Custom application console and stderr

Question

I'm writing a class to display a custom console from inside the application. I'm also using glog to log messages to a file and at the same time to stderr. How can I have my console class listen to stderr?

I thought of making a custom fstream and the doing something like:

CustomStream cs;
auto original_buf = std::cerr.rdbuf(cs.rdbuf());

and having calls to the stream operator << sent to the console class.

Or subclassing directly from std::filebuf and calling:

CustomFilebuf fb;
auto original_buf = std::cerr.rdbuf(&fb);

Is this the right way to do it? I searched for some sample code but couldn't find very much.

Edit1: I tried using stream tee, but glog logs with stderr and not std::cerr, so I wasn't able to grab any data.

Answer 1

I am uncertain if this is relevant to your question, but...

ISO C99 says in 7.19.5.3, paragraph 6:

When a file is opened with update mode ('+' as the second or third character in the above list of mode argument values), both input and output may be performed on the associated stream. However, output shall not be directly followed by input without an intervening call to the fflush function [...], and input shall not be directly followed by output without an intervening call to a file positioning function, unless the input operation encounters end-of-file.

Also some say reading from stderr is "undefined behavior"...

Though you can read from stderr, as long as you flush before reading:

fwrite("x", 1, 1, stderr);
fflush(stderr);
fgetc(stderr);

Also take a look at How do I read stdout/stderr output of a child process correctly?

---

For anyone who would like to redirect stdout to a console Window in a Win32 app there is AllocConsole.

I even created a simple (trivial) function to redirect stdout to the console Window:

#include <fstream>
#include <io.h>
#include <fcntl.h>

#define cMaxConsoleLines 500

void ReadyConsole() {

    short int hConHandle;
    long lStdHandle;

    CONSOLE_SCREEN_BUFFER_INFO coninfo;

    FILE *fp;

    // Allocate a console for the program
    AllocConsole();

    // set the screen buffer to be big enough to let us scroll text
    GetConsoleScreenBufferInfo(GetStdHandle(STD_OUTPUT_HANDLE), &coninfo);

    coninfo.dwSize.Y = cMaxConsoleLines; // The max number of lines for the console!

    SetConsoleScreenBufferSize(GetStdHandle(STD_OUTPUT_HANDLE), coninfo.dwSize);


    // Redirect STDOUT to the console
    lStdHandle = (long)GetStdHandle(STD_OUTPUT_HANDLE);
    hConHandle = _open_osfhandle(lStdHandle, _O_TEXT);

    fp = _fdopen(hConHandle, "w");  // Writing to the console

    *stdout = *fp;

    setvbuf(stdout, NULL, _IONBF, 0);
    // -------------------------------


    // Redirect STDIN to the console
    lStdHandle = (long)GetStdHandle(STD_INPUT_HANDLE);
    hConHandle = _open_osfhandle(lStdHandle, _O_TEXT);

    fp = _fdopen(hConHandle, "r");  // Reading from the console

    *stdin = *fp;

    setvbuf(stdin, NULL, _IONBF, 0);
    // ------------------------------


    // Redirect STDERR to the console
    lStdHandle = (long)GetStdHandle(STD_ERROR_HANDLE);
    hConHandle = _open_osfhandle(lStdHandle, _O_TEXT);

    fp = _fdopen(hConHandle, "w");  // STDERR writing to the console!

    *stderr = *fp;

    setvbuf(stderr, NULL, _IONBF, 0);
    // ------------------------------


    // Point the console to STDIO
    std::ios::sync_with_stdio();

}

If you would like the console to be for Debug only, make sure to include <crtdbg.h>, which defines whether the app is in debug mode (for VC++), then, for example you could add:

#ifdef _DEBUG
// The file with the ReadyConsole function
#include <DebugStuff.h>
#endif

and to use it

#ifdef _DEBUG
ReadyConsole(); // Ready the console for debugging
#endif

#ifdef _DEBUG
fprintf(stdout, "Position, Line 1, DEBUG-INFO-HERE");
cout << "COUT is working!"; // NOTE, for cout, you will need <iostream>
#endif

Here is an additional little function (it logs the message to both stderr and stderr.log file)

void StdErr(char* Error) {

    fprintf(stderr, Error);
    FILE* FP = fopen("stderr.log", "a");
    fputs(Error, FP);
    fclose(FP);

}