Android Logcat very strange behavior when getting empty string

Question

So I came across something strange that made me loose some time. I have been trying to print the content of an ArrayList containing string elements, sometimes, an element might

Answer 1

That is an interesting question. I just tried this in LogRabbit and am able to see the same result.

I took a quick browse through the android source and see that Log.W(...) ends up in native code and getting handled in logd_write.c

This basically writes the data to /dev/log/main (or one of the other logs)

You can get those logs like this:

adb pull /dev/log/events .
adb pull /dev/log/main .
adb pull /dev/log/radio .
adb pull /dev/log/system .

You will need to press cntl-C otherwise the copy will happen forever.

Looking in the raw log in /dev/log/main I see the message does get logged:

 <8b>F×U^_<8c>^Y^U^Emfl_MessageList^@Before Empty^@^R^@^@^@!z^@^@!z^@^@
 <8b>F×U^_<8c>^Y^U^Emfl_MessageList^@^@^]^@^@^@!z^@^@!z^@^@
 <8b>F×U^_ <8c>^Y^U^Emfl_MessageList^@After Empty^@7^@^@^@^@^E^@^@^Z^E^@^@

That gets decoded by struct found in logger.h So I think this is a problem in adb. pull the source code from here: (looks like quite a few of undocumented commands there)

This is the primary function

static int logcat(TransportType transport, const char* serial, int argc, const char** argv) {
    char* log_tags = getenv("ANDROID_LOG_TAGS");
    std::string quoted = escape_arg(log_tags == nullptr ? "" : log_tags);

    std::string cmd = "shell:export ANDROID_LOG_TAGS=\"" + quoted + "\"; exec logcat";

    if (!strcmp(argv[0], "longcat")) {
        cmd += " -v long";
    }

    --argc;
    ++argv;
    while (argc-- > 0) {
        cmd += " " + escape_arg(*argv++);
    }

    return send_shell_command(transport, serial, cmd);
}

Looking in there I see that all logcat does is basically this:

adb shell
> exec logcat

So I think the root of the problem is in logcat itself. Logcat.cpp calls into log_read.c

Based on my quick read through things what I think is happening is the message is not terminated properly. The empty message does not show up until another message is appended and the first message overruns and shows the second message because it has the appropriate termination.