Implementing pthread_self() in C

Question

I'm trying to implement pthread_self() in C but I'm confused on what exactly it does. I'm aware that it returns the thread ID but is that ID a memory location because it returns a pthread_t which I'm not sure how to interpret. Additionally, how would I go about retrieving the id of the thread, do I just create a new thread and return it?

Answer 1

pthread_self() returns the ID of the thread. Please check man pages for pthread_self and pthread_create.

man 3 pthread_self
man 3 pthread_create

For pthread_create(), the first argument is of type pthread_t. It is assigned the ID of the newly created thread. This ID is used to identify the thread for other pthread functions. The abstract type of pthread_t is implementation dependent.

Before returning, a successful call to pthread_create() stores the ID of the new thread in the buffer pointed to by thread; this identifier is used to refer to the thread in subsequent calls to other pthread functions.

pthread_self returns the same ID, what pthread_create stores in the first argument "thread"

       int pthread_create(pthread_t *thread, const pthread_attr_t *attr,
                          void *(*start_routine) (void *), void *arg);
       pthread_t pthread_self(void);

In my system pthread_t type is "unsigned long int"

/usr/include/x86_64-linux-gnu/bits/pthreadtypes.h:typedef unsigned long int pthread_t;

In the following example, the value returned by pthread_self() and th1 are same.

// main function:
    pthread_t th1;

    if(rc1 = pthread_create(&th1, NULL, &functionC1, NULL))
    {
           printf("Thread creation failed, return code %d, errno %d", rc1, errno);
    }
    printf("Printing thread id %lu\n", th1);

// Thread function: 
    void *functionC1(void *)
    {
            printf("In thread function Printing thread id %lu\n", pthread_self());
    }

    Output:
    Printing thread id 140429554910976
    In thread function Printing thread id 140429554910976

Please check blog Tech Easy for more information on threads.