputs(). They both can throw exceptions. I have never seen this before, in any application or language, where occasionally it fails to open stderr.

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• Araq (orginal) [2023-12-13T07:44:24+01:00] view original

  writeFile doesn't throw ValueError, but IOError.

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• stbalbach (orginal) [2023-12-13T18:35:21+01:00] view original

  Excellent thanks for catching that. I did some research and found this:

  try:
      writeFile(filename, text)
    except Exception as e:
      echo e.msg
  Run

  I'm try it and see what happens, because it seems more robust for any type of Exception, future proof. If the same problem I'll change to IOError. Still, I wonder why it fails writing to stderr.

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• shirleyquirk (orginal) [2023-12-13T20:26:53+01:00] view original

  I wonder why it fails writing to stderr.

  its even more of a head scratcher how it can write the exception to stderr after it couldnt write to stderr

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• stbalbach (orginal) [2023-12-16T17:17:41+01:00] view original

  This appears resolved by setting the exception per above. Now looks like:

  #
  # showException
  #
  proc showException(msg: string) =
    try:
      stderr.write(msg)
    except Exception as e:
      discard e.msg              # optionally log or email
  
  #
  #  >*
  #
  # Write 'text\n' to 'filename', overwrite previous content. Close on finish.
  #  example:
  #    "Hello" & " world" >* "/tmp/test.txt"
  #    "Hello" >* "/dev/stderr"
  #
  proc `>*`*(text, filename: string): bool {.discardable.} =
    
    var text = text & "\n"
    try:
      writeFile(filename, text)
    except Exception as e:
      showException(e.msg)
      return false
    return true
  
  Run

  I suspect there was never an actual error writing to stderr, but because of the wrong exception value, it caused something unexpected.

  It would be nice if when there is an incorrect value, it defaults to "except Exception", to protect programmers from themselves. Or gave a compile warning if possible. I don't know how the exception system works underneath.

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