mpi_barrier函式 用於乙個通訊子中所有程序的同步，呼叫函式時程序將處於等待狀態，直到通訊子中所有程序 都呼叫了該函式後才繼續執行。
如：
#include "stdio.h"
#include "string.h"
#include "mpi.h"
int main(int agc,char *agv)
編譯執行結果為：
process 3: 0
process 3: 1
process 3: 2
process 3: 3
process 3: 4
waiting.....
process 3: 5
process 3: 6
process 3: 7
process 3: 8
process 3: 9
process 0: 0
process 0: 1
process 0: 2
process 0: 3
process 0: 4
waiting.....
process 0: 5
process 0: 6
process 0: 7
process 0: 8
process 0: 9
process 1: 0
process 1: 1
process 1: 2
process 1: 3
process 1: 4
waiting.....
process 1: 5
process 1: 6
process 1: 7
process 1: 8
process 1: 9
process 2: 0
process 2: 1
process 2: 2
process 2: 3
process 2: 4
waiting.....
process 2: 5
process 2: 6
process 2: 7
process 2: 8
process 2: 9
可以看到程序直接將0-9列印出來，中間並沒有等待。現在將mpi_barrier的注釋去掉，即：
#include "stdio.h"
#include "string.h"
#include "mpi.h"
int main(int agc,char *agv)
列印輸出如下：
process 3: 0
process 3: 1
process 3: 2
process 3: 3
process 3: 4
waiting.....
process 0: 0
process 0: 1
process 0: 2
process 0: 3
process 0: 4
waiting.....
process 1: 0
process 1: 1
process 1: 2
process 1: 3
process 1: 4
waiting.....
process 2: 0
process 2: 1
process 2: 2
process 2: 3
process 2: 4
waiting.....
process 1: 5
process 1: 6
process 1: 7
process 1: 8
process 1: 9
process 3: 5
process 3: 6
process 3: 7
process 3: 8
process 3: 9
process 0: 5
process 0: 6
process 0: 7
process 0: 8
process 0: 9
process 2: 5
process 2: 6
process 2: 7
process 2: 8
process 2: 9
可以看出來程序在輸出0-4之後等待其它程序完成列印0-4操作之後，再列印5-9.
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