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<base href="http://llvm.org/bugs/" />
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - scan-build memory leak false positive"
href="http://llvm.org/bugs/show_bug.cgi?id=16113">16113</a>
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<th>Summary</th>
<td>scan-build memory leak false positive
</td>
</tr>
<tr>
<th>Product</th>
<td>clang
</td>
</tr>
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<th>Version</th>
<td>3.2
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
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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>
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<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>Consider the following C program (key locations marked by comments):
#include <stdlib.h>
#include <inttypes.h>
size_t do_stuff(void **out, unsigned m)
{
size_t n;
if (m >= SIZE_MAX/4) /* (A) */
return -1;
n = (size_t)m * 4; /* (X) */
*out = malloc(n); /* (B) */
if (!*out) /* (C) */
return -1;
return n; /* (Y) */
}
int main(int argc, char **argv)
{
void *buf;
size_t n;
n = do_stuff(&buf, argc);
if (n == (size_t)-1) /* (D) */
return EXIT_FAILURE; /* (E) */
free(buf);
return EXIT_SUCCESS;
}
The report produced by scan-build suggests that a memory leak is possible by
the following path:
1: false branch at (A)
2: allocate memory at (B)
3: false branch at (C)
4: true branch at (D)
5: memory not freed at (E)
But this is clearly a false positive: taking the false branch at (A) means
that m is less than SIZE_MAX/4, so the result of the multiplication at (X)
must be less than 4*(SIZE_MAX/4). Thus, n must be less than 4*(SIZE_MAX/4),
which implies that n must be less than SIZE_MAX. (size_t)-1 is equal to the
maximum representable value of size_t as per C's rules of signed-to-unsigned
conversions, i.e., (size_t)-1 is equal to SIZE_MAX. Hence, we can conclude
that n is not ever equal to (size_t)-1 at /* (Y) */.
The first 3 steps of this path imply that we got to the return at (Y) in
do_stuff, which we have established returns a value not equal to (size_t)-1.
It is therefore impossible to take the true branch at (D) and thus the
conclusion of the analyzer is incorrect.</pre>
</div>
</p>
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