Question

It is well known that there are three default I/O streams which are mapped to predefined objects in the standard library:

• 0: std::istream std::cin
• 1: std::ostream std::cout
• 2: std::ostream std::cerr and std::ostream std::clog

However, from within (e.g.) a bash script you can create additional streams (3,4,…).

So, can you create an extra output stream with the descriptor 3 and bind it to an std::ostream custom object? If so, how? std::ofstream doesn’t do the trick as it would create a file with the name “3” which is not what I want.

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Edit: It doesn’t have to be portable. It suffices if it works on POSIX.

Answer 1

It is not possible if you need your program to be portable. The C++ 11 Standard does not specify a unified way of doing that.

However, you can define your own output stream buffer which overrides the overflow() and xsputn() virtual functions and writes each character or sequence of characters to the stream with the specified descriptor using system-specific API.

Something along these lines:

class my_ostream_buf : public std::streambuf
{

public:

    my_ostream_buf(int fd) : _fd(fd) { }

protected:

    virtual int_type overflow (int_type c)
    {
        if (c != EOF)
        {
            char ch = c;
            if (write(_fd, &ch, 1) != 1)
            {
                return EOF;
            }
        }

        return c;
    }

    // This is not strictly necessary, but performance is better if you
    // write a sequence of characters all at once rather than writing
    // each individual character through a separate system call.
    virtual std::streamsize xsputn(const char* s, std::streamsize num)
    {
        return write(_fd, s, num);
    }

private:

    int _fd = 0;    

};

And this is how you would use it:

using namespace std;

int main()
{
    int fd = ...; // Any file descriptor
    my_ostream_buf buf(fd);

    ostream os(&buf); // Take care of the lifetime of `buf` here, or create your
                      // own class that derives from ostream and encapsulates an
                      // object of type my_ostream_buf

    os << "Hello" << endl;
}