文件锁定程序未锁定文件

问题描述

我制作了一个程序来在 c 程序中提供建议文件锁定，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};
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);
}
}

程序被制作成当有另一次尝试在另一个进程中打开该特定文件时，它会发出建议锁定。然而，新用户（来自新进程）可以编辑文件。

但是程序运行不正常。任何人都可以帮我找到代码中的错误。我无法确定我做错了什么？

解决方法

那个程序还能编译吗？建议锁只是建议性的，它们不会阻止其他进程打开文件或进行读/写。它们只告诉您何时使用文件描述符安全。因此，使用它们的方法是仅在您实际拥有锁时才进行读/写操作。

在这种情况下“安全”的含义不能有一个通用的答案，这取决于用例。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。