https://bugs.llvm.org/show_bug.cgi?id=44675
Bug ID: 44675
Summary: False positive on std::data(std::string&)
Product: clang
Version: 7.0
Hardware: PC
OS: Linux
Status: NEW
Severity: enhancement
Priority: P
Component: Static Analyzer
Assignee: dcough...@apple.com
Reporter: cassio.n...@gmail.com
CC: dcough...@apple.com, llvm-bugs@lists.llvm.org
The below presents 3 versions of a function named copy which attempts to get a
pointer to the first element of a string.
VERSION == 1: uses std::data(std::string&) and gives a false positive on "Use
of memory after it is freed"
VERSION == 2: uses str::string::data() and no warning is issued.
VERSION == 3: uses an inline copy-and-pasted implementation of libstdc++'s
std::data. Again, no warning is issued.
<FILE name="main.cpp">
#include <string>
#include <cstring>
#include <iterator>
namespace foo {
// Copy-and-pasted from libstdc++
template <typename _Container>
constexpr auto
data(_Container& __cont) noexcept(noexcept(__cont.data()))
-> decltype(__cont.data())
{ return __cont.data(); }
}
#define VERSION 1
void copy(std::string& tgt, char* src, unsigned n) {
tgt.resize(n);
#if VERSION == 1
std::memcpy(std::data(tgt), src, n); // scan-build:
warning: Use of memory after it is freed
#elif VERSION == 2
std::memcpy(tgt.data(), src, n); // scan-build: No
bugs found.
#else
std::memcpy(foo::data(tgt), src, n); // scan-build: No
bugs found.
#endif
}
void test(char* src, unsigned n) {
std::string tgt;
copy(tgt, src, n);
}
</FILE>
Analysed with:
$ scan-build clang++ -g -std=c++17 -c main.cpp -o main.o
FWIW:
a) Obviously, VERSION 2 is a good workaround. However, in my real code (not in
this self-contained, reduced test case) the function is a template on T and T
== std::string is just one possible instantiation. (Among others like
std::vector<char>, char[N], ... for which no warning is issued) Therefore,
fixing the false positive would be helpful to generic code.
b) In C++17 std::string::data returns char* in contrast to const char* as in
previous standards.
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