root/filelock.c

DEFINITIONS

1. main

#include<fcntl.h>
#include<unistd.h>
#include<errno.h>
typedef struct uinfo {
        char name[20];
        int accesscount;
} userinfo;

userinfo rec;


int main() {
        struct flock fl;
        int fd,n,r;

        fd=open("userdb.txt",O_RDWR);

        scanf("%d",&n);
        fl.l_type=F_RDLCK; fl.l_whence=SEEK_SET; 
        fl.l_start=(n-1)*sizeof(userinfo); fl.l_len=sizeof(userinfo);

        r=fcntl(fd,F_SETLK,&fl);
        if (r<0) {
                perror("read lock");
                return (-1);
        } 
        lseek(fd,(n-1)*sizeof(userinfo),SEEK_SET);
        read(fd,&rec,sizeof(rec));
        rec.accesscount++;
        fl.l_type=F_WRLCK;
        r=fcntl(fd,F_SETLK,&fl);
        if (r<0) {
                perror("write lock");
                return (-1);
        } 
        sleep(1);
        lseek(fd,-1*sizeof(userinfo),SEEK_CUR);
        write(fd,&rec,sizeof(rec));
        fl.l_type=F_UNLCK;
        r=fcntl(fd,F_SETLK,&fl);

        return 0;
}