linux fcntl 文件锁,七、Linux系统编程-文件和IO(五)fcntl函数及常用操作、文件锁

一、fcntl函数及常用操作

(1)、函数声明

#include

#include

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

(2)、功能：操纵文件描述符，改变已打开的文件的属性

复制文件描述符 F_DUPFD(long)

文件描述符标志 F_GETFD(void) F_SETFD(long)

文件状态标志 F_GETFL(void) F_SETFL(long)

文件锁 F_GETLK, F_SETLK,F_SETLKW

示例：

1、复制文件描述符

#include

#include

#include

#include

#include

#include

#include

#include

#define ERR_EXIT(m)

do

{

perror(m);

exit(EXIT_FAILURE);

}while(0)

int main()

{

int fd;

char buf[1024];

fd = open("aa.txt",O_WRONLY);

if (fd == -1)

ERR_EXIT("open error");

close(1);

fcntl(fd,F_DUPFD,0);

dup(fd);

printf("AAAGGn");

return 0;

}2、设置清除文件状态标志

#include 

#include

#include

#include

#include

#include

#include

#include

#define ERR_EXIT(m)

do

{

perror(m);

exit(EXIT_FAILURE);

}while(0)

void set_flag(int fd,int flag);

void cls_flag(int fd,int flag);

int main()

{

char buf[512];

read(0,buf,5);

printf("%sn",buf);

set_flag(0,O_NONBLOCK);

cls_flag(0,O_NONBLOCK);

read(0,buf,5);

return 0;

}

void set_flag(int fd, int flag)

{

int val;

val = fcntl(fd,F_GETFL,0);

if (val == -1)

ERR_EXIT("fcntl get flag error");

val |= flag;

if (fcntl(fd,F_SETFL,val) < 0)

ERR_EXIT("fcntl set flag error");

}

void cls_flag(int fd,int flag)

{

int val;

val = fcntl(fd,F_GETFL,0);

if (val == -1)

ERR_EXIT("fcntl get flag error");

val &= ~flag;

if (fcntl(fd,F_SETFL,val) < 0)

ERR_EXIT("fcntl set flag error");

}

3、设置清除文件文件锁

文件锁结构：

struct flock {

...

short l_type; /* Type of lock: F_RDLCK,

F_WRLCK, F_UNLCK */

short l_whence; /* How to interpret l_start:

SEEK_SET, SEEK_CUR, SEEK_END */

off_t l_start; /* Starting offset for lock */

off_t l_len; /* Number of bytes to lock */

pid_t l_pid; /* PID of process blocking our lock

(F_GETLK only) */

...

};设置和清除锁示例：

#include

#include

#include

#include

#include

#include

#include

#include

#define ERR_EXIT(m)

do

{

perror(m);

exit(EXIT_FAILURE);

} while(0)

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

{

int fd;

if ((fd = open("aa.txt",O_CREAT | O_RDWR | O_TRUNC,0644)) == -1)

ERR_EXIT("open error");

struct flock lock;

memset(&lock,0,sizeof(lock));

lock.l_type = F_WRLCK;

lock.l_whence = SEEK_SET;

lock.l_start = 0;

lock.l_len = 0;

if (0 != (fcntl(fd,F_SETLK,&lock)) ) //如果设置F_SETLKW，则会阻塞直到获得锁

ERR_EXIT("lock error");

getchar();

lock.l_type = F_UNLCK;

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

ERR_EXIT("unlock error");

write(fd,"AAA",3);

return 0;

}设置一个排他锁之后再次执行这个程序就会出现lock error: Resource temporarily unavailable错误。之后输入一个字符解锁之后，其他进程才能访问该文件。