Why can't node.js emit data event on child_process.stdout?

Here is my Node.JS code

var rec = spawn('rec.exe');
rec.stdout.setEncoding('ascii');
rec.stdout.on('data', function (data) {
console.log(data);
    //do something
}
);

And my C++ program (compiled by Cygwin) writes something to stdout using printf().

if(bufsize!=0&&rec[0]=='0')
    {
    printf("%s\n",rec);
    //printf("Received,write into exchange file\n");
    //fp=fopen("recvdata.txt","w");
    //fprintf(fp,"%s\n",rec);
        //fclose(fp);
}

But the data event is never emitted.

I'm sure there is something wrong with my C++ code because it works with some other commands like ping.

Then I noticed that in this case, the event is emitted

if(fd==-1)
{
    printf("Failed,again\n");
    return 0;
}

It means when the process exits, everything works fine. But that's not what I want.

Can someone help me? Thanks.