Understanding #pragma omp parallel

Question

I am reading about OpenMP and it sounds amazing. I came at point where the author states that #pragma omp parallel can be used to create a new team of threads. So I wanted to know what difference does #pragma omp parallel mean here. I read that #pragma omp for uses the current team of threads to process a for loop.So I have two examples

First simple example:

 #pragma omp for
 for(int n=0; n<10; ++n)
 {
   printf(" %d", n);
 }
 printf(".\n");

Second example

 #pragma omp parallel
 {
  #pragma omp for
  for(int n=0; n<10; ++n) printf(" %d", n);
 }
 printf(".\n");

My question is are those thread created on the fly every time or once when an application starts also when or why would I want to create a team of more threads ?

Answer 1

Your first example wouldn't compile like that. The "#pragma omp for" advises the compiler to distribute the work load of the following loop within the team of threads which you have to create first. A team of threads is created with the "#pragma omp parallel" statement as you use it in the second example. You can combine the "omp parallel" and "omp for" directives by using "#pragma omp parallel for" The team of threads are created after the parallel statement and are valid within this block.