Error: "sync variables cannot currently be written"

Question

One of my students is having a severe problem with their project (assignment is: http://turing.plymouth.edu/~kgb1013/?course=4310&project=0). They (and I) are getting a weird error when compiling with my test file, testSemaphore.chpl, available at: http://turing.plymouth.edu/~kgb1013/4310/semaphoreProject/testSemaphore.chpl . The error is this:

$CHPL_HOME/modules/standard/IO.chpl:3038: error: sync variables cannot currently be written - apply readFE/readFF() to those variables first

I've had them remove the body of all of their methods, but they're still getting the error. Here is the body of their code:

use Time;

class Semaphore {
    var gate1$ : sync int;

    //Constructor
    proc Semaphore(){
    }

    //secondary Constructor
    proc Semaphore(givenTokens : int){
    }

    //returns the number of tokens available
    proc getNumTokens() : int {
        return 1;
    }

    //gives out a token
    proc p() {

    }

    //returns a token
    proc v() {
    }

}

Why are they getting this error?

What version of Chapel are you using? For clarification, does the body of code posted produce the error for you? It is compiling and running for me on the `master` branch and `1.16.0` — ben-albrecht, Feb 26 '18 at 16:28
Sorry, I had misinterpreted the code provided. I have reproduced the error now. — ben-albrecht, Feb 26 '18 at 16:34

score 4 · Accepted Answer · answered Feb 26 '18 at 16:37

Sorry you're having trouble! It turns out that some of this would be better as a GitHub issue on the Chapel project, but let me first explain what's happening.

The class Semaphore includes a sync variable field. Meanwhile, the default implementation for writing a class is to write each field. So the error message in IO.chpl is trying to say that it doesn't have a reasonable way of outputting a sync variable. E.g.

var x: sync int;
writeln(x);

gives the same error. I think it would be reasonable to open up a GitHub issue on the Chapel project about how inscrutable this error is (and how it doesn't report a line number that is useful to you). Note that I personally used chpl testSemaphore.chpl --print-callstack-on-error to better understand the error - it often helps to add --print-callstack-on-error when you get an error referring to an internal/standard module.

Now, as for the assignment, there are two ways to resolve it:

Adjust testSemaphore.chpl to not print out the semaphore representation in 'writeln' calls. I commented out both writelns starting with "Testing that" and got it to compile.
Adjust the class Semaphore to include a writeThis to replace the compiler-generated write-each-field default, as below:

Here is an example of such a writeThis (see also The readThis, writeThis, and readWriteThis methods )

class Semaphore {
  var gate1$ : sync int;
  // Other methods as before
  proc writeThis(f) {
    f <~> "sync semaphore";
  }
}

Oh, it's because he didn't add a writeThis method! Got it! I added a bit to the project description to make sure people skip it less often. — Kyle, Feb 26 '18 at 19:04

Error: "sync variables cannot currently be written"

1 Answers1