文件锁定程序不锁定文件

Question

我已经制作了一个程序来在 c 程序中锁定咨询文件，下面是我为该任务编写的代码。

#include <stdio.h>
#include <stdlib.h>
#include <sys/file.h>
#include <fcntl.h>
#include <unistd.h>
#include <errno.h>


int controller, fsize;
char data[1000], ch;
int fd ;
                  /* l_type, l_whence, l_start, l_len, l_pid*/
struct flock fl = {F_UNLCK, SEEK_SET,    0,       0,      0};
void Edit();
void Delete();
void Exit();
void Lock();


void main()
{

do
{
    clrscr();
    fd = open("demo.txt", "w");
    FILE *fp = fdopen("demo.txt", "w");
    if (fd != NULL)
    {
        printf("Opening the file!\n");
        fd = fdopen("demo.txt", "r");
        printf("We can open the file in write mode, meaning no other process has lock enabled on it.");
        printf("Contents of the file are: \n\t");
        readfile(fd);
        fclose(fd);
    }
    else if ((fd = fopen("demo.txt", "r"))!= NULL)
    {
        fd = fopen("demo.txt", "r");
        printf("We can open the file in read mode, meaning some other process has locked the write permissions on it.");
        printf("Contents of the file are: \n\t");
        readfile(fd);
        fclose(fd);
    }
    else
    {
        printf("File does not exist.\n");
    }
    printf("\n\t\t***** WELCOME USER! THIS IS A SIMPLE TEXT EDITOR *****");

    Lock();
    Operations();
}
while(1);
}

void Lock()
{
fd = fopen("demo.txt", "w");
fl.l_type = F_RDLCK;
fl.l_pid = getpid();
if (fcntl(fd, F_SETLK, &fl) == -1)
{
    printf("Can't set exclusive lock\n");
}
else if(fl.l_type!=F_UNLCK)
{
    printf("File has been exclusively locked by the process %d\n", fl.l_pid);
    fl.l_type = F_UNLCK;
    printf("File Unlocked! Other processes can execute on the file now.");
}
else
{
    printf("File is not locked\n");
}
fclose(fd);
}

void clrscr()
{
system("@cls||clear");
}

void Operations()
{
    printf("\n\n\tOperations you can perform here:\n\t\n");
    printf("\n\t1.ADD TO FILE\n\t2.DELETE THE FILE\n\t3.EXIT\n");
    printf("\n\tEnter your choice: ");
    scanf("%d",&controller);
    switch(controller)
    {
    case 1:
        Add();
        break;
    case 2:
        remove("demo.txt");
        break;
    case 3:
        exit(0);
    }
}



void Add()
{
    fd = fopen("demo.txt", "a");
    printf("Enter contents to store in file : \n");
    fgets(data, 1000, stdin);
    fputs(data, fd);
    fclose(fd);
    printf("Data added to the file successfully");
    fd = fopen("demo.txt", "r");
    readfile(fd);
    fclose(fd);
}



void readfile(FILE *fPtr)
{
char c = getc(fd);
while (c != EOF)
{
    printf("%c", c);
    c = getc(fd);
}
}

该程序是这样设计的，当在另一个进程中再次尝试打开该特定文件时，它会发出建议锁定。然后，新用户（来自新进程）可以编辑该文件。

但程序运行不正常。谁能帮我找出代码中的错误。我无法确定我做错了什么？

Answer 1

该程序甚至可以编译吗？咨询锁只是咨询性的，它们不会阻止其他进程打开文件或进行读/写。它们只会告诉您何时可以安全 使用文件描述符。所以使用它们的方法是，只有在你真正拥有锁时才进行读/写。

在这种情况下，“安全”的含义不能有一个通用的答案，它取决于用例。但通常意味着程序员已确保文件在获取锁之前和释放锁之后处于一致状态。