linux使用pid file

UNIX环境编程里面，讲了如何使用pid文件，但是比较隐晦，今天整理一下：

原文如下：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <errno.h>
#include <fcntl.h>

#define PIDFILE "deamon.pid"

#define write_lock(fd,offset,whence,len) \
lock_reg(fd,F_SETLK,F_WRLCK,offset,whence,len)

int lock_reg(int fd, int cmd, int type, off_t offset, int whence, off_t len){
    flock lock;

    lock.l_type = type; // F_RDLCK, F_WRLCK, F_UNLCK
    lock.l_start = offset; // type offset, relative to l_whence
    lock.l_whence = whence;  // SEEK_SET, SEEK_CUR, SEEK_END
    lock.l_len = len;

    return (fcntl(fd, cmd, &lock));
}

// -rw-r--r-- 
// 644
#define FILE_MODE S_IRUSR|S_IWUSR|S_IRGRP|S_IROTH

int main(int argc, char** argv){
    int fd;
    if((fd = open(PIDFILE, O_WRONLY | O_CREAT, FILE_MODE)) < 0){
        printf("open file error!");
        exit(errno);
    }
    
    if(lock_reg(fd, 0, SEEK_SET, 0) < 0){
        if(errno == EACCES || errno == EAGAIN){
            printf("deamon is already running!\n");
            exit(0);
        }
        else{
            printf("write lock error!\n");
            exit(errno);
        }
    }

    if(ftruncate(fd, 0) < 0){
        printf("trancate error!\n");
        exit(errno);
    }

    char buf[11];
    snprintf(buf, sizeof(buf), "%d", getpid());
    if(write(fd, buf, strlen(buf)) != strlen(buf)){
        printf("write our pid error!\n");
        exit(errno);
    }

    int val;
    if((val = fcntl(fd, F_GETFD, 0)) < 0){
        printf("fnctl F_GETFD error!\n");
        exit(errno);
    }
    val |= FD_CLOEXEC;
    if(fcntl(fd, F_SETFD, val) < 0){
        printf("fcntl F_SETFD error!\n");
        exit(errno);
    }

    sleep(3);
    printf("exit!");

    exit(0);
}

直接调用如下：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <errno.h>
#include <fcntl.h>

int main(int argc, char** argv){
    int fd;
    
    // -rw-r--r-- 
    // 644
    int mode = S_IRUSR|S_IWUSR|S_IRGRP|S_IROTH;
    const char* file_name = "deamon.pid";
    
    // open pid file
    if((fd = open(file_name, O_WRONLY | O_CREAT, mode)) < 0){
        printf("open file error!");
        exit(errno);
    }
    
    // require write lock
    flock lock;

    lock.l_type = F_WRLCK; // F_RDLCK, F_WRLCK, F_UNLCK
    lock.l_start = 0; // type offset, relative to l_whence
    lock.l_whence = SEEK_SET;  // SEEK_SET, SEEK_CUR, SEEK_END
    lock.l_len = 0;
    
    if(fcntl(fd, F_SETLK, &lock) < 0){
        if(errno == EACCES || errno == EAGAIN){
            printf("deamon is already running!\n");
            exit(0);
        }
        else{
            printf("write lock error!\n");
            exit(errno);
        }
    }

    // truncate file
    if(ftruncate(fd, 0) < 0){
        printf("trancate error!\n");
        exit(errno);
    }

    // write the pid
    char buf[11];
    snprintf(buf, sizeof(buf), "%d", getpid());
    if(write(fd, buf, strlen(buf)) != strlen(buf)){
        printf("write our pid error!\n");
        exit(errno);
    }

    // auto close when fork child process.
    int val;
    if((val = fcntl(fd, F_GETFD, 0)) < 0){
        printf("fnctl F_GETFD error!\n");
        exit(errno);
    }
    val |= FD_CLOEXEC;
    if(fcntl(fd, F_SETFD, val) < 0){
        printf("fcntl F_SETFD error!\n");
        exit(errno);
    }

    sleep(3);
    printf("exit!");

    exit(0);
}