pthreads-1 :Creation of a thread using pthreads

pthreads are posix threads supported by the unix based systems. The pthread library allows us to create multiple threads in the user space and execute varions operations in each of the thread. Let us see how we can create a thread using posix thread library.
The function to create a thread is

int pthread_create(pthread_t *thread_id, const pthread_attr_t *thread_attr, void *(*thread_fn) (void *), void *thread_arg);

Arguments:

thread_id: The id assigned to the new thread that gets created is filled in this variable. thread_attr: Various attributes of thread to control the operations and values of the thread. If NULL is passed as the value then it the default arguments are aset. thared_fn: The function that will be exectued in the thread. thread_arg: The arguments that will be passed to thread_fn. Returns 0 on successful creation of the thread.

Thus to create a thread
Declare a variable of kind pthread_t

pthread_t thread_id.

Declare a variable of kind pthread_attr_t

pthread_attr_t *thread_attr

Create a function which needs to be executed in the thread.
For e.g.

static void *hello(void *) { char *ret; ret="hello" printf("Hello from the thread"); return ret; }

We will need pass a pointer to the above funtion to pthread_create and as we are not sending any arguments to the function we can pass NULL in the arg field of pthread_crate. Thus to create thread with default attributes and which will execute the function hello will look as follows

pthread_create(&thread_id,NULL,&hello,NULL)

Once the thread is created by the parent thread should wait for the completion of the child thread Because if the parent thread gets killed, the child thread will automatically be killed and hence the operations of the child thread will never be executed.
To make the parent thread wait for the children threads we make use of the function pthread_join .

int pthread_join(pthread_t thread_id, void **ret);

Arguments:

thread_id: The thread id returned by pthread_create ret: The value returned by the child thread gets filled in this variable if it is not set to NULL.

Thus to make the parent thread to wait for the child thread
We will need a variable

void *ret

ptread_join will look as follows.

pthread_join(thread_id,ret);

Thus the whole code to create a thread using pthread create will look as follows.
create_thread.c

#include <stdio.h> #include <pthread.h> #include <stdlib.h> void * hello(void *temp) { char *ret; ret="Finished"; printf("\n Hello from thread \n"); return ret; } main() { pthread_t thread_id; pthread_attr_t attr; int ret; void *res; ret=pthread_create(&thread_id,NULL,&hello,NULL); printf("\n Created thread"); pthread_join(thread_id,&res); printf("Thread returned: %s \n",(char *)res); }

To compile a program that used the pthread library we need to pass the option -pthread to gcc to ling the required libraries and compile it successfully.

cc -pthread create_thread.c -o create_thread

The output should be:

created thread Hello from thread Thread returned: Finished

We can see that the parent thread creates a thread and waits for it to complete before exiting. If we removed the pthread_join(and the print of return value) from our program the output would look as below.

Created thread

As the parent thread did not wait for the child thread, the child thread never got a chance to exit as it got killed as soon as the parent exited.