Disabling stdout buffering of a forked process

I wrote a code in C/C++ which forks a child process, duplicates the stdin/stdout into a pipe ends and calls execvp.

Everything is working fine (i.e. the output from stdin/err/out is captured by the parent process)

The problem is that the child stdout is buffered.

so if the child code looks like this:

printf("Enter any key and hit ENTER:\n");
fgets(line);
printf("read: %s\n", line);
exit(0);

In the parent process I don't see the line 'Enter any key:' - it will be "flushed" only after the program calls exit (which auto flushes the stdout buffer) or an explicit call to 'flush(stdout)' is added

I did some research and tried adding a call to disable the stdout buffering by adding a call to:

setvbuf(stdout, NULL, _IONBF, 0); just before calling execvp(...) in the parent process

so the relevant code looks now like this:

int rc = fork();
if ( rc == 0 ) {
    // Child process
    if(workingDirectory.IsEmpty() == false) {
        wxSetWorkingDirectory( workingDirectory );
    }
    int stdin_file  = fileno( stdin  );
    int stdout_file = fileno( stdout );
    int stderr_file = fileno( stderr );

    // Replace stdin/out with our pipe ends
    dup2 ( stdin_pipe_read,  stdin_file );
    close( stdin_pipe_write );

    dup2 ( stdout_pipe_write, stdout_file);
    dup2 ( stdout_pipe_write, stderr_file);
    close( stdout_pipe_read );

    setvbuf(stdout, NULL, _IONBF, 0);

    // execute the process
    execvp(argv[0], argv);
    exit(0);

}

With no luck.

Any ideas?

EDIT:

here is a sample of the parent code, the only thing needs changing is the path to the child executable:

#include <unistd.h>
#include <signal.h>
#include <sys/types.h>
#include <sys/select.h>
#include <errno.h>
#include <sys/wait.h>
#include <string>
#include <string.h>
#include <cstdio>

static int   r