OpenMP“未使用的变量”编译错误

Question

i tried to write a c++ programm using openmp for parallelization. Unfortunately i get a compiling error which i dont understand. I have listed the g++ command, the problematic lines of code and the error message. If i missed to give important information please let me know.

g++ -o Pogramm -Wall -fopenmp Programm.cpp

#pragma omp parallel
int id,nths,tnbr;
id=omp_get_thread_num();
nths=omp_get_num_thread();

Tree.cpp:52:7: warning: unused variable 'id' [-Wunused-variable]

error: 'id' was not declared in this scope id=omp_get_thread_num();

Can someone tell me why 'id' ist not undeclared?

Answer 1

According to your code, the scope of the parallel region, which is the scope where you define id, only includes the subsequent line, ie the line where you define id. Therefore, when you use the id variable outside, you get an undefined variable error. Besides, you are also getting an unused id variable warning because you are not using it in the parallel region (where you could use it).

Most probably you just forgot to add the curly braces to enlarge the scope to be parallelized alltogether, ie

#pragma omp parallel
{
  int id,nths,tnbr;
  id=omp_get_thread_num();
  nths=omp_get_num_thread();
  ...
}

A minimal working example is:

#include<iostream>
#include<omp.h>
using namespace std;
int main() {
#pragma omp parallel
    {
        int id,nths,tnbr;
        id=omp_get_thread_num();
        nths=omp_get_num_threads();
        cout << "id, nths: " << id << nths << endl;
    }
    return 0;
}

This can be successfully compiled, eg using g++ v. 4.8.5

g++ main.cpp -fopenmp -Wall