Redirecting stdout from another program in C++

Question

I'm writing a unit test and therefore, cannot change the code within the file that I'm testing. The code that I'm testing has messages in cout that I am trying to redirect into a file to check to make sure that the program is outputting the right messages. Does anyone have a way to redirect stdout in another program that won't cause a lag? I have tried freopen() and that causes my program to hang for some reason.

Consider [this](http://stackoverflow.com/questions/3661106/overlapped-readfileex-on-child-process-redirected-stdout-never-fires). Definitly not a dupe, but related. Might help. — John Dibling, Jun 07 '12 at 22:40
This question is answered [here](http://stackoverflow.com/questions/5257509/freopen-equivalent-for-c-streams). — jxh, Jun 07 '12 at 22:38
After redirecting, did you call `flush()` after the code under test had run its full course? — wallyk, Jun 07 '12 at 23:19

Jonathan Wakely · Accepted Answer · 2012-06-07T23:15:34.240

2

You could create a filebuf then replace cout's streambuf with it:

{
  std::filebuf f;
  f.open("output.txt", std::ios::out);
  std::streambuf* o = std::cout.rdbuf(&f);
  std::cout << "hello" << std::endl;  // endl will flush the stream
  std::cout.rdbuf(o);
}

You need to restore cout's original streambuf again (or set it to a null pointer) or it will probably crash when the global streams are flushed and destroyed, because the filebuf will already have gone out of scope.

edited Jun 07 '12 at 23:15

answered Jun 07 '12 at 22:40

Jonathan Wakely

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Is `.rdbuf(0)` intended to be `.rdbuf(o)`? – ildjarn Jun 07 '12 at 22:41
This has caused it to start to hang again somehow. Is there something you know of that cause it not to hang? – Annika Peterson Jun 07 '12 at 23:03
It works for me in a small test. Have you run it in a debugger to see where it hangs? – Jonathan Wakely Jun 07 '12 at 23:17
I think the reason it may be hanging is that it is running within another set of macros that I cannot change that also use stdout. Is that possible? – Annika Peterson Jun 07 '12 at 23:22
Apparently my standard-fu is failing me here -- where does the standard say that `cout.rdbuf(0)` is equivalent to what you have? – ildjarn Jun 07 '12 at 23:26
@ildjarn, I didn't say it's equivalent, I said it "works" i.e. it won't crash. Leaving `cout` referring to the `filebuf` after it goes out of scope is unsafe, restoring the original streambuf _or_ clearing `cout`'s streambuf avoids that problem. – Jonathan Wakely Jun 08 '12 at 00:02
Yes, but "works" is a meaningless word without context/guarantees... I understand the rationale for needing to unset the `filebuf`, but I'm unclear as to why `cout.rdbuf(0);` should work. – ildjarn Jun 08 '12 at 00:04
@Annika, anything else directly using `stdout` _should_ be unaffected, replacing the streambuf affects operations done through the `cout` object, not the `stdout` file descriptor. – Jonathan Wakely Jun 08 '12 at 00:05
@ildjarn, you want [ios::Init] i.e. 27.5.3.1.6 in C++11, which says that after the last `Init` is destroyed `cout.flush()` is called, which is UB if `cout.rdbuf()` is a dangling pointer, but according to [ostream.unformatted] 27.7.3.7 paragraph 7 is a no-op if `rdbuf()` is null. Happy? You owe me two standard references now ;) – Jonathan Wakely Jun 08 '12 at 00:18
I've paid my standard-reference dues on this site already, but I'll keep that in mind. ;-] Thanks for the answer. :-] – ildjarn Jun 08 '12 at 00:20
Turns out that .rdbuf(0) was part of the problem and only .rdbuf(o) worked for me. I also had another error that was causing the hanging. Thanks for your help! – Annika Peterson Jun 11 '12 at 17:56

jxh · Answer 2 · 2012-06-08T02:02:08.547

You can use 'open()' and 'dup2()'. You can use the helpers provided below. An example of how to use them:

void
code_to_test ()
{
    std::cout << "Here we go" << std::endl;
    std::cerr << "Danger danger" << std::endl;
}

run_test(code_to_test);

The run_test helper invokes redirection, and runs the test code.

template <typename TEST> void
run_test (TEST t, bool append = false) {
    flush_output();
    Redirect o(1, "/tmp/test_stdout", append);
    Redirect e(2, "/tmp/test_stderr", append);
    t();
    flush_output();
}

The flush_output helper flushes the streams.

void flush_output () {
    fflush(stdout);
    fflush(stderr);
    std::cout.flush();
    std::cerr.flush();
}

The Redirect class effects the redirection in the constructor. It restores the original descriptors back in the destructor.

class Redirect
{
    int m_what;
    int m_old_what;
public:
    Redirect (int what, std::string where, bool append = false)
        : m_what(what), m_old_what(dup(what)) {
        int flags = O_CREAT|O_WRONLY;
        if (append) flags |= O_APPEND;
        int f = open(where.c_str(), flags, 0660);
        dup2(f, m_what);
        close(f);
    }
    ~Redirect () {
        dup2(m_old_what, m_what);
        close(m_old_what);
    }
};

Redirecting stdout from another program in C++

2 Answers2

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