"Cannot import url", "File outside of allowed directories" using --standard-json

Question

I'm trying to understand how to use solidity's JSON input-output API. Here's my json file;

{
  "language": "Solidity",
  "sources": {
    "StandardToken.sol": {
      "urls": [
        "file:///home/myuser/myproject/contracts/vendor/StandardToken.sol"
      ]
    }
  }
}

My best guess as to the "correct" command is;

cat input.json | solc --allow-paths /home/myuser/myproject/contracts --standard-json

I've tried lots of different relative paths, ending with or without a trailing slash, running the command in different directories, etc. But no matter what combination I try, I get back a json object with this error message;

"Cannot import url (\"file:///home/myuser/myproject/contracts/vendor/StandardToken.sol\"): File import callback not supported"

I was getting this on solc version 0.4.24, but I upgraded to 0.5.1+commit.c8a2cb62.Linux.g++ and get the same issue.

Strangely, I get a similar message with solcjs;

$ cat input.json | solcjs --standard-json
Cannot import url (\"file:///home/myuser/myproject/contracts/vendor/StandardToken.sol\"): File import callback not supported

There seem to be a lot of github issues mentioning these error messages, but I can't make use of any of them.

Answer 1

When I change input.json to the following, without "file://" and with back-slashes instead of forward-slashes, solc is able to import the URL.

{
  "language": "Solidity",
  "sources": {
    "StandardToken.sol": {
      "urls": [
        "\home\myuser\myproject\contracts\vendor\StandardToken.sol"
      ]
    }
  }
}