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Invalid free while performing a std::string assign with -O2 set in g++

Answer 1

A:

You should try to reproduce the crash with a self-contained example. Using the information you gave us, I tested the following code.

Note that I needed a split function. Not knowing what you used, I took one from this question. If you use boost::split or a split function you wrote yourself, please include it in your question.

#include <iostream>
#include <sstream>
#include <string>
#include <vector>

std::vector<std::string> &split(const std::string &s, char delim, std::vector<std::string> &elems) {
    std::stringstream ss(s);
    std::string item;
    while(std::getline(ss, item, delim)) {
        elems.push_back(item);
    }
    return elems;
}

std::vector<std::string> split(const std::string &s, char delim) {
    std::vector<std::string> elems;
    return split(s, delim, elems);
}

bool GetStreamName(const char* pRawPath, std::string& pStreamName)
{
    std::vector<std::string> parts = split(pRawPath,'/'); // line 260
    pStreamName = parts.back(); // line 261

    parts = split(pStreamName.c_str(),':');
    pStreamName = parts.back();
    return true;
}

int main()
{
    std::string streamName;
    GetStreamName("/some:weird/path/of:x/a:/stream:foo", streamName);
    std::cout << streamName << std::endl;
}

Try to run this code (with -O2, with any g++ optimizations you can think of, with valgrind, etc.) and see if it crashes again.

Using a simple, self-contained example, you can determine if the source of the crashes is an error in your code or anything else (such as conflicting C++ libraries, as it was in your previous question).

Danilo Piazzalunga 2010-02-23 13:18:51

I tried this with my version of the split function and I did not get a crash nor did I get a warning in valgrind and so the code does not seem to be the problem directly. I think it could be a linking or .so issue, but I'm really not sure?

bbazso 2010-02-23 15:48:14

Try running your code again in valgrind. If valgrind warns you about invalid memory access, you have found your bug.However, if you still have the problem described in http://stackoverflow.com/questions/2303740/c-program-always-crashes-while-doing-a-stdstring-assign, most certainly the problem is in your environment and not in your code.

Danilo Piazzalunga 2010-02-23 17:55:49

If you can, please try compiling and running your crashing code on another system (possibly using the default compiler installed with the distro) and see if you still get the crash.

Danilo Piazzalunga 2010-02-23 17:59:45

I've tried it on Linux RH 4.2, 4.4 and Ubuntu 9.04 and I still get the problem, but the packages and libs are very similar and not all of them are the versions that come with the distro and so there could be a problem there, but I have no idea how to converge on something!

bbazso 2010-02-23 18:19:49

Did you use the distribution's g++ compiler at least in one case? Ubuntu 9.04 includes gcc 4.3.3, RHEL 4.2 has gcc 4.0.1 and RHEL 4.4 uses gcc 4.1.0.Did you perform these tests on the same machines or on different machines?

Danilo Piazzalunga 2010-02-23 22:52:37

Answer 2

+2 A:

If it's not guarantied that split doesn't return empty vector I'd add a check if it's empty. Otherwise assigning back() of empty vector to a string could lead to the behavior that you receive.

bool GetStreamName(const char* pRawPath, std::string& pStreamName)
{
    std::vector<std::string> parts = split(pRawPath,'/'); // line 260
    if( parts.size() == 0 )
    {
        cout << "Cannot parse '" << pRawPaths << "'" << endl;
        return false;
    }
    pStreamName = parts.back(); // line 261

    parts = split(pStreamName.c_str(),':');
    pStreamName = parts.back();
    return true;
}

Dmitry Yudakov 2010-02-23 13:32:53

When I step through the code the split function returns a vector that is not empty and contains the correct string and so this does not seem to be the problem, but +1 since this is a good check to have in the code!

bbazso 2010-02-23 15:03:19

Good catch! IIRC, calling `back()` on an empty vector results in undefined behavior. +1

Danilo Piazzalunga 2010-02-23 17:51:25

I changed the code to add in a check like you mentioned and I was never returning an empty vector and so this was not the problem. However, I tested the case when the vector is empty and the program crashes in both -O0 and -O2. Good find! The weird thing is that the program does not crash anymore when the string passed into this function is initialized as per: std::string lStreamName = ""; ??? I have no idea why? Any ideas ?

bbazso 2010-02-23 19:39:14

To be honest I missed the part with only -O2 build crash on the first reading. Could you try instead of the string reference to return the string? I know your way is valid, but I wonder if the compiler could optimize it in some weird way.In gdb you could also use 'step' instead of 'c'(continue) and to see step by step what leads to the crash.

Dmitry Yudakov 2010-02-23 21:50:12

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Invalid free while performing a std::string assign with -O2 set in g++

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