[swift-dev] print buffering

Greg Parker gparker at apple.com
Fri Apr 1 17:56:32 CDT 2016

> On Apr 1, 2016, at 2:52 PM, Drew Crawford via swift-dev <swift-dev at swift.org> wrote:
> We noticed today when running a swift CLI program to a pipe that stdout is fully buffered (e.g. not unbuffered, not line-buffered).  So just now we committed unbuffering IO to a bunch of CLI programs.
> Is this the right default behavior for Swift?
> I realize this is a Cism with a long history, but I think an argument can be made that the Cism is surprising (in that: it surprised us). I think most Swift developers would assume "print" to be line-buffered.
> Requiring programs to opt-in to linebuffering so they can work via pipe feels wrong to me.  Am I crazy?

We coincidentally got a Radar about this a few days ago.

    func testExample() {
        print("foo")  // doesn't appear before the crash

Making print() less than fully buffered (either unconditionally or conditional on stdout being sufficiently terminal-like) ought to be reasonable. I mirrored the bug report to https://bugs.swift.org/browse/SR-1127 .

Greg Parker     gparker at apple.com     Runtime Wrangler

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