进程间通信-文件锁

来源：互联网发布：家电销量数据编辑：程序博客网时间：2024/04/30 01:59

当多个进程都需要对同一个文件进行读写操作的时候，为了确保在进程读写完成前不被其他进程修改，可以使用文件锁机制。

通过函数实现：

int fcntl(int fd, int cmd, struct flock *lock);

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) */
               ...
           };

注意：如果len==0，表示锁定从起点开始到文件尾的区域

锁的兼容性

当前文件区域加读锁加写锁

无锁可以可以

有读锁可以不可以

有写锁不可以不可以

例子：

#include <stdarg.h>#include <stdio.h>#include <sys/types.h>#include <unistd.h>#include <stdlib.h>#include <signal.h>#include <fcntl.h>/*define a function pid_printf*/void pid_printf(char *format, ...){    va_list ap;    va_start(ap, format);    printf("[%d]%d:", getpid(),__LINE__);    vprintf(format, ap);}int set_lock(int fd, int cmd, int type, off_t offset, int whence, off_t len){    struct flock lock;    lock.l_type = type;    lock.l_start = offset;    lock.l_whence = whence;    lock.l_len = len;        return fcntl(fd, cmd, &lock);}/*readlock*/#define read_lock(fd, offset, whence, len) \    set_lock(fd, F_SETLK, F_RDLCK, offset, whence, len)/*readlock wouldblock*/#define readw_lock(fd, offset, whence, len) \    set_lock(fd, F_SETLKW, F_RDLCK, offset, whence, len)/*writelock*/#define write_lock(fd, offset, whence, len) \    set_lock(fd, F_SETLK, F_WRLCK, offset, whence, len)/*writelock wouldblock*/#define writew_lock(fd, offset, whence, len) \    set_lock(fd, F_SETLKW, F_WRLCK, offset, whence, len)/*unlock*/#define un_lock(fd, offset, whence, len) \    set_lock(fd, F_SETLK, F_UNLCK, offset, whence, len)pid_t test_lock(int fd, int type,  off_t offset, int whence, off_t len){    struct flock lock;    lock.l_type = type;    lock.l_start = offset;    lock.l_whence = whence;    lock.l_len = len;        if(fcntl(fd, F_GETLK, &lock) == -1)    {        perror("fcntl");        return -1;    }    if(lock.l_type == F_UNLCK)    {        return 0;    }    return lock.l_pid;}int main(int argc, char *argv[]){    int fd;    pid_t pid;    pid_t lpid;        /*create a file*/    fd=creat("/tmp/flock_test", 0660);    if(fd == -1)    {        perror("create flock_test error!");        return -1;    }    pid_printf("/tmp/flock_test created, fd=[%d]\n", fd);        /*write lock*/    pid_printf("add writelock now\n");    write_lock(fd, 0, SEEK_SET, 0);        /*creat process*/    pid = fork();    if(pid==0)    {        /*子进程会继承父进程的环境和上下文中的大部分内容，包括文件描述符。此时父子进程享有相同的文件偏移量，执行相同的程序读取文件中的字符。程序执行结果是随机的，可能是父进程先读，产生一个偏移，再由子进程读其相邻字符*/        /*while(1)        {            lpid = test_lock(fd, F_RDLCK, 0,SEEK_SET, 0);            if(lpid > 0)            {                pid_printf("process  [%d] locked /tmp/flock_test\n", lpid);            }            else if(lpid ==0)            {                pid_printf("/tmp/flock_test unlock\n");                break;            }            else            {                break;            }            sleep(1);        }*/        writew_lock(fd, 0, SEEK_SET, 0);        pid_printf("subprocess get writelock\n");    }    else if(pid != -1)    {        sleep(3);        pid_printf("unlock write lock of /tmp/flock_test now!\n");        /*unlock*/        un_lock(fd, 0, SEEK_SET, 0);        /*wait subprocess*/        wait(pid, NULL);    }    else    {        printf("there was an error with fork\n");    }        close(fd);    return 0; }/*涉及到的函数主要是fcntl()的应用*/

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