OpenMP threadprivate问题error: automatic variable ‘a1’ cannot be ‘threadprivate’

Question

RT

    int a1;
    #pragma omp threadprivate(a1)

error: automatic variable ‘a1’ cannot be ‘threadprivate’

貌似加上static 就可以了。 threadprivate必须要全局变量或静态变量？

但是线程内的一份拷贝而已，又不能改变a1的值，那这和private还有啥区别呢？

Answer 1

threadprivate指定的私有变量是全生命周期的，而不是仅限于当前语句块。
比如下面这段程序：

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

int a = 10, i;

int main(){
#pragma omp threadprivate(a)
    {
#pragma omp parallel for
        for(i = 0; i<4; i++){
            i = omp_get_thread_num();
            a = i+2;
        }
    }
    printf("%d\n", a);
#pragma omp threadprivate(a)
    {
#pragma omp parallel for
        for(i = 0; i<4; i++){
            printf("tid = %d, a = %d\n", omp_get_thread_num(), a);
        }
    }
    return 0;
}

其输出是：

2
tid = 2, a = 4
tid = 0, a = 2
tid = 3, a = 5
tid = 1, a = 3