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Re: [Xen-devel] Build errors with latest xen-unstable from staging

Ian Campbell wrote:

On Sun, 2011-02-06 at 19:01 +0000, Kamala Narasimhan wrote:
FYI - Pulled the latest xen-unstable from staging to sync some patches and got
these trivial errors while compiling -

xl_cmdimpl.c: In function âprint_domain_vcpuinfoâ:
xl_cmdimpl.c:3351: warning: âfirstsetâ may be used uninitialized in this 
xl_cmdimpl.c:3351: note: âfirstsetâ was declared here
xl_cmdimpl.c:3350: warning: âbitmaskâ may be used uninitialized in this function
xl_cmdimpl.c:3350: note: âbitmaskâ was declared here
xl_cmdimpl.c:3350: warning: âpmapâ may be used uninitialized in this function
xl_cmdimpl.c:3350: note: âpmapâ was declared here

GCC version - 4.2.4.  Initializing the three variables it complained about fixed
the issue.

They are actually initialised before use, during the first pass through
the for loop when i==0 and state==0, but I can see how gcc would be
unable to figure that out (in fact I'm not sure about firstset myself).
I agree with Ian, looking through the code again I can confirm that they are _not_ used uninitialized. Even firstset is set when the state switches from 0 to 1 and is only used later in state 1 and 3, so it is safe. It seems like the compiler is not smart enough to recognize this, especially the modulo operation could be a barrier for the analysis, I think. According to the manpage those warning are only emitted on -O, which I left out for my testing, so I didn't spot this warning. Sorry.

To appease the compiler I would ack the fix, though from an academic point of view it is not necessary.

In the Linux kernel they have a macro to annotate such instances:
         * A trick to suppress uninitialized variable warning without 
generating any
         * code
        #define uninitialized_var(x) x = x
Do we want something similar?
I don't think so. x = x looks more like a hack, if we can easily initialize the variables, we should do so.


  If this trivial change should require a signed off line, here it is
- Signed-off-by: Kamala Narasimhan <kamala.narasimhan@xxxxxxxxxx>

Always just assume a change does.

diff -r 7ada6faef565 tools/libxl/xl_cmdimpl.c
--- a/tools/libxl/xl_cmdimpl.c  Sun Feb 06 17:26:31 2011 +0000
+++ b/tools/libxl/xl_cmdimpl.c  Sun Feb 06 13:53:50 2011 -0500
@@ -3347,8 +3347,8 @@ static void print_bitmap(uint8_t *map, i
 static void print_bitmap(uint8_t *map, int maplen, FILE *stream)
     int i;
-    uint8_t pmap, bitmask;
-    int firstset, state = 0;
+    uint8_t pmap = 0, bitmask = 0;
+    int firstset = 0, state = 0;

     for (i = 0; i < maplen; i++) {
         if (i % 8 == 0) {


Andre Przywara
AMD-Operating System Research Center (OSRC), Dresden, Germany

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