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_IROTHint 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);}