[LLVMbugs] [Bug 12801] New: possibly spurious warning about garbage value

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=12801

             Bug #: 12801
           Summary: possibly spurious warning about garbage value
           Product: clang
           Version: trunk
          Platform: Macintosh
        OS/Version: MacOS X
            Status: NEW
          Severity: normal
          Priority: P
         Component: Static Analyzer
        AssignedTo: kremenek at apple.com
        ReportedBy: tjw at me.com
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified


Created attachment 8537
  --> http://llvm.org/bugs/attachment.cgi?id=8537
example

clang-sa warns about garbage values even if they are never used. This is
causing new spurious warnings in a third party library that we use, which had
previously built w/o these warnings.

In the attached example, the output argument pX of read_byte() is not written
to unless the fread succeeds. It then returns READ_OK and the caller checks for
this, only using (for some value of "using") the value of its local in this
case. The eventual output of read_short() won't be changed by this value being
garbage, even though some operations are done on it (the addition of the second
read byte).

I guess this might not be the case if overflow traps are on, but I'm not sure
if clang or the C spec says anything about this. Maybe this is mandated by the
spec.

For now, we'll rewrite the library code to be less "clever" anyway =)

clang version 3.2 (trunk 156620)
Target: x86_64-apple-darwin11.3.0
Thread model: posix

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