Is "std::cout" usable in Android-ndk

Question

In Android-ndk, we could use "__android_log_write", "__android_log_print", ... etc to output messages to the "LogCat" window. How about if I use "std::cout" to output some strings ? E.g.

std::cout << "some strings" << std::endl;

Where would the strings be sent.

It seems that Android does not have Console Applications and the above strings may not be sent. Could I redirect the "stdout" to a file so that sending strings to "std::cout" is equivalent to logging messages ?

Answer 1

You can create a class derived from std::streambuf which uses the Android specific functions to send the produced sequence of characters. I don't know where the default implementation of std::cout sends characters on Android, however. Basically, this would look something like this:

class androidbuf : public std::streambuf {
public:
    enum { bufsize = 128 }; // ... or some other suitable buffer size
    androidbuf() { this->setp(buffer, buffer + bufsize - 1); }

private:
    int overflow(int c)
    {
        if (c == traits_type::eof()) {
            *this->pptr() = traits_type::to_char_type(c);
            this->sbumpc();
        }
        return this->sync()? traits_type::eof(): traits_type::not_eof(c);
    }

    int sync()
    {
        int rc = 0;
        if (this->pbase() != this->pptr()) {
            char writebuf[bufsize+1];
            memcpy(writebuf, this->pbase(), this->pptr() - this->pbase());
            writebuf[this->pptr() - this->pbase()] = '\0';

            rc = __android_log_write(ANDROID_LOG_INFO, "std", writebuf) > 0;
            this->setp(buffer, buffer + bufsize - 1);
        }
        return rc;
    }

    char buffer[bufsize];
};

To actually set up std::cout to write to this stream buffer, you would do something like this in your main() function:

int main() {
    std::cout.rdbuf(new androidbuf);
    ...
}

This create a memory leak for the one androidbuf stream which is, however, somewhat intentional: the stream may be written to after main() is exited and it is flushed when when std::cout gets destroyed. If you don't want this, you could either restore std::cout's original stream buffer or set it to null and delete the return from rdbuf():

   // avoid a one-time resource leak but don't get output afterwards:
   delete std::cout.rdbuf(0);

Answer 2

According to the Android documentation, stdout & stderr output to /dev/null. You can use the Android Debug Bridge to achieve what you want.

By default, the Android system sends stdout and stderr (System.out and System.err) output to /dev/null. In processes that run the Dalvik VM, you can have the system write a copy of the output to the log file. In this case, the system writes the messages to the log using the log tags stdout and stderr, both with priority I. To route the output in this way, you stop a running emulator/device instance and then use the shell command setprop to enable the redirection of output. Here's how you do it:

$ adb shell stop
$ adb shell setprop log.redirect-stdio true
$ adb shell start

The system retains this setting until you terminate the emulator/device instance. To use the setting as a default on the emulator/device instance, you can add an entry to /data/local.prop on the device.