memory sanitizer gives spurious error?

[nathansalwen]

Hi,

It is possible that I misunderstand C++ order of execution but I receive an incomprehensible (to me) uninitialized data error with following code. I am using clang 4.0 on Ubuntu 17.04

$ cat foo.cpp
#include
#include

const char * foo[] =
{
"AAA",
"BBB"
};

const int numfoo = sizeof(foo)/sizeof(const char *);

typedef std::map<int, int> other_map;
class bar : other_map
{
public:
bar(const char * b[], int numb)
{
for (int i = 0; i < numb; i++)
{
std::cout << b[i] << std::endl;
}
}
};

bar c(foo, numfoo);

int main(int argc, char** argv) {
return 0;
}
$ clang -g -fsanitize=memory foo.cpp -lstdc++
$ ./a.out
Uninitialized bytes in __interceptor_memcmp at offset 236 inside [0x7ffc47b0e0c0, 256)
==19320==WARNING: MemorySanitizer: use-of-uninitialized-value
#0 0x7fe423328e75 in std::ctype::_M_widen_init() const (/usr/lib/x86_64-linux-gnu/libstdc++.so.6+0xb6e75)
#1 0x7fe423384b87 in std::basic_ostream<char, std::char_traits >& std::endl<char, std::char_traits >(std::basic_ostream<char, std::char_traits >&) (/usr/lib/x86_64-linux-gnu/libstdc++.so.6+0x112b87)
#2 0x490b41 in bar::bar(char const**, int) /home/nks/foo.cpp:20:31
#3 0x41a624 in __cxx_global_var_init.2 /home/nks/foo.cpp:25:5
#4 0x41a67d in _GLOBAL__sub_I_foo.cpp /home/nks/foo.cpp
#5 0x49209c in __libc_csu_init (/home/nks/a.out+0x49209c)
#6 0x7fe42238137f in __libc_start_main /build/glibc-cxyGtm/glibc-2.24/csu/../csu/libc-start.c:247
#7 0x41a6a9 in _start (/home/nks/a.out+0x41a6a9)

SUMMARY: MemorySanitizer: use-of-uninitialized-value (/usr/lib/x86_64-linux-gnu/libstdc++.so.6+0xb6e75) in std::ctype::_M_widen_init() const
Exiting
$ clang --version
clang version 4.0.0-1ubuntu1 (tags/RELEASE_400/rc1)
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin

[chefmax]

To use MSan with C++ you need to use MSanitized libc++ (see https://github.com/google/sanitizers/wiki/MemorySanitizerLibcxxHowTo for details).

[nathansalwen]

Thank you! I see I missed it in the Wiki.