Here I will consider firstprivate and lastprivate. Recall one of the earlier entries about private variables. When a variable is declared as private, each thread gets a unique memory address of where to store values for that variable while in the parallel region. When the parallel region ends, the memory is freed and these variables no longer exist. Consider the following bit of code as an example:

#include <stdio.h>
#include <stdlib.h>
#include <omp.h>


int main(void){
int i;
int x;
x=44;
#pragma omp parallel for private(x) 
for(i=0;i<=10;i++){
x=i;
printf("Thread number: %d     x: %d\n",omp_get_thread_num(),x);
}
printf("x is %d\n", x);


}

Yields…

Thread number: 0     x: 0
Thread number: 0     x: 1
Thread number: 0     x: 2
Thread number: 3     x: 9
Thread number: 3     x: 10
Thread number: 2     x: 6
Thread number: 2     x: 7
Thread number: 2     x: 8
Thread number: 1     x: 3
Thread number: 1     x: 4
Thread number: 1     x: 5
x is 44

You’ll notice that x is exactly the value it was before the parallel region.

Suppose we wanted to keep the last value of x after the parallel region. This can be achieved with lastprivate. Replace private(x) with lastprivate(x) and this is the result:

Thread number: 3     x: 9
Thread number: 3     x: 10
Thread number: 1     x: 3
Thread number: 1     x: 4
Thread number: 1     x: 5
Thread number: 0     x: 0
Thread number: 0     x: 1
Thread number: 0     x: 2
Thread number: 2     x: 6
Thread number: 2     x: 7
Thread number: 2     x: 8
x is 10

Notice that it is 10 and not 8. That is to say, it is the last iteration which is kept, not the last operation. Now what if we replace lastprivate(x) with firstprivate(x). What do you think it will do? This:

Thread number: 3     x: 9
Thread number: 3     x: 10
Thread number: 1     x: 3
Thread number: 1     x: 4
Thread number: 1     x: 5
Thread number: 0     x: 0
Thread number: 0     x: 1
Thread number: 0     x: 2
Thread number: 2     x: 6
Thread number: 2     x: 7
Thread number: 2     x: 8
x is 44

If you were like me, you were expecting to get the value 0 i.e. the value of x on the first iteration. NO

firstprivate Specifies that each thread should have its own instance of a variable, and that the variable should be initialized with the value of the variable, because it exists before the parallel construct.

That is, every thread gets its own instance of x and that instance equals 44.

The parallel directive #pragma omp parallel makes the code parallel, that is, it forks the master thread into a number of parallel threads, but it doesn’t actually share out the work.
What we are really after is the parallel for directive, which we call a work-sharing construct. Consider

#include <iostream>
#include <omp.h>

using namespace std;
main (void)
{
	int i;
	int foo;
#pragma omp parallel for
  for(i=1;i<10;i++){
#pragma omp critical
{
foo=omp_get_thread_num();
	  cout << "Loop number: "<< i << " " << "Thread number: " << foo << endl;
}
}
}

The for directive applies to the for loop immediately preceding it. Notice how we don’t have to outline a parallel region with curly braces {} following this directive in contrast to before. This program yields:

[michael@michael lindonslog]$ ./openmp 
Loop number: 1 Thread number: 0
Loop number: 8 Thread number: 3
Loop number: 2 Thread number: 0
Loop number: 3 Thread number: 0
Loop number: 9 Thread number: 3
Loop number: 6 Thread number: 2
Loop number: 4 Thread number: 1
Loop number: 7 Thread number: 2
Loop number: 5 Thread number: 1
[michael@michael lindonslog]$ 

Notice what I said about the order. By default, the loop index i.e. “i” in this context, is made private by the for directive.

At the end of the parallel for loop, there is an implicit barrier where all threads wait until they have all finished. There are however some rules for the parallel for directive

1. The loop index, i, is incremented by a fixed amount each iteration e.g. i++ or i+=step.
2. The start and end values must not change during the loop.
3. There must be no “breaks” in the loop where the code steps out of that code block. Functions are, however, permitted and run as you would expect.
4. The comparison operators may be < <= => >

There may be times when you want to perform some operation in the order of the iterations. This can be achieved with an ordered directive and an ordered clause. Each thread will wait until the previous iteration has finished it’s ordered section before proceeding with its own.

int main(void){
int i,a[10];
#pragma omp parallel for ordered 
for(i=0;i<10;i++){
a[i]=expensive_function(i);
#pragma omp ordered
printf("Thread ID: %d    Hello World %d\n",omp_get_thread_num(),i);
}
}

Will now print out the Hello Worlds in order. N.B. There is a penalty for this. The threads have to wait until the preceding iteration has finished with its ordered section of code. Only if the expensive_function() in this case were expensive, would this be worthwhile.