On Fri, Apr 26, 2024 at 11:20:33AM -0300, Fabiano Rosas wrote:
> If the user is not passing in a file name which QEMU can open at will,
> we must then require that the user pass the two file descriptors with
> the flags already properly set. We'll use the already existing fdset +
> QMP add-fd infrastructure for this.
Yes I remember such requirement that one extra fd is needed for direct-io,
however today when I looked closer at the man page it looks like F_SETFL
works with O_DIRECT too?
F_SETFL (int)
Set the file status flags to the value specified by arg.
File access mode (O_RDONLY, O_WRONLY, O_RDWR) and file
creation flags (i.e., O_CREAT, O_EXCL, O_NOCTTY, O_TRUNC) in
arg are ignored. On Linux, this command can change only the
O_APPEND, O_ASYNC, O_DIRECT, O_NOATIME, and O_NONBLOCK flags.
It is not possible to change the O_DSYNC and O_SYNC flags;
see BUGS, below.
====8<====
$ cat fcntl.c
#define _GNU_SOURCE
#include <stdio.h>
#include <fcntl.h>
#include <assert.h>
#include <unistd.h>
int main(void)
{
int fd, newfd, ret, flags;
fd = open("test.txt", O_RDWR | O_CREAT, 0660);
assert(fd != -1);
flags = fcntl(fd, F_GETFL);
printf("old fd flags: 0x%x\n", flags);
newfd = dup(fd);
assert(newfd != -1);
flags = fcntl(newfd, F_GETFL);
printf("new fd flags: 0x%x\n", flags);
flags |= O_DIRECT;
ret = fcntl(newfd, F_SETFL, flags);
flags = fcntl(fd, F_GETFL);
printf("updated new flags: 0x%x\n", flags);
return 0;
}
$ make fcntl
cc fcntl.c -o fcntl
$ ./fcntl
old fd flags: 0x8002
new fd flags: 0x8002
updated new flags: 0xc002
====8<====
Perhaps I missed something important?
--
Peter Xu