c - 除非在使用输出重定向时调用显式 fflush 函数，否则线程不会给出输出

Question

我正在编写一个多线程程序。程序结构如下所示

int main()
{

    printf("To Call threads\n");
    thread1=pthread_create(&trd1,NULL,process1,(void *)sleepTimeForP1);
    thread2=pthread_create(&trd2,NULL,process2,(void *)sleepTimeForP2);
    pthread_join(trd1, NULL);
    pthread_join(trd2, NULL);
    return 0;
}

线程的代码是

void *process1 (void *sleepTimeForP1)
{
    int *tsleepTimeForP1 = (int *)sleepTimeForP1;
    while(1)
    {
        pthread_mutex_lock(&lock);
        printf("inside thread\n");
        pthread_mutex_unlock(&lock);
        sleep(1);
    }

}

该代码工作正常，并在未完成输出重定向时打印输出。但是当给出输出重定向时，文件中没有输出。现在，如果我在每个 printf() 语句之后给出显式 fflush(stdout)，我就会得到所需的输出。为什么我需要给出明确的 fflush() ？

Answer 1

来自 C99 §7.19.3[7]:

As initially opened, the standard error stream is not fully buffered; the standard input and standard output streams are fully buffered if and only if the stream can be determined not to refer to an interactive device.

“完全缓冲”(相对于“行缓冲”):C99 §7.19.3[3]

When a stream is fully buffered, characters are intended to be transmitted to or from the host environment as a block when a buffer is filled. When a stream is line buffered, characters are intended to be transmitted to or from the host environment as a block when a new-line character is encountered.

因此，当标准输出是交互式的(例如 shell)时，它是行缓冲的。但是，当您将它重定向到一个文件时，它会变成完全缓冲的，您必须使用 fflush。