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Re: [libvirt] [PATCH V6 3/3] Add support for file descriptor sets



On 02/15/2013 10:01 PM, Eric Blake wrote:
On 02/14/2013 05:00 AM, Stefan Berger wrote:
+static char *
+qemuCommandPrintFDSet(int fdset, int fd, int open_flags, const char *name)
+{
+    const char *mode = "";
+    virBuffer buf = VIR_BUFFER_INITIALIZER;
+
+    if (name) {
+        switch ((open_flags & O_ACCMODE)) {
+        case O_RDONLY:
+            mode = "RDONLY:";
+            break;
+        case O_WRONLY:
+            mode = "WRONLY:";
+            break;
+        case O_RDWR:
+            mode = "RDWR:";
+            break;
Is it worth a default case when the mode is unrecognized?  Then again,
unless the Linux kernel ever gains O_SEARCH/O_EXEC support, there
probably won't ever be any code hitting the default case.

Then we can leave it as-is I suppose. We can always treat other flags separately in this case statement if needed in the future.


+        }
+    }
+
+    virBufferAsprintf(&buf, "set=%d,fd=%d%s%s", fdset, fd,
+                      (name) ? ",opaque=" : "",
+                      mode);
+    if (name)
+        virBufferEscape(&buf, ',', ",", "%s", name);
Slightly easier to read as:

virBufferAsprintf(&buf, "set=%d,fd=%d", fdset, fd);
if (name)
     virBufferEscape(&buf, ',', ",", ",opaque=%s", name);

Something like this, yes. 'mode' still needs to be printed in the if(name) part but cannot be part of virBufferEscape.

Rather than having the user supply a sentinel, would it be better to
have the user provide nopenFlags?  That is, when opening a single fd,
passing '&mode, 1' is easier than passing 'int[] { mode, -1}',
especially if we don't want to use C99 initializer syntax.  For that
matter, would it be any easier to pass a flags instead of a mode, where
we have the bitwise-or of:

QEMU_USE_RDONLY = 1 << 0, // O_RDONLY
QEMU_USE_RDWR   = 1 << 1, // O_RDWR
QEMU_USE WRONLY = 1 << 2, // O_WRONLY

on the grounds that writing 'QEMU_USE_RDONLY | QEMU_USE_RDWR' looks a
little cleaner than writing '(int[]){O_RDONLY, O_RDWR, -1}' (no
temporary arrays required).

For that we would need additional code everywhere where we need to convert these QEMU_USE_* to the POSIX flags by bitwise sampling the flags in a loop,which is practically everywhere where the POSIX flags are understood today, e.g., qemuOpenFile(). I am not sure it will make things easier.

   Stefan


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