Client

Objectives

• Interact with structs
• Handle raw data in a buffer
• Send data over a network socket

Getting Started

Use the GitHub Classroom link posted in the Learning Suite for the lab to accept the assignment. Next, ssh into your Raspberry Pi using VSCode and clone the repository in your home directory. This lab should be done in VSCode on your Raspberry Pi. Make sure the lab is top level folder of your VSCode editor.

Overview

You have come very far in understanding how to control your computer! Up until this point, we have covered programming peripheral devices, handling inputs from a user, and many other skills that allow you to control your Pi Z2W to the fullest! In this lab, we will take you from influencing processes on your own system to interacting with programs on other systems! In this lab you will gain practice with network programming by sending some of your saved photos over a network to a server.

Config Struct

IP Addresses and Ports

Before we can access a program on a different computer, we need to know where that computer lives. In order to know where that computer lives, we need to know that computer’s address. For example, when you are on your web browser and you want to visit https://byu.edu, the browser will use a special protocol to extract the IP address from the domain name that you provided in the URL bar.

If you want to find an IP address of a certain website (most popular ones have many), you can use ping and observe the output:

$ ping byu.edu

PING byu.edu (128.187.16.184) 56(84) bytes of data.
64 bytes from byu.com (128.187.16.184): icmp_seq=1 ttl=58 time=2.53 ms
64 bytes from email.byu.edu (128.187.16.184): icmp_seq=2 ttl=58 time=2.92 ms
64 bytes from email.byu.edu (128.187.16.184): icmp_seq=3 ttl=58 time=11.1 ms

Note that 128.187.16.184 is the IP address of the server that responded to our ping.

Now that we know the address of the computer that we want to talk to, we need to know what port is being used by the program we want to communicate with. A port is a special window that a program uses to communicate with other programs. For example, a web server that is hosting a website normally does this under port 80 or 443 (if you are using HTTPS instead of HTTP). Every time you use ssh on your computer, you are talking to the remote computer over port 22 by default.

When we connect to another computer that is listening on a specific port, we refer to the listening computer as the server and the computer that is trying to connect to the server as the client.

Struct Data Members

As mentioned previously, in order to talk to a server, we need to know several things about it. For this lab, the Config struct in client.h contains the information you will need in order to connect to the server:

typedef struct Config {
    char *port;
    char *host;
    uint8_t *payload;
    uint32_t payload_size;
    char *hw_id;
} Config;

The location you will be connecting to to upload your photos in this lab will be:

• Host: ecen224.byu.edu (Note: this host is only available from the campus network)
• Port: 2240

Network Socket

Now that we know the server name and port that we want to connect to, how do we go about doing that? The answer is quite simple and more surprising than you would think. Talking to a different computer in C is much like writing or reading data to a file. However, instead of writing to a file, we will be writing to a socket.

When we want to write to a file in C, we need to use the fopen() function to open the file. In network programming, it is a bit more complicated. However, for this lab, you will not be expected to create a network socket by yourself. The function client_connect(), which connects to a server port and opens a socket, has been written for you. Similar to how fopen() returns a FILE pointer that you use to interact with a file, client_connect() returns a file descriptor (with the data type of int) that lets your program keep track of which port your computer will use to talk to the remote computer.

Sending Data

While in previous labs you used write() to write to a file, for network programming, we use send() to write to a socket (see the link in Explore More for more details about this function). Look at the tutorial below to see how the function behaves and you will notice it is very close to writing to a file. However, be careful. Since this is network programming, there is no guarantee that when you call send() that it will send all the data in the buffer you tried to send. You will be responsible for writing the logic to ensure that all of the data is sent correctly by implementing a function called client_send_image in client.c.

The standard send function will return how much of the data actually got sent. If the amount of data you were expecting to send is not the same as what send returned, that means not all of the data got sent. For example, if I am sending data that is 100 bytes long and I call send and it only returns 50, that means only the first 50 bytes got sent. I need to call send again, passing the rest of the data. You will need to keep calling send until the total bytes sent is equal to the total bytes you were expecting to send. Here is some pseudo-code showing this concept:

int socket = ...
uint8_t *data = ...
int data_length = ...
int total_sent = 0
int num_sent = 0

while total_sent < data_length:
    num_sent = send(socket, data + total_sent, data_length - total_sent, 0)

    if num_sent == 0:
        print("Error while sending data")
        break

    total_sent += num_sent

In this lab, you will need to send your homework ID and your image data. The easiest way of doing this is combining the data into one block of data (using malloc and memcpy). Your homework ID must go first, followed by the image data.

The results will be displayed at http://ecen224.byu.edu:2241/<homework_id>, replacing <homework_id> with your actual homework ID. For example, if your homework ID was 123456789, then the URL where your pictures are stored are at http://ecen224.byu.edu:2241/123456789. Remember, this server is only available when you are on the campus network.

Deallocating Resources

When calling functions that create or allocate system resources, you need to remember to free those resources when you are done with them. If this is not done, unexpected behavior may occur on your system. So remember the following:

Requirements

• Copy your main.c, viewer folder, and lib folder from the previous lab into the cloned repository. Make sure to keep the client.h and client.c files in your lib folder. You can run something like this:

cp ~/camera/main.c ~/client    # Copy main.c
cp -r ~/camera/viewer ~/client # Copy viewer folder 
cp -r ~/camera/lib ~/client    # Copy lib folder

• Add lib/client.h to your list of #includes in your main.c.

• Same as last lab, when you press the center button, you should take a picture and show the picture, allowing the user to apply different filters. When you press the center button again, instead of exiting the picture and going to the menu right away, first connect to the server and send the picture. Once you have sent the picture, show the menu. Specifically, when the center button gets pressed while showing the picture you should do the following:

  • Create an empty Config struct and load it with the appropriate data. The port, host, and hw_id can be hard-coded. Use the picture buffer obtained in the last lab using the camera_capture_data function, fill in the payload field. The size of the payload should be the size of the image data.

  • Start the client connection using client_connect() and passing the Config struct. This function is implemented for you and will print out the status of the connection to the server.

  • Call the client_send_image function to send the data, passing the Config struct. You will need to implement this function yourself. This function will combine the homework ID and the image data into one buffer and send it to the server using the send function. You will need to make sure all of the data is sent correctly.

  • Call the client_receive_response function. This function is implemented for you and will print out the response from the server. This function can help debug any problems you might have with sending your image data.

  • Close the connection using client_close(). You will need to implement this function yourself.

  • Show the menu.

Submission

• Answer the questions in the README.md.

• To pass off to a TA, demonstrate your doorbell running your program that fulfills all of the requirements outlined above. You must also show the TAs your implementation of the client_send_image function and where you close the socket.

• To successfully submit your lab, you will need to follow the instructions in the Lab Setup page, especially the Committing and Pushing Files section.

Explore More!

• memcpy() in C

• send() in C

• Ultimate Guide to Network Programming in C

• Code for the server part of this lab