Pthreads, Part 1: Introduction

Intro to Threads

What is a thread?

A thread is short for 'thread-of-execution'. It represents the sequence of instructions that the CPU has (and will) execute. To remember how to return from function calls, and to store the values of automatic variables and parameters a thread uses a stack.

What is a Lightweight Process (LWP)? How does it relate to threads?

Well for all intents and purposes a thread is a process (meaning that creating a thread is similar to fork) except there is no copying meaning no copy on write. What this allows is for a process to share the same address space, variables, heap, file descriptors and etc.

The actual system call to create a thread is similar to fork; it's clone. We won't go into the specifics but you can read the man pages keeping in mind that it is outside the direct scope of this course.

LWP or threads are preferred to forking for a lot of scenarios because there is a lot less overhead creating them. But in some cases (notably python uses this) multiprocessing is the way to make your code faster.

How does the thread's stack work?

Your main function (and other functions you might call) has automatic variables. We will store them in memory using a stack and keep track of how large the stack is by using a simple pointer (the "stack pointer"). If the thread calls another function, we move our stack pointer down, so that we have more space for parameters and automatic variables. Once it returns from a function, we can move the stack pointer back up to its previous value. We keep a copy of the old stack pointer value - on the stack! This is why returning from a function is very quick - it's easy to 'free' the memory used by automatic variables - we just need to change the stack pointer.

In a multi threaded program, there are multiple stack but only one address space. The pthread library allocates some stack space (either in the heap or using a part of the main program's stack) and uses the clone function call to start the thread at that stack address. The total address space may look something like this.

How many threads can my process have?

You can have more than one thread running inside a process. You get the first thread for free! It runs the code you write inside 'main'. If you need more threads you can call pthread_create to create a new thread using the pthread library. You'll need to pass a pointer to a function so that the thread knows where to start.

The threads you create all live inside the same virtual memory because they are part of the same process. Thus they can all see the heap, the global variables and the program code etc. Thus you can have two (or more) CPUs working on your program at the same time and inside the same process. It's up to the operating system to assign the threads to CPUs. If you have more active threads than CPUs then the kernel will assign the thread to a CPU for a short duration (or until it runs out of things to do) and then will automatically switch the CPU to work on another thread. For example, one CPU might be processing the game AI while another thread is computing the graphics output.

Simple Usage

Hello world pthread example

To use pthreads you will need to include pthread.h AND you need to compile with -pthread (or -lpthread) compiler option. This option tells the compiler that your program requires threading support

To create a thread use the function pthread_create. This function takes four arguments:

int pthread_create(pthread_t *thread, const pthread_attr_t *attr,
                   void *(*start_routine) (void *), void *arg);

• The first is a pointer to a variable that will hold the id of the newly created thread.
• The second is a pointer to attributes that we can use to tweak and tune some of the advanced features of pthreads.
• The third is a pointer to a function that we want to run
• Fourth is a pointer that will be given to our function

The argument void *(*start_routine) (void *) is difficult to read! It means a pointer that takes a void * pointer and returns a void * pointer. It looks like a function declaration except that the name of the function is wrapped with (* .... )

Here's the simplest example:

#include <stdio.h>
#include <pthread.h>
// remember to set compilation option -pthread

void *busy(void *ptr) {
// ptr will point to "Hi"
    puts("Hello World");
    return NULL;
}
int main() {
    pthread_t id;
    pthread_create(&id, NULL, busy, "Hi");
    while (1) {} // Loop forever
}

If we want to wait for our thread to finish use pthread_join

void *result;
pthread_join(id, &result);

In the above example, result will be null because the busy function returned null. We need to pass the address-of result because pthread_join will be writing into the contents of our pointer.

See Pthreads Part 2