The basic parallel construct

Let's see the classic OpenMP "Hello World!" program:

#include <stdio.h>

int main(void)
{
    #pragma omp parallel
    {
        printf("Hello World!\n");
    }

    return 0;
}

Check the kernel part:

#pragma omp parallel
{
    printf("Hello World!\n");
}

The whole part is called construct. The first line, "#pragma omp parallel" is compiler directive, and parallel is the directive name. The rest part, "{ ... }", is called structured block. The directive applies structured block, which means "{ printf("Hello World!\n"); }" will run in parallelism (In this simple example, the '{' and '}' are not necessary, but I think it is a good habit to keep them.).

Compile and run the program on one 2-core machine:

# gcc -fopenmp hello.c
# ./a.out
Hello World!
Hello World!

The "Hello World!" is printed twice. Run it on another 24-core machine:

# gcc -fopenmp hello.c
# ./a.out
Hello World!
Hello World!
......
Hello World!

This time, "Hello World!" is outputted 24 times. It seems the thread number is always equal to CPU cores, but we should not assume it. The algorithm of generating how many threads is a little complicated, and the spec has a detailed description of it.

We can use num_threads clause to set the requested number of threads. Change the directive as following:

#pragma omp parallel num_threads(4)

compile again and run the program on both the 2-core and 24-core machines, and the "Hello World!" is outputted 4 times. Please notice that the Operating System will try best to allocate num_threads threads, but due to the limitation on the resources, it is not guaranteed. For example:

#pragma omp parallel num_threads(300000)

The above directive will cause program crash on my 2-core machine.

In fact, the general format OpenMP directive for C/C++ is like this:

#pragma omp directive-name [clause[ [,] clause] ... ] new-line 

"#pragma omp" identifies that this is used for OpenMP; every directive must have one directive-name (e.g., parallel.), and 0 or more clauses (e.g., num_threads.) follow it. At the end of directive, there should be a new-line character, e.g, on Unix, it is \n.