How should I fix valgrind's uninitialised value error?

Question

I have written a small application which works at some point with binary data. In unit tests, I compare this data with the expected one. When an error occurs, I want the test to display the hexadecimal output such as:

Failure
      Expected: string_to_hex(expected, 11)
      Which is: "01 43 02 01 00 65 6E 74 FA 3E 17"
To be equal to: string_to_hex(writeBuffer, 11)
      Which is: "01 43 02 01 00 00 00 00 98 37 DB"

In order to display that (and to compare binary data in the first place), I used the code from Stack Overflow, slightly modifying it for my needs:

std::string string_to_hex(const std::string& input, size_t len)
{
    static const char* const lut = "0123456789ABCDEF";

    std::string output;
    output.reserve(2 * len);
    for (size_t i = 0; i < len; ++i)
    {
        const unsigned char c = input[i];
        output.push_back(lut[c >> 4]);
        output.push_back(lut[c & 15]);
    }

    return output;
}

When checking for memory leaks with valgrind, I fould a lot of errors such as this one:

Use of uninitialised value of size 8
at 0x11E75A: string_to_hex(std::__cxx11::basic_string, std::allocator > const&, unsigned long)

I'm not sure to understand it. First, everything seems initialized, including, I'm mistaken, output. Moreover, there is no mention of size 8 in the code; the value of len varies from test to test, while valgrind reports the same size 8 every time.

How should I fix this error?

Are you sure that `len` is indeed `input` length? Why are you now obtaining string length directly or not checking that `len` is not greater than length? — user7860670, Apr 06 '19 at 21:07
@VTT: since I'm manipulating binary data, nothing prevents it from containing 0x00 inside, so there is no way for the app to determine the length *from* the string. Lengths are therefore determined independently of the content of the data, and yes, I'm sure that in the tests, `len` is the correct value every time. — Arseni Mourzenko, Apr 06 '19 at 21:24
Presence of terminating null has no effect on the length reported by the string object so it is trivial to determine the length. Anyway, you should probably provide mcve. I'm string to suspect that you pass a pointer to array of chars which gets implicitly converted to `std::string` cropping it to first `\0` so `len` does end up greater than actual string length. — user7860670, Apr 06 '19 at 21:27
@VTT: checked. This is exactly what happens. Replacing `input` type by `const char *` fixes the issue. Would you like to write the answer so I could upvote and accept it? — Arseni Mourzenko, Apr 06 '19 at 21:32

score 1 · Accepted Answer · answered Apr 06 '19 at 21:50

So this is one of the cases where passing a pointer to char that points to buffer filled with arbitrary binary data into evil implicit constructor of std::string class was causing string to be truncated to first \0. Straightforward approach would be to pass a raw pointer but a better solution is to start using array_view span or similar utility classes that will provide index validation at least in debug build for both input and lut.

How should I fix valgrind's uninitialised value error?

1 Answers1