I'm porting some code to MacOS (13 in this case) and I'm having an issue with something that seems strange. Setting O_NONBLOCK on a pty fails with
Inappropriate ioctl for device (25)
I'm doing:
rv = fcntl(fd, F_GETFL, 0);
if (rv == -1)
return -1
rv |= O_NONBLOCK;
if (fcntl(fd, F_SETFL, rv) == -1)
return -1;
In fact, if I pass in 0 for "rv" to fcntl() it gives the same failure. Just for pty masters, not for anything else I've seen.
I need it to be non-blocking to avoid lockups.
I was able to figure this out. It turn out that posix_openpt() is broken on macos, the file descriptor it returns isn't useful for much. If you use openpty(), that file descriptor works ok. So it appears to be a bug in macos with posix_openpt().
So it appears to be a bug in macos with
posix_openpt.
I’m not a pseudo-terminal expert, so I’m not in a position to say whether or not Posix guarantees that posix_openpt will behave as you expect. However, standard practice is to open the client file descriptor before you start playing around with the server one. Consider this code:
import Foundation
func setNonBlocking(_ fd: CInt) -> CInt {
var flags = fcntl(fd, F_GETFL)
assert(flags >= 0)
flags |= O_NONBLOCK
let ok = fcntl(fd, F_SETFL, flags) >= 0
return ok ? 0 : errno
}
func main() {
let server = posix_openpt(O_RDWR | O_NOCTTY)
print("server before:", setNonBlocking(server))
var ok = grantpt(server) >= 0
assert(ok)
ok = unlockpt(server) >= 0
assert(ok)
let client = open(String(cString: ptsname(server)), O_RDWR | O_NOCTTY, 0)
assert(client >= 0)
print("server after: ", setNonBlocking(server))
}
main()
It prints:
server before: 25
server after: 0
So, once you have client set up, you can make server non-blocking. Indeed, this is exactly what openpty does under the covers, which you can see in the Darwin source.
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—
Quinn “The Eskimo!” @ Developer Technical Support @ Apple
let myEmail = "eskimo" + "1" + "@" + "apple.com"
Thanks a bunch for looking at this. It's the client open that's needed before anything will work, just the grantpt() and unlockpt() is not enough.
POSIX is kind of vague, but I would interpret it as saying that whatever is returned from posix_openpt() is a normal file descriptor that can be used for normal purposes. But it's on the margins. I would say it's a bug, but that's me :).
The way it works in MacOS is kind of unfortunate, really. There are reasons to create a pty that don't require the creating program to open the client. There are programs that create a pty so so that other programs that can only talk to serial ports can be used without a serial port, for instance.
But at least I (and the rest of the world) know now.
-corey