https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=289441
Bug ID: 289441
Summary: /dev/dsp O_NONBLOCK flag effective when set by
fctnl(), ineffective when set by open()
Product: Base System
Version: 14.3-RELEASE
Hardware: amd64
OS: Any
Status: New
Severity: Affects Some People
Priority: ---
Component: kern
Assignee: [email protected]
Reporter: [email protected]
Here, the O_NONBLOCK flag is set by the open() call, and the fcntl() with
F_GETFL proves it is set, but write() will still block:
---snip---
int audio_fd, ret;
audio_fd = open("/dev/dsp", O_WRONLY | O_NONBLOCK, 0);
ret = fcntl(audio_fd, F_GETFL, 0);
printf("flag = 0x%08x\n", ret);
ret = write(audio_fd, ..., ...);
---snip---
However, if O_NONBLOCK is set by fcntl() with F_SETFL, then write() becomes
non-blocking, correctly returning -1 with errno==EAGAIN when the buffer is
full, instead of blocking:
---snip---
int audio_fd, ret;
audio_fd = open("/dev/dsp", O_WRONLY, 0);
ret = fcntl(audio_fd, F_SETFL, O_NONBLOCK);
ret = fcntl(audio_fd, F_GETFL, 0);
printf("flag = 0x%08x\n", ret);
ret = write(audio_fd, ..., ...);
---snip---
The same value ("flag = 0x00000005") is printed out in both cases by printf(),
but it is only effective in actually making write() non-blocking when set by
fcntl() with F_SETFL, it does nothing when it's set by open().
Do any real applications pass O_NONBLOCK in open()? Yes, at least Wine does, in
file dlls/wineoss.drv/oss.c. This could be the cause of several sound bugs with
Wine on FreeBSD.
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