Redirect output streams to logcat

This guide will guide you on how to redirect the stdout and stderr streams to adb logcat on Android.

A complete project where this technique is applied can also be downloaded from the samples repo.

Redirect the stdout and stderr to logcat

The Node.js runtime and the Node.js console module use the process' stdout and stderr streams. Some code is needed to redirect those streams to the Android system log, so they can be viewed with logcat. C++ code to manage the redirection can be included by starting two background threads (one for stdout and the other for stderr), to provide a more pleasant Node.js debugging experience.

Add the helper functions to native-lib.cpp:

app/src/main/cpp/native-lib.cpp

#include <pthread.h>
#include <unistd.h>
#include <android/log.h>

//...

// Start threads to redirect stdout and stderr to logcat.
int pipe_stdout[2];
int pipe_stderr[2];
pthread_t thread_stdout;
pthread_t thread_stderr;
const char *ADBTAG = "NODEJS-MOBILE";

void *thread_stderr_func(void*) {
    ssize_t redirect_size;
    char buf[2048];
    while((redirect_size = read(pipe_stderr[0], buf, sizeof buf - 1)) > 0) {
        //__android_log will add a new line anyway.
        if(buf[redirect_size - 1] == '\n')
            --redirect_size;
        buf[redirect_size] = 0;
        __android_log_write(ANDROID_LOG_ERROR, ADBTAG, buf);
    }
    return 0;
}

void *thread_stdout_func(void*) {
    ssize_t redirect_size;
    char buf[2048];
    while((redirect_size = read(pipe_stdout[0], buf, sizeof buf - 1)) > 0) {
        //__android_log will add a new line anyway.
        if(buf[redirect_size - 1] == '\n')
            --redirect_size;
        buf[redirect_size] = 0;
        __android_log_write(ANDROID_LOG_INFO, ADBTAG, buf);
    }
    return 0;
}

int start_redirecting_stdout_stderr() {
    //set stdout as unbuffered.
    setvbuf(stdout, 0, _IONBF, 0);
    pipe(pipe_stdout);
    dup2(pipe_stdout[1], STDOUT_FILENO);

    //set stderr as unbuffered.
    setvbuf(stderr, 0, _IONBF, 0);
    pipe(pipe_stderr);
    dup2(pipe_stderr[1], STDERR_FILENO);

    if(pthread_create(&thread_stdout, 0, thread_stdout_func, 0) == -1)
        return -1;
    pthread_detach(thread_stdout);

    if(pthread_create(&thread_stderr, 0, thread_stderr_func, 0) == -1)
        return -1;
    pthread_detach(thread_stderr);

    return 0;
}

Start the redirection right begore starting the Node.js runtime:

    //Start threads to show stdout and stderr in logcat.
    if (start_redirecting_stdout_stderr()==-1) {
        __android_log_write(ANDROID_LOG_ERROR, ADBTAG, "Couldn't start redirecting stdout and stderr to logcat.");
    }

    //Start node, with argc and argv.
    return jint(node::Start(argument_count,argv));