Question

3. In the main function, pthread create is called to create four child threads. A data file is divided into four blocks. Each child thread handles one block hild . After all four child threads complete, the main thread print out a final result. Write pseudocode to solve the above program. In your code, OVERLOOK the detail of opening a file, divide a file or handle a block. For example, use the comment //open a file to indicate the action of open a file. You code must include pthread create. You can use pthread join or semaphore to synchronize the main thread and child threads. In youe the commnent hrad Jitn

Answer 1

Lets say the data file contains different numbers and we need to find the sum of all numbers in the file
Lets say The data in the file can divided into four equal parts
Now we'll see how to perform this operation in 4 threads
Assume :
   The data is of length 20
   Each block can contain 5 elements each
Operation :
   The threads have thread-id as 0,1,2,3
   using this we can vary the index of the array as follows
   if th-id = 1 , j = 5*1 = 5 to less than 5+5 = 10
   if thid = 2 , j = 5*2 = 10 to less than 5*2+5 = 15
   To generalize for given thread id th , j = 5*th to j<5*th+5 gives the elements in the array for that data block that is to be processed in that particular thread.

PSEUDOCODE :
Initialize global variable sum to 0
sum is used by all the threads
Read the data in the file into an array a
Define a function that is to be passed to thread
   function *sumT(void *threadid):
       1 . Initialize j
       2 . th = (int)threadid
       3 . Loop from j = 5*th to j < 5*th+5
               update sum with a(j)
       4 . print the sum that is calculated in this thread with thread-id
       5 . pthread_exit(NULL) ,exit the thread

Define Main
   main():
       1 . Initialize the number of threads you want
           pthread_t threads[4];
       2 . Loop from i = 0 to 3
               create a thread
               int j = pthread_create(&threads[i],NULL,sumT,(void*)i);
               if(j)
                   print ERROR and ERROR CODE j
                   exit(-1)
       3 . After all threads are executed we need to join them
       4 . Initialize k
       5 . Loop k = 0 to 3
               pthread_join(k,NULL);
       6 . Print the sum variable
       7 . Exit the main thread , pthread_exit(NULL)

Here sum is used as operation to perform in threads, According to your requirement you can change it
The concept remains the same, we need pthread variable for each different thread and a function to execute by the thread and a parameter for that function.2nd parameter to thread_create mostly stays NULL.