Threaded Client

Table of Contents

Objectives

  • Deal with technical debt
  • Learn to multitask in C
  • Create a POSIX thread with arguments

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

One of the last fundamentals to know about programming in general is how to make your program do multiple things at once. Until this point, we have written single-threaded programs. This means that our programs have only followed a single path of execution while it runs (i.e., the program can only focus on one task at a time). However, many of the problems that we solve in the real-world are much more complex and may require us to interact with many facets of the problem simultaneously. In the lab for this class, creating a useful smart doorbell is definitely one of those multifaceted problems. Consider the following:

  • How can I upload my photo to the server while viewing it?

  • How can I actively be monitoring the status of my upload while also drawing to my screen in a loop?

While considering the previous questions, you may have noticed that with your current skill set, these problems seem difficult to solve without a complex algorithm or multiple programs running at once. By using threads, we can sidestep a lot of this headache by grouping different tasks/functions into distinct threads and call them from our main thread.

Creating a Thread

Threads in Linux programming are known as POSIX threads (or pthreads for short). pthread_t is the data type used to uniquely identify a thread:

#include <pthread.h>

pthread_t my_thread;

This object is used inside a range of different functions found inside the pthread.h library. The most important function for this lab is pthread_create(), which allows us to spawn a new thread and provide the function that it will run. Consider the simple threading program below:

#include <stdio.h>
#include <unistd.h>   // Library that includes sleep()
#include <pthread.h>  // Library that includes pthreading types and functions

void *print_msg(void *unused)
{
    while(1)
    {
        printf("I'm in a thread!\n");
        sleep(2);     // Sleeps for 2 seconds
    }

    return NULL;
}

int main()
{
    pthread_t my_thread;
    pthread_create(&my_thread, NULL, print_msg, NULL);

    while(1)
    {
        printf("I'm in main!\n");
        sleep(1);
    }

    return 0;
}

To compile using the pthread.h library, you will need to add -pthread to your normal gcc compile command.

Let’s take a closer look at what is going on in the pthread_create() function. The arguments that are expected in the function are as follows:

  1. The address of your thread struct. This is how your program will know how to identify your thread in the program.

  2. This argument expects a special pthread_attr_t struct pointer. Since we are covering the basics of threading in this class, this field is ignored by putting a NULL in its place.

  3. A pointer to the function we are calling. Function pointers can be provided by giving the name of the function. Make sure you only provide the name, and don’t call the function (eg. print_msg instead of print_msg()). If you like, you can put a & in front of the function name to make it more obvious it’s a function pointer (eg. &print_msg), but this is not required by the compiler.

  4. A pointer you provide to this argument will be passed as an argument to the function you are running in the new thread. This is a great way to provide information to the new thread. Notice that for this example it is NULL since our function doesn’t require any special arguments. The next section has more information about how to pass arguments into a thread.

You’ll notice that both our main() function and our print_msg() function have while(1) loops that will run indefinitely. Because threading allows us to run different functions simultaneously, we will see output that looks something like this:

I'm in a thread!
I'm in main!
I'm in main!
I'm in a thread!
I'm in main!
I'm in main!
I'm in a thread!
I'm in main!
I'm in main!
I'm in a thread!
I'm in main!
I'm in main!
...

Passing in Arguments to a Thread

While the previous example may have been interesting in the sense that we’ve learned to use threads, it is pretty lackluster in the sense that we can’t get any information into or out of the thread. To do this, we will need to understand working with void * and allocating memory.

void *: The Generic Pointer

A void * is the generic pointer in C. It is used to store a memory address, but does not indicate the type of data stored at the address. This means that any type of pointer can be cast to or from a void * object. It is up to you, as the programmer, to know what you passed into the function and cast it to the correct pointer type. This datatype is very useful when using threading functions because it allows us to pass in the address of any type of data to the function we are spawning. The threading function also returns a void * data type, allowing it to pass back a pointer to any type of data. This accounts for why our print_msg() function in the previous example program’s return type was a void *.

