4: Threads
- Page ID
- 123276
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)When I mentioned threads in Section 2.3, I said that a thread is a kind of process. Now I will provide a more careful explanation.
When you create a process, the operating system creates a new address space, which includes the text segment, static segment, and heap; it also creates a new “thread of execution”, which includes the program counter and other hardware state, and the call stack.
The processes we have seen so far are “single-threaded”, which means that only one thread of execution runs in each address space. In this chapter, you will learn about “multi-threaded” processes that have multiple threads running in the same address space.
Within a single process, all threads share the same text segment, so they run the same code. But different threads often run different parts of the code.
And they share the same static segment, so if one thread changes a global variable, other threads see the change. They also share the heap, so threads can share dynamically-allocated chunks.
But each thread has its own stack, so threads can call functions without interfering with each other. Usually threads don’t access each other’s local variables (and sometimes they can’t).
The example code for this chapter is in the repository for this book, in a directory named counter. For information on downloading this code, see Section 0.2.
- 4.1: Creating threads
- This page explains Pthreads, or POSIX Threads, as the primary threading standard in C programming for UNIX systems. It details the inclusion of specific header files and calling relevant functions. The compilation process requires linking to the Pthread library using the -l option in gcc, and it illustrates this with an example of compiling an array source file while enabling debugging and optimization features.
- 4.2: Creating threads
- This page discusses a Pthread function, `make_thread`, which simplifies thread creation and error-checking. It introduces a `Shared` structure to manage shared data, specifically a counter, and a `make_shared` function for its allocation and initialization. Each thread runs an `entry` function that manipulates the counter using a `child_code` function. The page concludes with a loop illustrating the creation of multiple child threads using the `make_thread` function.
- 4.3: Joining threads
- This page describes a wrapper function for pthread_join that facilitates waiting for a thread's completion while checking for errors and exiting on failure. It details a loop for a parent thread to wait for multiple child threads sequentially, independent of their completion order. An example demonstrates compiling and executing the program, highlighting the non-deterministic output typical in multithreading scenarios.
- 4.4: Synchronization errors
- This page discusses the resolution of thread interference issues in a program that accesses a shared variable, counter. It introduces a mutex to synchronize access, ensuring only one thread can modify the counter at once. This implementation, part of the Shared struct, allows threads to lock and unlock the mutex during critical sections, resulting in a consistent output of the counter reflecting the number of executed threads.
- 4.5: Mutex
- This page discusses the implementation of a Mutex type as a wrapper for pthread_mutex_t from the POSIX threads API. It highlights issues with passing pthread_mutex_t by value and introduces a Mutex type for improved readability. A function, make_mutex, is created for safe allocation and initialization of the mutex, along with wrapper functions for locking and unlocking. All content is contained within mutex.c and mutex.h files.