Posix Threads

In this post I would demonstrate the usage of Posix threads through a very simple problem. I would also demonstrate how we can synchronize Posix threads using mutex. Though there are many synchronizing primitives available, I would stick to mutex for simplicity. Please note that the code presented here is only for demonstration purpose and is inefficient.

Before starting on the problem, let me give a brief introduction of Posix threads. Historically, hardware vendors developed their own proprietary implementations of threads. These implementations differed substantially from each other. So, it was very difficult to develop multi-threaded portable applications. So, a standard specification was required to standardize the thread interface and this interface is Posix thread.

Problem :

A string is to be processed by a given number of threads. The processing in this case would be printing of the string. Each thread should do specific amount of work before passing control to the next thread. The work in our case is the printing of characters. The control is to be passed to the next thread in a round robin manner till the string is processed completely.

Program should take 3 command line arguments.
<Program name> <Input String> <Amount of work> <Number of threads>
Here is the desired output for a sample run.
ankit@ankit:~/problems/p1$ ./driver ABCDEFGHILKLMN 2 3
Thread 1 : AB
Thread 2 : CD
Thread 3 : EF
Thread 1 : GH
Thread 2 : IL
Thread 3 : KL
Thread 1 : MN
ankit@ankit:~/problems/p1$

Note that each thread is getting control in a round robin manner. In the beginning, thread 1 printed "AB" since the amount of work specified is 2. Then thread 2 printed "CD", then thread 3 printed "EF". After this, control again came to thread 1.

This program should be complied using -lpthread option. eg: gcc driver.c -lpthread -o driver

This problem throws light on many basic aspects of Posix threads like thread creation, thread synchronization. Ideally the sleeping part in the program is not efficient. Instead we should wait the thread and signal when its turn come. But this is for demonstration purpose.