CODEWITHSUNDEEP
c++memory-managementmemory-leaks
Arseni Mourzenko
Arseni Mourzenko

Reputation: 52411

How should I fix valgrind's uninitialised value error?

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?

Upvotes: 0

Views: 180

Answers (1)

user7860670
user7860670

Reputation: 37600

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.

Upvotes: 1

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