读书笔记：第9章 记录上锁 (6)
《UNIX网络编程：卷2》P171：图9-11
记录上锁的一个常见用途是确保某个程序（如守护程序）在任何时刻只有一个副本在运行。
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/*
* onedaemon.c
* P171 图9-11 确保某个程序只有一个副本在运行
*/
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>
#include <string.h>
#include <errno.h>
#define read_lock(fd, offset, whence, len) \
lock_reg(fd, F_SETLK, F_RDLCK, offset, whence, len)
#define readw_lock(fd, offset, whence, len) \
lock_reg(fd, F_SETLKW, F_RDLCK, offset, whence, len)
#define write_lock(fd, offset, whence, len) \
lock_reg(fd, F_SETLK, F_WRLCK, offset, whence, len)
#define wrtiew_lock(fd, offset, whence, len) \
lock_reg(fd, F_SETLKW, F_WRLCK, offset, whence, len)
#define un_lock(fd, offset, whence, len) \
lock_reg(fd, F_SETLK, F_UNLCK, offset, whence, len)
#define is_read_lockable(fd, offset, whence, len) \
!lock_test(fd, F_RDLCK, offset, whence, len)
#define is_write_lockable(fd, offset, whence, len) \
!lock_test(fd, F_WRLCK, offset, whence, len)
#define FILE_MODE (S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH)
#define MAXLINE 1024
#define PATH_PIDFILE "pidfile"
int lock_reg(int fd, int cmd, int type, off_t offset, int whence, off_t len);
int lock_test(int fd, int type, off_t offset, int whence, off_t len);
int main(int argc, char *argv[])
{
int pidfd;
char line[MAXLINE];
/*
* 打开一个文件并为其上锁
*/
if ((pidfd = open(PATH_PIDFILE, O_RDWR | O_CREAT, FILE_MODE)) < 0) {
fprintf(stderr, "open error: %s\n", strerror(errno));
exit(1);
}
if (write_lock(pidfd, 0, SEEK_SET, 0) < 0) {
if (errno == EACCES || errno == EAGAIN) {
fprintf(stderr, "unable to lock %s, is %s already running ?\n", PATH_PIDFILE, argv[0]);
exit(1);
} else {
fprintf(stderr, "unable to lock %s\n", PATH_PIDFILE);
}
}
snprintf(line, sizeof(line), "%d\n", getpid());
/*
* 把文件截短，然后将本进程PID写入文件
*/
ftruncate(pidfd, 0);
if (write(pidfd, line, strlen(line)) < 0) {
fprintf(stderr, "write error: %s\n", strerror(errno));
exit(1);
}
pause();
exit(0);
}
int lock_reg(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));
}
int lock_test(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)
return(-1);
if (lock.l_type == F_UNLCK)
return(0);
return(lock.l_pid);
}
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运行程序：
$ ./onedaemon & 启动第一个副本 [1] 2847 $ cat pidfile 检查写入文件中的PID 2847 $ ./onedaemon & 尝试启动第二个副本 [2] 2851 $ unable to lock pidfile, is ./onedaemon already running ? [2]+ 退出 1 ./onedaemon