Fork-Join parallel execution model

OpenMP uses Fork-Join parallel execution model. Take the following code as an example:

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

int main(void)
{
    printf("Before: total thread number is %d\n", omp_get_num_threads());

    #pragma omp parallel
    {
        printf("Thread id is %d\n", omp_get_thread_num());
    }

    printf("After: total thread number is %d\n", omp_get_num_threads());

    return 0;
}

Build and run it on 2-core machine:

# gcc -fopenmp parallel.c
# ./a.out
Before: total thread number is 1
Thread id is 0
Thread id is 1
After: total thread number is 1

Let's analyze the code step by step:

(1) After starting the program, there is only main thread in progress, and it is called master thread. omp_get_num_threads() is used to get current thread number. So the first output line is:

Before: total thread number is 1

(2) When master thread encounters the following construct:

#pragma omp parallel
{
    printf("Thread id is %d\n", omp_get_thread_num());
}

It will spawn a group of child thread (in our case, just 1 child thread) to run the code in structured block concurrently, so master thread is parent thread simultaneously. The parent thread and group of child thread are called team together. Every thread will get a thread ID, which can be obtained from omp_get_thread_num() function. Master thread's ID is 0, other threads get ID from 1, 2, .... So the following outputs from threads make sense:

Thread id is 0
Thread id is 1

(3) After finish executing structured block, the progress transit from parallel to sequential running, and that's the meaning of "Fork-Join", so the last output shows only master thread is active here:

After: total thread number is 1

P.S., although the above log shows there is only one thread alive, but since most OpenMP implementation use thread pool to get performance gain, so the child thread is in idle state, not exit. Use gdb can verify it:

(1) Build program with debug information:

# gcc -g -fopenmp parallel.c

(2) Use gdb to debug program and set breakpoint in "return 0;":

# gdb a.out
......
(gdb) b parallel.c:15
Breakpoint 1 at 0x7f9: file parallel.c, line 15.
(gdb) r

(3) After the breakpoint is hit, check the thread number:

(gdb) i threads
  Id   Target Id         Frame
* 1    Thread 0x7ffff7febc00 (LWP 24412) "a.out" main () at parallel.c:15
  2    Thread 0x7ffff73cf700 (LWP 24416) "a.out" do_spin (val=8, addr=0x5555557566d4)
    at /build/gcc/src/gcc/libgomp/config/linux/wait.h:56

We can see there are 2 threads.