flock（）的范围是什么？

Question

我正在测试是否从其他线程锁定文件描述符会影响主线程。

#include <iostream>
#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/stat.h>
#include <sys/types.h>
//#include <share.h>
#include <stdio.h>
#include <pthread.h>
#include <errno.h>

void *print_message_function( void *ptr );

pthread_mutex_t mut = PTHREAD_MUTEX_INITIALIZER;

int main (int argc, char * const argv[]) {

    pthread_t thread1, thread2;
const char *message1 = "Thread 1";
const char *message2 = "Thread 2";
int  iret1, iret2;

    iret2 = pthread_create( &thread2, NULL, print_message_function, (void*) message1);

    if(iret2)
    {
        fprintf(stderr,"Error - pthread_create() return code: %d\n",iret2);
        fflush(stdout);
        exit(EXIT_FAILURE);
    }

        printf("pthread_create() for thread 2 returns: %d\n",iret2);

    pthread_mutex_lock(&mut);
    sleep(1);

    std::cout<<"Enter any key:"<<std::endl;
    std::cin >> input;

    if (fd2 = open( "file1",    O_RDWR | O_CREAT | O_TRUNC, 0700 | 0x10) == -1)
    {
        perror("Cannot open file 2.\n"); 
        fflush(stdout);
    }
    else
    {
        printf("File was opened 2!!\n");
        fflush(stdout);
        if(flock(fd2, LOCK_NB | LOCK_EX)==0)
        {
            printf("THE FILE WAS LOCKED 2.\n");
        }
        else if(errno == EAGAIN)
        {
            printf("The file is locked 2. Resource temporarily unavailable.\n");
        }
    }

    pthread_mutex_unlock(&mut);

    close(fd2);
    std::cout<<"File closed."<<std::endl;
    std::cin >> input;
    return 0;
}


void *print_message_function( void *ptr )
{
    char *message;
    message = (char *) ptr;
    printf("%s \n", message); 
    fflush(stdout);

    int fd1;

    if ((fd1 = open( "file1",   O_RDWR | O_CREAT | O_TRUNC, 0700 | 0x10)) == -1)
    {
        perror("thread: Cannot open file 1.\n");
        fflush(stdout);
        //std::cin >> input;
    }
    else
    {
        printf("File was opened 1!!\n");
        fflush(stdout);


        if(flock(fd1, LOCK_NB | LOCK_EX)==0)
        {
            printf("THE FILE WAS LOCKED 1.\n");
        }
        else if(errno == EAGAIN)
        {
            printf("The file is locked 1. Resource temporarily unavailable.\n");
        }
    }

    pthread_mutex_lock(&mut);
    pthread_mutex_unlock(&mut);

    printf("End of thread.\n");
}

输出：

[Session started at 2014-11-26 17:46:16 +0000.]
pthread_create() for thread 2 returns: 0
Thread 1 
File was opened 1!!
THE FILE WAS LOCKED 1.
Enter any key:
1
File was opened 2!!
THE FILE WAS LOCKED 2.
File closed.
End of thread.

我不明白为什么在线程内部使用的flock（）不会阻止在主线程中使用的flock（）获得锁。

从linux手册页：

由flock（）创建的锁与一个打开的文件描述关联（请参见open（2））。这意味着重复的文件描述符（例如，由fork（2）或dup（2）创建）引用相同的锁，并且可以使用这些描述符中的任何一个来修改或释放此锁。此外，通过对这些重复描述符中的任何一个进行显式LOCK_UN操作或在所有此类描述符都已关闭时释放锁定。

Answer 1

错别字。这行错误地设置了fd2（顺便说一句，您没有在发布的代码中声明）为0，即stdin：

if (fd2 = open( "file1",    O_RDWR | O_CREAT | O_TRUNC, 0700 | 0x10) == -1)

因此，fd1和fd2几乎可以肯定是指不同的文件和你的运动的点都将丢失。您的意思是说：

if ((fd2 = open("file1", ...)) == -1)