Brian Wainscott <[EMAIL PROTECTED]> writes:
> #include <stdio.h>
> main()
> {
> int i,j;
>
> for(i=j=0; i< 5; i++) {
> j=do_sub(j);
> }
>
> printf("Result = %d\n",j);
> }
> int do_sub(int in)
> {
> int out;
> //int junk=1; /* No errors */
> int junk; /* Errors */
>
> out = in+junk;
> return out;
> }
>
> has an obvious error ("junk" is not initialized). When I build this on my
> OpenSuSE 11.0 x86_64 system with the gcc 4.3.1 compiler, I either get no
> valgrind errors (if junk=1 is used), or I get this (when junk is not
> initialized):
>
> ==22349== Use of uninitialised value of size 8
> ==22349== at 0x4E6E423: (within /lib64/libc-2.8.so)
> ==22349== by 0x4E7166F: vfprintf (in /lib64/libc-2.8.so)
> ==22349== by 0x4E78FE9: printf (in /lib64/libc-2.8.so)
> ==22349== by 0x40055E: main (in /home/brian/tmp/tst)
>
> Now, this "error" is in the system libraries, and I don't KNOW that the
> error report is wrong. But I don't get ANY errors about the usage of junk,
> or anything else.
Errors are not reported until they are `visible'. The
uninitialized-ness of junk propagates to do_sub:out, and then main:j,
but it is only upon the observed access in the printf that the
uninitialized nature of main:j is visible.
See:
http://www.valgrind.org/docs/manual/mc-manual.html#mc-manual.value
HTH,
-tom
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