Dear Peff and Dscho On 27/06/2019 22:10, Jeff King wrote: > On Thu, Jun 27, 2019 at 02:00:54PM +0200, Johannes Schindelin wrote: > >>> We can use setvbuf() to toggle buffering back and forth, but I'm not >>> sure if there's a way to query the current buffering scheme for a stdio >>> stream. >> >> It also is not very safe, especially when we want to have this work in a >> multi-threaded fashion. > > I considered that, too, but I think it is safe. stdio has its own > locking, so every individual call is atomic. The potentially problematic > case would be where we switch back from line buffering to no-buffering, > and somebody else has written some content into our stack-based buffer > (that is about to go out of scope!). But I'd assume that as part of the > switch to no-buffering that any stdio implementation would flush out the > buffer that it's detaching from (while under lock). Nothing else makes > sense. The C standard section 7.19.5.6 says that The setvbuf function may be used only after the stream pointed to by a stream has been associated with an open file and before any other operation (other than an unsuccessful call to setvbuf) is performed on the stream. The is a note about the buffer that says The buffer has to have a lifetime at least as great as the open stream, so the stream should be closed before a buffer that has automatic storage duration is deallocated upon block exit. So changing the buffer in the way that has been proposed is undefined behavior on two counts I think. Best Wishes Phillip > That said... > >> I'd be much more comfortable with rendering the string into a buffer and >> then sending that buffer wholesale to stderr. > > It's sufficiently complex that I think I prefer to just use our own > buffer, too. > > It also makes it more likely for the newline and the message to end up > in an atomic write(), so if multiple threads _are_ writing to stderr > they'd be more likely to stay together. > > It does sound like people in the other part of the thread are OK with > just getting rid of the "_ln" functions altogether. > > -Peff >
