文件锁的应用

NAME
       fcntl - manipulate file descriptor

SYNOPSIS
       #include
       #include

       int fcntl(int fd, int cmd, ... /* arg */ );

DESCRIPTION
       fcntl()  performs  one  of the operations described below on the open file
       descriptor fd.  The operation is determined by cmd.
 

#include
#include
#include
#include
#include

#define FILE_NAME "flock.txt"

int set_lock(const int fd,const int type){
    printf("pid: %d\n",getpid());

    struct flock fflock;
    memset(&fflock ,0,sizeof(struct flock));
    fcntl(fd,F_GETLK,&fflock);
    if(fflock.l_type!=F_UNLCK){
        if(fflock.l_type==F_RDLCK){
            printf("flock has been set to read lock by %d\n",fflock.l_pid);
        }else if(fflock.l_type==F_WRLCK){
            printf("flock has been set to write lock by %d\n",fflock.l_pid);
        }
    }

    fflock.l_type=type;
    fflock.l_whence=SEEK_SET;
    fflock.l_start=0;
    fflock.l_len=0;
    fflock.l_pid=-1;

    if(fcntl(fd,F_SETLKW,&fflock)<0){
        printf("set lock failed\n");
        return -1; 
    }

    switch(fflock.l_type){
        case F_RDLCK:
            printf("read lock is set by %d\n",getpid());
            break;
        case F_WRLCK:
            printf("read lock is set by %d\n",getpid());
            break;
        case F_UNLCK:
            printf("read lock is set by %d\n",getpid());
            break;
    }
    printf("Process pid =%d out. \n",getpid());
    return 0;
}

int main(void){

    int fd=open(FILE_NAME,O_RDWR|O_CREAT,0666);
    if(fd<0){
	printf("file: %s open failed!!!\n",FILE_NAME);
	exit(-1);
    }

    //lock
    set_lock(fd,F_WRLCK);//对文件上写锁
    getchar();

    //unlock
    set_lock(fd,F_UNLCK);//解锁
    getchar();
  
    return 0;
} 

 对上述代码进行参数修改(F_RDLCK,F_WRLCK,F_UNLCK)可观察不同文件锁的权限影响.

结论:当对文件上写锁后,则不可以再对文件写或读.

当对文件上读锁后,则不可以再对文件写,但可以读.