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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - scan-build null dereference false positive"
href="http://llvm.org/bugs/show_bug.cgi?id=16129">16129</a>
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<th>Summary</th>
<td>scan-build null dereference false positive
</td>
</tr>
<tr>
<th>Product</th>
<td>clang
</td>
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<th>Version</th>
<td>3.2
</td>
</tr>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>Linux
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
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<th>Severity</th>
<td>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
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<th>Component</th>
<td>Static Analyzer
</td>
</tr>
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<th>Assignee</th>
<td>kremenek@apple.com
</td>
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<th>Reporter</th>
<td>nbowler@draconx.ca
</td>
</tr>
<tr>
<th>CC</th>
<td>llvmbugs@cs.uiuc.edu
</td>
</tr>
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<th>Classification</th>
<td>Unclassified
</td>
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<p>
<div>
<pre>Created <span class=""><a href="attachment.cgi?id=10575" name="attach_10575" title="Annotated source file.">attachment 10575</a> <a href="attachment.cgi?id=10575&action=edit" title="Annotated source file.">[details]</a></span>
Annotated source file.
Consider the following C program (key locations marked in comments):
#include <stdio.h>
#include <stdlib.h>
int main(int argc, char **argv)
{
int *x = NULL, n = argc;
if (!x && !argv[n]) /* (A) */
return EXIT_FAILURE;
if (argv[n]) /* (B) */
;
else
printf("%d\n", *x); /* (C) */
return EXIT_SUCCESS;
}
The scan-build report suggests that a NULL pointer dereference is possible by
the following path:
1: false branch at (A)
2: false branch at (B)
3: NULL dereference at (C).
This is clearly a false positive. By taking the false branch at A, that
implies that at least one of x and argv[n] is non-zero (ignore the fact that
in this test case both values are always zero). By taking the false branch at
B, this implies that argv[n] is zero. So given the first 2 decisions, we can
conclude that x must be non-zero at (C), and thus there is no NULL
dereference.</pre>
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