Example

Consider the following program:

#include <stdio.h>
#include <unistd.h>  // Library that includes sleep()
#include <stdlib.h>  // Library that includes malloc()
#include <pthread.h> // Library that includes pthreading types and functions

void *print_num(void *arg)
{
    while(1)
    {
        // Notice that we have to cast the void * back to an int * and then we use the * to get the value inside
        printf("The number is:\t%d\n", *((int *) arg));
        sleep(2);     // Sleeps for 2 seconds
    }

    return NULL;
}

int main()
{
    int *i = (int *)malloc(sizeof(int));    // Create an int pointer and give it enough space to hold an int.
    *i = 0;                                  // Set the value inside the pointer to 0

    pthread_t my_thread;

    // Notice that we have to cast i to a void *
    pthread_create(&my_thread, NULL, print_num, (void *) i); 

    while(1)
    {
        *i = (*i) + 1;                      // Increment the value inside of i
        printf("I'm in main!\n");
        sleep(1);
    }

    return 0;
}

Let’s take a closer look at this code example. The pthread_create() function is spawning a new thread to run the print_num function. We want to pass an integer to this function. Since we always use a void * to pass data to a function run by pthread_create(), we need to allocate memory for an integer and provide it’s address as the last argument to pthread_create().

You’ll notice some things about how we’ve done this:

  • We have allocated the space for the integer using malloc. While we could have simply declared an integer (int i;), this would create the integer on the stack. In general, it’s bad practice to pass addresses on the stack to other threads as they may continue to execute after stack variable goes out of scope. Another alternative to malloc is to declare the integer as a global variable.

  • To set the value of the integer to zero, we needed to refer to the pointer’s contents: *i = 0;

  • When we passed it into pthread_create(), we needed to cast it from a int * to a void *.

Now that we have provided the address of our integer to pthread_create(), let’s take a look at how it is received by the print_num() function:

  • The address is provided as an argument to the function with type void *.

  • To use arg we need to do two things:

    1. Cast arg back into the type that we expect (in this case an int *).

    2. Deference it with a * to get the integer value pointed to by this address (the * in front of the cast statement).

Return Values from Threads

To get a return value from a thread in C, you would need to use the pthread_join() function. For the purposes of this lab, we will not be using pthread_join(), but you can read more about it in the Explore More! section. pthread_join() is NOT recommended for this lab.

Technical Debt

Technical debt is the idea that when you code something for the first time, it is normally not the most polished code. When you need to go back and improve the functionality of your code, there will need to be some refactoring done.

You may have noticed at this point that threading is very function heavy. This means that to use it in your doorbell code, you will need to have nice functions that group code together for certain tasks. If you have not been using functions up to this point, you are likely in some technical debt. You will need to encapsulate some functionality into functions as described in the Requirements sections below.

Requirements

  • Copy all of your code, except for the README.md file from last lab into your newly cloned repository.

  • In order to use the threading library, you need to include the library in source code and your compilation.
    • Add #include <pthread.h> and #include <unistd.h> to your headers in main.c.
    • Add -pthread to the CFLAGS variable in your Makefile.
    • Before you go any farther, try making your project to make sure everything is still working.

  • Write a function called send_image. This function should take care of connecting to the server, sending the image, receiving the response, and closing the socket. You need to figure out the correct signature for this function.

  • Run your code using this newly created function (not using threads) to make sure that everything is still working as expected. Notice that sending data to the server is slow and your menu gets blocked while you are waiting to send the data.

  • Instead of calling send_image directly, use a thread to call the function instead. Now that calling send_image is no longer blocking, you might need to refactor your code to not call free right after the thread is started. Warning: you should not call any display functions in a thread. The display functions are not thread-safe, meaning that if multiple threads call the same function, weird things can happen. You should only call the display functions in your main loop.

  • Create a status bar the size of your highlight bar at the bottom of the screen with a blue background:

    • When your image is being sent in a thread, the text of the bar should say “Sending…” with the bar as a blue background.

    • When your image is sent successfully (i.e. the thread reached the end successfully), change the color of the status bar to green and show “Sent!” as the text. How are you going to know when the thread is done sending? A global variable that helps you to maintain what state you are in might be helpful.

    • The status bar should should stay green with the sent message for 2 seconds. After the 2 seconds hide the status bar. How do you know when it has been 2 seconds? Consider editing your send_image.

Here is a demo showing the different features of the lab:

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.

  • 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!