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[SR-6386] Note incorrect pass-through of variadic arguments #48936

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swift-ci opened this issue Nov 15, 2017 · 1 comment
Open

[SR-6386] Note incorrect pass-through of variadic arguments #48936

swift-ci opened this issue Nov 15, 2017 · 1 comment
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compiler The Swift compiler in itself improvement

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@swift-ci
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Previous ID SR-6386
Radar None
Original Reporter rmann (JIRA User)
Type Improvement
Additional Detail from JIRA
Votes 1
Component/s Compiler
Labels Improvement
Assignee None
Priority Medium

md5: c1e78ccbc6e54d3f8578b6e6c8b37666

relates to:

  • SR-8047 compiler should warn when implicitly casting Array to CVarArg

Issue Description:

I've had a long-working debugLog() method that looks like this:

func
debugLog<T>(_ inMsg: T, file inFile : String = #file, line inLine : Int = #line)
{
    let file = (inFile as NSString).lastPathComponent
     let s = "\(file):\(inLine)    \(inMsg)"
     print(s)
}

I wanted to add a version that works like String(format:):

func
debugLog(format inFormat: String, file inFile : String = #file, line inLine : Int = #line, _ inArgs: CVarArg...)
{
     let s = String(format: inFormat, inArgs)
     debugLog(s, file: inFile, line: inLine)
}

While this compiles and executes, all of the values are zero for this example:

let xc = CGFloat(1.0)
let yc = CGFloat(0.0)
let len = CGFloat(282.1364917907643)

debugLog(format: "Pixel %f, %f length too far %f", xc, yc, len)

Output:

FisheyeImageRenderer.swift:108    Pixel 0.000000, 0.000000 length too far 0.000000

Something is being misinterpreted in the passing of inArgs: CVarArg.... The solution is to call

let s = String(format: inFormat, arguments: inArgs)

The compiler, perhaps, should warn of this mistake.
@swift-ci
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Comment by Federico Cappelli (JIRA)

I don't think this is a bug.

When you pass a `CVarArg` to `String(format: inFormat, inArgs)` at that point `inArgs` is already an array.

If you check the Swift implementation of os_log(...) in https://github.com/apple/swift/blob/6e7051eb1e38e743a514555d09256d12d3fec750/stdlib/public/Darwin/os/os_log.swift you can see how this is done:

public func os_log(
  _ message: StaticString,
  dso: UnsafeRawPointer? = #dsohandle,
  log: OSLog = .default,
  type: OSLogType = .default,
  _ args: CVarArg...)
{
  guard log.isEnabled(type: type) else { return }
  let ra = _swift_os_log_return_address()


  message.withUTF8Buffer { (buf: UnsafeBufferPointer<UInt8>) in
    // Since dladdr is in libc, it is safe to unsafeBitCast
    // the cstring argument type.
    buf.baseAddress!.withMemoryRebound(
      to: CChar.self, capacity: buf.count
    ) { str in
      withVaList(args) { valist in                           <<<<<<<<HERE
        _swift_os_log(dso, ra, log, type, str, valist)
      }
    }
  }
}

@swift-ci swift-ci transferred this issue from apple/swift-issues Apr 25, 2022
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@swift-ci