在Linux下,如果不加限制,同一个程序,可以有多个运行实例,也称之为进程.它们都有一样的名字,执行着一样的代码段.不同的是,它们拥有不同的pid以及进程空间.有时候,希望同一时间只能创建一个进程.下面这段示例代码就加了这样一个限制.
核心点:
1. 进程在启动时,判断/tmp/my_pid_file是否存在;如果不存在,则将当前进程的pid写入,程序继续运行;
2. 如果/tmp/my_pid_file已经存在但无内容,则将当前进程的pid写入,程序继续运行;
3. 如果/tmp/my_pid_file已经存在且有内容,读取并判断该值指向的pid是否正在运行。如果没有,则将当前进程的pid写入,程序继续运行;否则,当前进程退出。使用了 kill(pid, 0) 进行检测.
4. 在收到SIGINT和SIGTERM时,先删除/tmp/my_pid_file,再退出.
single_app.c:
#include <stdio.h>
#include <stdlib.h>
#include <signal.h>
#include <string.h>
#include <errno.h>
#include <fcntl.h>
#include <sys/types.h>
#include <unistd.h>
#define MY_PID_FILE "/tmp/my_pid_file"
#define BUF_LEN_FOR_PID 64
static int write_pid_into_fd(int fd, pid_t pid)
{
int ret = -1;
char buf[BUF_LEN_FOR_PID] = {0};
/* Move cursor to the start of file. */
lseek(fd, 0, SEEK_SET);
sprintf(buf, "%d", pid);
ret = write(fd, buf, strlen(buf));
if(ret <= 0) { /* Write fail or write 0 byte */
if(ret == -1)
perror("Write "MY_PID_FILE" fail\n");
ret = -1;
} else {
printf("Create "MY_PID_FILE" ok, pid=%d\n", pid);
ret = 0;
}
return ret;
}
/*
* Create MY_PID_FILE, write pid into it.
*
* @return: 0 is ok, -1 is error.
*/
static int create_pid_file(pid_t pid)
{
int fd, ret;
char buf[BUF_LEN_FOR_PID] = {0};
fd = open(MY_PID_FILE, O_WRONLY | O_CREAT | O_EXCL, 0666); /* rw-rw-rw- */
if(fd == -1) {
perror("Create "MY_PID_FILE" fail\n");
return -1;
}
ret = flock(fd, LOCK_EX);
if(ret == -1) {
perror("flock "MY_PID_FILE" fail\n");
close(fd);
return -1;
}
ret = write_pid_into_fd(fd, pid);
flock(fd, LOCK_UN);
close(fd);
return ret;
}
/*
* If pid file already exists, check the pid value in it.
* If pid from file is still running, this program need exit();
* If it is not running, write current pid into file.
*
* @return: 0 is ok, -1 is error.
*/
static int check_pid_file(int fd, pid_t pid)
{
int ret = -1;
pid_t old_pid;
char buf[BUF_LEN_FOR_PID] = {0};
ret = flock(fd, LOCK_EX);
if(ret == -1) {
perror("flock "MY_PID_FILE" fail\n");
return -1;
}
ret = read(fd, buf, sizeof(buf)-1);
if(ret < 0) { /* read error */
perror("read from "MY_PID_FILE" fail\n");
ret = -1;
} else if(ret > 0) { /* read ok */
old_pid = atol(buf);
/* Check if old_pid is running */
ret = kill(old_pid, 0);
if(ret < 0) {
if(errno == ESRCH) { /* old_pid is not running. */
ret = write_pid_into_fd(fd, pid);
} else {
perror("send signal fail\n");
ret = -1;
}
} else { /* running */
printf("Program already exists, pid=%d\n", old_pid);
ret = -1;
}
} else if(ret == 0) { /* read 0 byte from file */
ret = write_pid_into_fd(fd, pid);
}
flock(fd, LOCK_UN);
return ret;
}
/*
* It will create the only one pid file for app.
*
* @return: 0 is ok, -1 is error.
*/
static int init_pid_file()
{
pid_t pid;
int fd, ret;
pid = getpid();
fd = open(MY_PID_FILE, O_RDWR);
if(fd == -1) { /* open file fail */
if(errno == ENOENT) { /* No such file. Create one for this program. */
ret = create_pid_file(pid);
} else {
perror("open "MY_PID_FILE" fail\n");
ret = -1;
}
} else { /* pid file already exists */
ret = check_pid_file(fd, pid);
close(fd);
}
return ret;
}
static void sigHandler(int sig)
{
if(sig == SIGINT || sig == SIGTERM)
remove(MY_PID_FILE);
_exit(0);
}
int main()
{
if(-1 == init_pid_file()) {
exit(-1);
}
/* Ctrl + C */
if(signal(SIGINT, sigHandler) == SIG_ERR) {
exit(-1);
}
/* kill pid / killall name */
if(signal(SIGTERM, sigHandler) == SIG_ERR) {
exit(-1);
}
while(1)
sleep(3);
return 0;
}