Knowing how to properly use threads should be part of every computer science and engineering student repertoire. This tutorial is an attempt to help you become familiar with multi-threaded programming with the POSIX (Portable Operating System Interface) threads, or pthreads. This tutorial explains the different tools defined by the pthread library, shows how to use them, and gives examples of using them to solve real life programming problems.
Technically, a thread is defined as an independent stream of instructions that can be scheduled to run as such by the operating system.
A thread is a semi-process that has its own stack, and executes a given piece of code. Unlike a real process, the thread normally shares its memory with other threads (where as for processes we usually have a different memory area for each one of them). A Thread Group is a set of threads all executing inside the same process. They all share the same memory, and thus can access the same global variables, same heap memory, same set of file descriptors, etc. All these threads execute in parallel (i.e. using time slices, or if the system has several processors, then really in parallel).
Single- and Multi-Threaded Processes
In order to take full advantage of the capabilities provided by threads, a standardized programming interface was required. For UNIX systems, this interface has been specified by the IEEE POSIX 1003.1c standard (1995). Implementations which adhere to this standard are referred to as POSIX threads, or Pthreads. Most hardware vendors now offer Pthreads in addition to their proprietary threads.
If implemented correctly, threads have some advantages over processes. Compared to the standard fork(), threads carry a lot less overhead.
Remember that fork() produces a second copy of the calling process. The parent and the child are completely independent, each with its own address space, with its own copies of its variables, which are completely independent of the same variables in the other process.
Threads share a common address space, thereby avoiding a lot of the inefficiencies of multiple processes.
On the other hand, because threads in a group all use the same memory space, if one of them corrupts the contents of its memory, other threads might suffer as well. With processes, the operating system normally protects processes from one another, and thus if one corrupts its own memory space, other processes won't suffer.
Example 1: A responsive user interface
One area in which threads can be very helpful is in user-interface programs. These programs are usually centered around a loop of reading user input, processing it, and showing the results of the processing. The processing part may sometimes take a while to complete, and the user is made to wait during this operation. By placing such long operations in a separate thread, while having another thread to read user input, the program can be more responsive. It may allow the user to cancel the operation in the middle.
Example 2: A graphical interface
In graphical programs the problem is more severe, since the application should always be ready for a message from the windowing system telling it to repaint part of its window. If it's too busy executing some other task, its window will remain blank, which is rather ugly. In such a case, it is a good idea to have one thread handle the message loop of the windowing systm and always ready to get such repain requests (as well as user input). Whenever this thread sees a need to do an operation that might take a long time to complete (say, more then 0.2 seconds in the worse case), it will delegate the job to a separate thread.
Example 3 : A Web server
When a multi-threaded program starts executing, it has one thread running,
which executes the main() function of the program. This is already a
full-fledged thread, with its own thread ID. In order to create a new thread,
the program should use the
pthread_create() function.
Here is how to use it:
#include <stdio.h> /* standard I/O routines */
#include <pthread.h> /* pthread functions and data structures */
/* function to be executed by the new thread */
void* PrintHello(void* data)
{
int my_data = (int)data; /* data received by thread */
pthread_detach(pthread_self());
printf("Hello from new thread - got %d\n", my_data);
pthread_exit(NULL); /* terminate the thread */
}
/* like any C program, program's execution begins in main */
int main(int argc, char* argv[])
{
int rc; /* return value */
pthread_t thread_id; /* thread's ID (just an integer) */
int t = 11; /* data passed to the new thread */
/* create a new thread that will execute 'PrintHello' */
rc = pthread_create(&thread_id, NULL, PrintHello, (void*)t);
if(rc) /* could not create thread */
{
printf("\n ERROR: return code from pthread_create is %d \n", rc);
exit(1);
}
printf("\n Created new thread (%d) ... \n", thread_id);
pthread_exit(NULL); /* terminate the thread */
}
Understanding the simple threaded program above. While it does not do anything useful, it will help you understand how threads work. Let us take a step by step look at what the program does.
main() we declare a variable called
thread_id, which
is of type pthread_t.
This is basically an integer used to identify the
thread in the system.
After declaring thread_id, we call the pthread_create()
function to create a real, living thread.
pthread_create()
gets 4 arguments The first argument is
a pointer to thread_id, used by pthread_create()
to supply the program with the thread's identifier.
The second argument is used to set some
attributes for the new thread. In our case we supplied a NULL pointer to
tell pthread_create() to use the default values.
Notice that PrintHello() accepts a void * as an argument and
also returns a void * as a return value. This shows us that it is possible
to use a void * to pass an arbitrary piece of data to our new thread, and
that our new thread can return an arbitrary piece of data when it finishes.
How do we pass our thread an arbitrary argument? Easy. We use the fourth
argument to the pthread_create() call. If we do not want
to pass any data to the new thread, we set the fourth argument to NULL.
pthread_create() returns zero on success and a non-zero value
on failure.
pthread_create() successfully returns, the program will consist of
two threads. This is because the main program
is also a thread and it executes the code in the
main() function in parallel to the thread it creates.
Think of it this way: if you write a program that does not use POSIX threads
at all, the program will be single-threaded (this single thread is called
the "main" thread).
pthread_exit() causes the current thread
to exit and free any thread-specific resources it is taking.
In order to compile a multi-threaded program using gcc,
we need to link it with the pthreads library. Assuming you have this library
already installed on your system, here is how to compile our first program:
The source code for this program may be found in the
hello.c file.
gcc hello.c -o hello -lpthread
pthread_self
A thread can get its own thread id
by calling pthread_self(), which returns the thread id:
Use it as
pthread_t pthread_self();
pthread_t tid;
tid = pthread_self();
There are several ways for threads
to terminate. One way to safely terminate is to call the
pthread_exit routine (the equivalent of exit for processeas).
It is necessary to use pthread_exit at the end of the main program.
Otherwise, when it exits, all running threads will be killed.
The pthread_join()
function for threads is the equivalent of wait() for processes.
A call to pthread_join
blocks the calling thread until the thread with identifier equal to the first
argument terminates.
#include <stdio.h> /* standard I/O routines */
#include <pthread.h> /* pthread functions and data structures */
void* PrintHello(void* data)
{
pthread_t tid = (pthread_t)data; /* data received by thread */
pthread_join(tid, NULL); /* wait for thread tid */
printf("Hello from new thread %d - got %d\n", pthread_self(), data);
pthread_exit(NULL); /* terminate the thread */
}
/* like any C program, program's execution begins in main */
int main(int argc, char* argv[])
{
int rc; /* return value */
pthread_t thread_id; /* thread's ID (just an integer) */
int tid;
tid = pthread_self();
rc = pthread_create(&thread_id, NULL, PrintHello, (void*)tid);
if(rc) /* could not create thread */
{
printf("\n ERROR: return code from pthread_create is %d \n", rc);
exit(1);
}
sleep(1);
printf("\n Created new thread (%d) ... \n", thread_id);
pthread_exit(NULL);
}pthread_join() is the identifier of the thread to join.
The second argument is a void pointer.
pthread_join(pthread_t tid, void * return_value);
return_value pointer is non-NULL, pthread_join will place
at the memory location pointed to by return_value,
the value passed by the thread tid through the pthread_exit call.
Since we don't care about return value of the main thread, we set it to NULL.
At any point in time, a thread is either joinable or detached (default state is joinable).
Joinable threads must be
reaped or killed by other threads (using pthread_join) in order to free memory resources.
Detached threads cannot be reaped or killed by other threads, and resources are automatically
reaped on termination. So unless threads need to synchronize among themselves, it is
better to call
instead of pthread_detach(pthread_self());
pthread_join.