Threads$()$

$$ In system programming, threads are the smallest unit of execution within a process. A thread is a sequence of

instructions that can be scheduled and executed independently by the operating system's scheduler. Multiple

threads can exist within a single process, sharing the process's resources such as memory, file descriptors, and

global variables

Thread vs$.$ Process:

$$ Process: A process is an independent program in execution. It has its own memory space, system resources,

and execution context.

$$ Thread: A thread is a lightweight execution unit within a process. Multiple threads within the same process

share the process's resources but have their own execution state $($program counter, stack, and registers$).$

Types of Threads:

$$ User Threads: Managed at the user level by libraries without kernel support. They are fast but can block the

entire process if one thread makes a blocking system call.

$$ Kernel Threads: Managed directly by the operating system kernel. The kernel schedules them, and blocking

in one thread doesn$$t affect other threads in the process.

$5$

Program to create Threads in Linux

$$ Syntax$-$

#include

int pthread$\_$create$($pthread$\_$t $*$thread$,$ const pthread$\_$attr$\_$t $*$attr$,$ void $*(*$start$\_$routine$)($void $*),$ void arg$)$;

pthread$\_$t $*$thread:

Meaning: A pointer to a pthread$\_$t variable, which will hold the thread identifier $($ID$)$ of the newly created thread. After

the thread is successfully created, this variable will store the ID of the new thread.

Usage: You pass the address of a pthread$\_$t variable, like &thread, where thread is declared as pthread$\_$t thread;.

const pthread$\_$attr$\_$t $*$attr:

Meaning: A pointer to a thread attribute object $($pthread$\_$attr$\_$t$),$ which specifies the thread's attributes $($e$.$g$.,$ whether the

thread is joinable, stack size, scheduling policies$).$ You can specify attributes such as whether the thread is detached or

joinable.

Usage: If you want to use default thread attributes, pass NULL. If you need specific attributes, create and initialize a

pthread$\_$attr$\_$t object

$6$

void $*(*$start$\_$routine$)($void $*)$:

Meaning: A pointer to the function that the thread will execute. This function should take a void$*$ argument and

return a void$*.$ This function is called the "start routine," and the thread begins executing at this point.

Usage: You pass the name of the function you want the thread to run.

void $*$arg:

Meaning: A pointer to the argument that will be passed to the start$\_$routine function. This is how you provide data

to the thread function when the thread starts running.

Usage: You can pass any type of data $($e$.$g$.,$ integer, structure$)$ by casting it to void$*.$ If you don't need to pass an

argument, you can pass NULL.

Note: To compile any program which involves creation of thread$($s$)$ use pthread library $($lpthread$)$

Suppose the program is named $$Thread$.$c$,$ then to compile write

$gcc Thread.c $-$lpthread

To run the command remains same

$$./$a$.$out

$7$

Thread$()$ Example$-1$

#include

#include

#include

#include

void $*$thread$\_$function$($void $*$arg$)$;

int i$,$j;

int main$()\{$

pthread$\_$t a$\_$thread; $//$thread declaration

pthread$\_$create$($&a$\_$thread, NULL, thread$\_$function, NULL$)$;

$//$thread is created

pthread$\_$join$($a$\_$thread, NULL$)$; $//$process waits for thread to finish $.//$Comment this line to see the

difference

printf$("$Inside Main Program

$")$;

for$($j$=20$;j$<25$;j$++)$

$\{$

printf$("\%$d

$",$j$)$;

sleep$(1)$;

$\}$

$\}$

$8$

void $*$thread$\_$function$($void $*$arg$)\{$

$//$ the work to be done by the thread is defined in this

function

printf$("$Inside Thread

$")$;

for$($i$=0$;i$<5$;i$++)$

$\{$

printf$("\%$d

$",$i$)$;

sleep$(1)$;

$\}$

$\}$

Example $2,$ Thread returns a value to main$()$

#include

#include

#include

#include

#include

void $*$thread$\_$function$($void $*$arg$)$;

int i$,$n$,$j;

int main$()\{$

char $*$m$="5"$;

pthread$\_$t a$\_$thread; $//$thread declaration

void $*$result;

pthread$\_$create$($&a$\_$thread, NULL, thread$\_$function, m$)$; $//$thread is

created

pthread$\_$join$($a$\_$thread, &result$)$;

printf$("$Thread joined

$")$;

for$($j$=20$;j$<25$;j$++)$

$\{$

printf$("\%$d

$",$j$)$;

sleep$(1)$;

$\}$

printf$("$thread returned $\%$s

$",($char $*)$result$)$;

$\}9$

void $*$thread$\_$function$($void $*$arg$)\{$

int sum$=0$;

n$=$atoi$($arg$)$;

for$($i$=0$;i