Reading pipe from child process only works once

[JitteryJoe]

Hey cplusplus community!

I'm not really experienced with unix programming and encountered a bug I can't track down when working with pipes.
What I want to do: Fork a child process, send data to this process by pipe A (the process does some processing then) and receive resulting data by pipe B. The code below works fine when forking for the first time. On the second try however no data is received by the child process, hence the loop while (std::cin >> lineInput || result == "") runs endless. When using read instead of cin I receive some data, but not all of it (already on the first try). I appreciate any kind of idea or suggestion!

std::string result;

pid_t pid;
int pipeA[2];
int pipeB[2];


if(pipe(pipeA) < 0) perror("pipe a failed"); // To Child
if(pipe(pipeB) < 0) perror("pipe b failed"); // From Child

pid = fork();
if(pid == -1){
	perror("fork failed");
	exit(1);
}
else if(pid == 0){
	if(close(pipeA[1])<0) perror("c close a1 failed"); // No Writing in A
	if(close(pipeB[0])<0) perror("c close b0 failed"); // No Reading B
	if(dup2(pipeA[0], STDIN_FILENO)==-1) perror("c dub a failed");
	if(dup2(pipeB[1], STDOUT_FILENO)==-1) perror("c dub b failed");
	
	std::cerr << "child process starting ..." << std::endl;

	std::stringstream sstr;
	sstr << frequency;
	execl("../helloworld/app", "app", (char*)0);

	if(close(pipeA[0])<0) perror("c close a0 failed");
	if(close(pipeB[1])<0) perror("c close b1 failed");

	std::cerr << "child stopping ..." << std::endl;
	exit(1);
}
else{
	if(close(pipeA[0])<0) perror("close a0 failed"); // No Reading from A
	if(close(pipeB[1])<0) perror("close b1 failed"); // No Writing to B
	//if(dup2(pipeA[1], STDOUT_FILENO)==-1) perror("dub a failed"); // Write A
	if(dup2(pipeB[0], STDIN_FILENO)==-1) perror("dub b failed"); // Read B

	// write pipe
	FILE* stream;
	stream = fdopen (pipeA[1], "w");
	if(stream == 0) perror("stream failed");
	fwrite(&midi[0], midi.size(), sizeof(BYTE), stream);
	fflush(stream);
	fclose(stream);

	std::cout << "Start reading from position: " 
                  << lseek(pipeB[0], 0, SEEK_CUR) << std::endl;

	// read pipe
	char buf[1024];
	if(toFile == "") {

		// works second time, but not complete?
		//while (read(pipeB[0], &buf, 1024) > 0 || result == "")
		//    result += buf;

		// Does not work second time, no chars received
		std::string lineInput;
		while (std::cin >> lineInput || result == "")
			result.append(lineInput);
	}
	// close pipes
	//if(close (pipeA[1])<0) perror("close a1 failed");
	if(close (pipeB[0])<0) perror("close b0 failed");
	if(kill(pid,SIGKILL)<0) perror("sigkill failed");
}