strdup with stringstreams causes valgrind errors

Question

I have the following problem: I am writing a C++ program that has to wrap around a C library, so when I interact with the library I always have to use char* instead of std::string for all operations. In order to avoid working with char* as much as possible, I do the formatting with stringstreams, for example like this:

#include <iostream>
#include <sstream>
#include <string.h>
#include <cstdlib>

using namespace std;

int main(int argc, char** argv)
{
  ostringstream str;

  str << argv[0] << "+" << "hello";

  const char *s = str.str().c_str();

  char *y = strdup(s);

  // this I would give to a library function

  cout << y << endl;

  free(y);

  return 0;
}

As far as the output goes, the program correctly outputs "./test+hello". However, valgrind gives me a lot of errors of the type

==30350== Invalid read of size 1
==30350==    at 0x402B858: __GI_strlen (in /usr/lib/valgrind/vgpreload_memcheck-x86-linux.so)
==30350==    by 0x4213475: strdup (in /usr/lib/libc-2.16.so)
==30350==    by 0x41B2604: (below main) (in /usr/lib/libc-2.16.so)
==30350==  Address 0x4341274 is 12 bytes inside a block of size 25 free'd
==30350==    at 0x4029F8C: operator delete(void*) (in /usr/lib/valgrind/vgpreload_memcheck-x86-linux.so)
==30350==    by 0x410387A: std::string::_Rep::_M_destroy(std::allocator<char> const&) (in /usr/lib/libstdc++.so.6.0.17)
==30350==    by 0x41B2604: (below main) (in /usr/lib/libc-2.16.so)

What am I doing wrong?

Answer 1

 const char *s = str.str().c_str();

The str() returns a string object. You get a pointer to some internal data of it using c_str, then at the end of the line the string object is deleted. But you still have a pointer to the deleted internal string.

You need to do it like this -

std::string s = str.str();
const char* s = s.c_str()

to ensure that the string isn't deleted.