i am trying to override predefined functions such as strcmp, getenv, etc. i already override some of them using LD_PRELOAD options. but i can't override string class, how can i override c++ string class

i already tried something like this;

class string{
    char* str;
    string (char* str2){
        str = str2;
        std::cout << str2 << std::endl ;
    bool operator==(char* str2)  {
        std::cout << "c++ => " << str << "==" << str2 << std::endl;
        return false; 
    compare(char* str2)  {
        std::cout << "c++ => " << str << "==" << str2 << std::endl;
        return 0;

but it doesn't work i can't print the string that compared or even defined.


g++ -g -Wall -shared -fPIC -ldl -o bypasscpp.so bypass.cpp

And running program with;

LD_PRELOAD="./bypasscpp.so" ./app
  • 3
    It seems you're trying to achieve something else and I feel this may not be the best way to achieve it. You may want to consider describing your end goal and look for alternative solutions.
    – NirIzr
    Jun 4, 2018 at 21:14
  • @NirIzr i already tried many different solutions this is my last solution. I need to override these classes. Jun 4, 2018 at 22:50
  • @Mustafa Adeniz Are these class methods virtual ?
    – w s
    Jun 5, 2018 at 6:07
  • 1
    @MustafaAkdeniz: seriously, Nirlzr is right. If you (also!) described what you want to achieve rather than prescribing a particular method, we would be in a better position to help. Just because this is your last straw doesn't mean there aren't better ways to achieve what you want. Why else would you be asking here?
    – 0xC0000022L
    Jun 5, 2018 at 8:49
  • 1
    @malikcjm can you please put it in the answer box for future visitors?
    – Igor Skochinsky
    Jun 7, 2018 at 7:46

1 Answer 1


Are you trying to override std::string? Please check https://en.cppreference.com/w/cpp/string/basic_string. std::string is just a typedef for std::basic_string<char> which means that std::string is not dynamically linked. In other words those methods you try to override are statically linked into you ./app.

  • std::string is a name for the char-specialization of the std::basic_string template class. And while it can be statically linked, it is also provided as a function by the libstdc++6.so as evidenced by running readelf --dyn-syms /usr/lib/x86_64-linux-gnu/libstdc++.so.6|awk '$4 ~ /^FUNC/ {print $8}'|cut -d @ -f 1|c++filt|grep 'std::basic_string<char' (tested on Ubuntu 16.04).
    – 0xC0000022L
    Jun 7, 2018 at 8:13

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