Java / C socket, sending multiple variables and receiving it on the client side in differend variables

So I am trying to write a program that uses some form of FTP and other things, this is only early stage and I have encountered some problems with the write method in java and the recv function in C.
I dont really know how to explain it so I attached the code and one example.

 public void run() {
        try {


        System.out.println(getName() + " Waiting...");

        synchronized (serverSocket) {
            client = serverSocket.accept();
        }

        System.out.println("Starting Client Connetion with " + getName());

        bufferedReader = new BufferedReader(new InputStreamReader(client.getInputStream()));
        dOut = new DataOutputStream(client.getOutputStream());



        dOut.write(fileName.getBytes());
        dOut.flush();
        String idk = String.valueOf(fileArray.length);
        dOut.write(idk.getBytes());
        dOut.flush();
        Thread.sleep(500);
        dOut.write(fileArray);
        dOut.flush();
        
        System.out.println("Connection with " + getName() + " has ended");
    } catch(IOException | InterruptedException e) {
        System.out.println("CLIENT ERROR");
    } finally {
        clientMap[threadNumber] = false;
        try {
            client.close();
            bufferedReader.close();
            dOut.close();
        } catch (IOException e) {
            e.printStackTrace();
        }

    }
}

this is the function that handles the clients, I want to be able to send in this order and store in different variables on the client side.

• The file name
• File Size
• binary file content

on the other hand i have the receiving non-blocking socket on windows 10 :

void receiveFile(SOCKET ClientSocket) {
char filename_buffer[20];
int fileSize = 0;
unsigned char* receivedFile;
int iResult = 0;
int count = 0;
int total = 0;
////////////////////RECEIVE THE NAME OF THE FILE/////////////////////////////
memset(filename_buffer, '\0', 20);
iResult = recv(ClientSocket, filename_buffer, 20, 0);
if (iResult == -1) {
	printf("Recv Name : error");
	closesocket(ClientSocket);
	return NULL;
}
printf("File name : %s\nName Length : %d\n", filename_buffer, strlen(filename_buffer));

////////////////////RECEIVE THE SIZE OF THE FILE/////////////////////////////
char size[10];
memset(size, '\0', 10);
iResult = recv(ClientSocket, size, 10, 0);
if (iResult == -1) {
	printf("Recv size : error\n");
	closesocket(ClientSocket);
	return NULL;
}

fileSize = arrayToInt(size);
if (fileSize == 0) {
	printf("File Size : error");
	closesocket(ClientSocket);
	return NULL;
}
printf("File size : %d\n", fileSize);

////////////////////RECEIVE THE CONTENT OF THE FILE/////////////////////////////
receivedFile = calloc(fileSize, sizeof(unsigned char));
while ((count = recv(ClientSocket, receivedFile, fileSize, 0)) > 0) {
	total += count;
}

if (count == -1) {
	printf("Recv : error\n");
	closesocket(ClientSocket);
	return NULL;
}

printf("File content : \n");
for (int i = 0; i < fileSize;i++) {
	printf("%c", receivedFile[i]);
}

printf("\nTotal file bytes received : %d\n", total);

closesocket(ClientSocket);

writeBinaryFile(filename_buffer, receivedFile, fileSize);
printf("Successful write...\n");

}

The idea of using 2 different languages is pure didactic, I kinda love C but I need to learn a higher level language.

The problem I have is that when I receive the input from the Java-server on the C-client its all messed up the filename_buffer variable contains some of the size input in it and the size contains some of the file content.

I do not understand how to solve this without using the Thread.sleep() method and wait until I receive everything in filename_buffer, wait for the socket buffer to clear and send the file size variable, wait for everything to clear and then send the file content.

One example is this :
having file[] = "AAAAAA";

dOut.write("send_this.jpg".getBytes());
dOut.write(String(file.length).getBytes());
dOut.write(file)  // (as bytes);

// The client receives

filename_buffer = "send_this.jpg";
size = 6AAAAAA;
receivedFile = "AA..";

And using Thread.sleep everything works as expected but I am not so sure its the right way to do this.