Client

Objectives

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

Introduction

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 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” within the internet. Just like a virtual city, all computers connected to the wider internet have an address for you to find them! 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 translate the domain name you provided in the URL bar (byu.edu) to an IP address (128.187.16.184) which represents the actual virtual address of the computer.

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.

In addition to the address of the computer, we need to know what port is being used by the program we want to communicate with.

A port is a number representing a virtual entry point that a program uses to communicate with other programs. It acts like just a port at a dock, where certain ships (applications) are directed to certain ports to control traffic flow and security.

For example, a web server that is hosting a website normally does this under port 80 or 443. The default port for an ssh connection (like the one between your lab computer and your RPi) is port 22. To improve security, computers usually refuse connections or traffic directed at the wrong port.

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. In this lab, the Config struct in the provided 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 actually form a connection and transfer data? The answer is surprisingly simple: 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, opening a socket is a bit more complicated than opening a file, so we have provided The function client_connect(), which connects to a server port and opens a socket. 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

We used fwrite() to write to a file; in network programming we use send() to write to a socket (see the link in Explore More for more details about this function).

Be Careful! While send() always tries to send all the data you give it, the socket can’t guarantee that the all the data in your buffer will be sent at once. To help us keep track, send() returns the number of bytes successfully sent.

For example, if I am sending data that is 100 bytes long, I call send(), and it returns 50, then only the first 50 bytes were sent. I now need to call send() again, except this time I only pass in bytes 51 through 100 from my buffer. I repeat this process until the total number of bytes sent over all my send() calls equals 100.

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. Read the code below to see how the function behaves and get an idea of how to overcome this hurdle (this is just an example, but it works almost identically in your code).

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 together, will no null terminator or space between them. 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:

Procedure

Requirements

• 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.

Pass Off & Submission

• Your program must compile without warnings or errors. Your Makefile has the -Werror flag to ensure that it doesn’t.
• Pass off to a TA by demonstrating your doorbell running your program that fulfills all of the requirements outlined above.
• Make sure to upload your changes back to your GitHub repository. Follow the instructions on the Github Setup page.
• Take the Pass off Quiz on Learning Suite.
• Follow the instructions to update your README.md file with the new features of this lab.

Explore More

• memcpy() in C
• send() in C
• Ultimate Guide to Network Programming in C
• Code for the server part of this lab