flock

//flock文件上锁，是能够让多个进程安全，合理并按预料同时访问同一文件的方法。

//GNU/Linux====p175

#include<unistd.h>

#include<sys/file.h>

#include<sys/stat.h>

#include<sys/types.h>

#include<stdio.h>

#include<stdlib.h>

void setlock(int fd,int type){

struct flock lock;

char msg[80];

lock.l_whence = SEEK_SET;

lock.l_start = 0;

lock.l_len = 1;

while(1){

lock.l_type = type;

if((fcntl(fd,F_SETLK,&lock))==0)

return;

fcntl(fd,F_GETLK,&lock);

if(lock.l_type != F_UNLCK){

switch(lock.l_type){

case(F_RDLCK):

sprintf(msg,"read lock already set by %d\n",lock.l_pid);

break;

case(F_WRLCK):

sprintf(msg,"write lock already set by %d\n",lock.l_pid);

break;

}

puts(msg);

getchar();

}

}

}

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

int fd;

if((fd=open(argv[1],O_RDWR|O_CREAT,0666))<0){

perror("open ");

exit(-1);

}

setlock(fd,F_RDLCK);

printf("PID %d read %s\n",getpid(),argv[1]);

getchar();

setlock(fd,F_UNLCK);

printf("PID %d unlocked %s\n",getpid(),argv[1]);

getchar();

setlock(fd,F_WRLCK);

printf("PID %d write %s\n",getpid(),argv[1]);

getchar();

close(fd);

exit(0);

}