Linux分段错误与std :: string :: iterator

Question

I keep getting unusual segmentation faults inside libc.so.6 on a CentOS 6.4 64bit machine. This is the backtrace that gdb most often reports:

0x00007ffff60d9b3f in memcpy () from /lib64/libc.so.6
(gdb) backtrace
#0  0x00007ffff60d9b3f in memcpy () from /lib64/libc.so.6
#1  0x00000000004b6a6b in std::string::_S_construct<__gnu_cxx::__normal_iterator<char*, std::string> > ()
#2  0x00000000004b719b in NewsMAIL::SMTPClient::receiveLine(std::basic_string<char, std::char_traits<char>, std::allocator<char> >*) ()
#3  0x00000000004b776f in NewsMAIL::SMTPClient::handleResponse() ()

And this is the code in question that seems to trigger the segfault:

bool SMTPClient::receiveLine(std::string* Line)
{
    static std::string Buffer;
    std::string::iterator iter;

    while((iter = std::find(Buffer.begin(), Buffer.end(), '\n')) == Buffer.end()) {

        char Bucket[MAX_BUCKET_SIZE + 1] = {};
        int BytesRecv = read(m_Socket, Bucket, MAX_BUCKET_SIZE);

        //Did we get a socket error?
        if(BytesRecv == -1) {

            //This is generally considered a bad thing..
            *Line = Buffer;
            Buffer = std::string("");
            return false;
        }

        Bucket[BytesRecv] = 0;
        Buffer += Bucket;
    }

    *Line = std::string(Buffer.begin(), iter);
    Buffer = std::string(iter + 1, Buffer.end());

    return true;
}

Sometimes it works 100% without any failures so it is not everytime unfortunately. The above code is a slightly modified version of this: https://stackoverflow.com/a/1584620/3133245

Does anyone have any thoughts on why this might be happening? I am compiling with g++ 4.7.2

Thanks!

Nate

Answer 1

Using a static variable (Buffer) is not thread safe. Could cause a crash.

You should add a check that Line is not NULL .

BTW, the line Buffer = std::string(""); could be Buffer.clear();

Answer 2

In addition to the static variable issue, are you sure that the data that is received contains no embedded NULL characters?

If the resulting Buffer contains embedded NULL bytes, this line will not do the correct concatenation using the += operator:

Buffer += Bucket;

The += overload assumes that Bucket is a c-style string, thus the first NULL byte encountered will be used as the terminator when the concatenation occurs.

Taking a glance at the code, it would seem to be the case that if the Bucket does indeed contain embedded NULL chracters, doing the above concatenation could result in your "iter" iterator pointing passed the end() of Buffer (in those lines after the while() loop).

Instead, you can do this:

Buffer.append(Bucket, BytesRecv)

This guarantees that all characters that Bucket is addressing will be concatenated onto the existing string.

But before making any changes, make sure you know exactly what the issue is, especially since you stated the error doesn't happen very often. Changing around code without first knowing the true cause of the error may just mask the error, thus making it harder to diagnose the real issue.