Redirecting stderr to stdout using string stream

Question

I have a code like this

int main()
{
    std::stringstream oss;
    std::cerr.rdbuf( oss.rdbuf() );

    std::cerr << "this goes to cerr";
    std::cout << "[" << oss.str() << "]";
}

But i get the output of the program as

[this goes to cerr]Segmentation fault

How does the program segfault?

@Prasanth: Please try to post a minimal, compilable and executable example that reproduces the error. Otherwise, we can only guess. — Björn Pollex, May 24 '11 at 09:29
yes.. gdb gives this.. (gdb) run [here's some text] Program received signal EXC_BAD_ACCESS, Could not access memory. Reason: 13 at address: 0x0000000000000000 0x00007fff80e8c5f8 in std::ostream::flush () — Prasanth Madhavan, May 24 '11 at 09:32
possible duplicate of [Is it possible to "prepare" input from cin?](http://stackoverflow.com/questions/6013638/is-it-possible-to-prepare-input-from-cin) — Nawaz, May 24 '11 at 09:38

score 12 · Accepted Answer · edited May 23 '17 at 12:07

12

This is because you do not restore the buffer of cerr before your program exits. Do it like this:

#include <iostream>
#include <sstream>

int main()
{
  std::stringstream oss;
  std::streambuf* old = std::cerr.rdbuf( oss.rdbuf() );

  std::cerr << "this goes to cerr";
  std::cout << "[" << oss.str() << "]";
  std::cerr.rdbuf(old);
}

See this answer of mine for a solution that is exception safe.

edited May 23 '17 at 12:07

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answered May 24 '11 at 09:33

Björn Pollex

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this works.. thankyou.. although this is just for redirection of cerr to stdout.. it doesnt work for redirecting stderr.. why is that so? – Prasanth Madhavan May 24 '11 at 09:40
@Prasanth: It works for redirecting any `iostream` to any other `iostream`, but it requires some minor modifications. – Björn Pollex May 24 '11 at 09:44

sehe · Answer 2 · 2011-05-24T10:19:51.937

The other answer correctly address the how does this program segfault part of your question. However, I feel that the real question Redirecting stderr to stdout using string stream.. deserves a better answer:

You may simplify the whole shebang and make it scale and perform a infitely better better by just aliasing cerr to cout:

#include <iostream>

int main()
{
    std::cerr.rdbuf(std::cout.rdbuf());
    std::cerr << "this goes to cerr";
}

If you really want to be explicit:

    std::cerr.copyfmt(std::cout);
    std::cerr.clear(std::cout.rdstate());
    std::cerr.rdbuf(std::cout.rdbuf());

You can verify that the text is actually received on stdout when run

Redirecting stderr to stdout using string stream

2 Answers2