swift "print" doesn't appear in STDOut but 3rd party c library logs do when running in docker on ECS

Question

So locally when in dev through xcode or compiled with SPM the console logs appear as expected.

i.e with SPM locally everything is fine

swift build --configuration release 
.build/release/Myapp # prints to console

but when I am running the executable through a docker container running on ECS (linux I suppose), I don't see the logs generated by my Swift code but I do see stderr being printed by 3rd party libraries (i.e. libssl is printing errors) as well as shell logs when starting the application

For example:

Dockerfile

FROM swift
WORKDIR /app

COPY Package.swift ./
COPY Sources ./Sources
COPY Tests ./Tests
RUN swift package clean
RUN swift build --configuration release
RUN chmod +x start.sh
CMD ["start.sh"] # just a wrapper to see if "echo" works

in start.sh

# prints as expected
echo "hi this will print"
# nothing in the executable will print though
.build/release/MyApp

score 4 · Accepted Answer · answered Jul 17 '20 at 14:04

Had the same issue, I filed a radar, and Apple answered:

When piped to another process print is buffered, so no characters appear until the buffer is filled up. (When piped to the terminal we only buffer until we hit a newline.)

You can get the behavior you want by calling setbuf(stdout, nil) once at startup:

 import Darwin
 setbuf(stdout, nil)

score 3 · Answer 2 · answered Jul 24 '19 at 12:09

I'm not sure why it isn't showing up in standard output, but you can do this:

import Foundation
internal struct FileHandleOutputStream: TextOutputStream {
    private let fileHandle: FileHandle
    let encoding: String.Encoding

    init(_ fileHandle: FileHandle, encoding: String.Encoding = .utf8) {
        self.fileHandle = fileHandle
        self.encoding = encoding
    }

    mutating func write(_ string: String) {
        if let data = string.data(using: encoding) {
            fileHandle.write(data)
        }
    }
}
internal var STDERR = FileHandleOutputStream(.standardError)
internal var STDOUT = FileHandleOutputStream(.standardOutput)

which allows you to do:

print("error message", to: &STDERR)

to print to standard error.

I know this is just a workaround, but it may be better than nothing.

Had a same issue without any Dockers. Simply running `swift run ...` printed all as expected, but with `swift run ... > out.txt 2> err.txt` everything stopped working. `FileHandleOutputStream` did the job — Tim, Feb 06 '20 at 15:35

score 0 · Answer 3 · answered Sep 11 '20 at 13:44

With this code, you gonna override all the prints so it will work on Linux and Mac Xcode. I used the response from @sam and added the override print

import Foundation

internal var STDERR = FileHandleOutputStream(.standardError)
internal var STDOUT = FileHandleOutputStream(.standardOutput)

internal struct FileHandleOutputStream: TextOutputStream {
    private let fileHandle: FileHandle
    let encoding: String.Encoding

    init(_ fileHandle: FileHandle, encoding: String.Encoding = .utf8) {
        self.fileHandle = fileHandle
        self.encoding = encoding
    }

    mutating func write(_ string: String) {
        if let data = string.data(using: encoding) {
            fileHandle.write(data)
        }
    }
}

///
/// Override of the print so it can work with Linux and MAC
/// https://stackoverflow.com/questions/39026752/swift-extending-functionality-of-print-function
public func print(_ items: Any..., separator: String = " ", terminator: String = "\n") {
    
    let output = items.map { "\($0)" }.joined(separator: separator)
    
    #if os(Linux)
        print(output, to: &STDERR)
    #else
        Swift.print(output, terminator: terminator)
    #endif
}

swift "print" doesn't appear in STDOut but 3rd party c library logs do when running in docker on ECS

3 Answers3

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