who命令简单实现

/*
 *【带缓冲who命令简单实现】
 *who:用来查看当前有哪些用户登录到了本台机器上。
 *添加 whats_my_line支持who am i命令
 */
#include<stdio.h>
#include<stdlib.h>
#include<fcntl.h>
#include<utmp.h>
#include<time.h>

#define    UTMP_SIZE    (sizeof(struct utmp))
#define    UTMP_NRECS    16

char utmp_buf[UTMP_NRECS*UTMP_SIZE];

int cur_rec = 0;    //当前记录
int num_recs = 0;    //记录个数

struct utmp * utmp_next(int fd_utmp);
int utmp_reload(int fd_utmp);

char * whats_my_line(int fd);
void show_info(struct utmp * utbuf_fp);


int main(int argc,char *argv[])
{
    int fd_utmp;
    char *my_line = NULL;

    if(argc == 3)                    //who am i情况
        my_line = whats_my_line(0);

    if( (fd_utmp = open(UTMP_FILE,O_RDONLY)) == -1)
    {
        perror("open UTMP_FILE failed");
        exit(1);
    }

    struct utmp * utmp_p;
    while( (utmp_p = utmp_next(fd_utmp)) != NULL)
    {
        if(my_line == NULL || strcmp(utmp_p->ut_line,my_line) == 0)
            show_info(utmp_p);
    }

    if(close(fd_utmp) == -1 )
    {
        perror("close file failed");
        exit(1);
    }
    return 0;
}

struct utmp * utmp_next(int fd_utmp)
{
    if( (cur_rec == num_recs) && (utmp_reload(fd_utmp)==0) )    //缓冲区数据取完
        return (struct utmp *)NULL;

    return (struct utmp *)&utmp_buf[cur_rec++ * UTMP_SIZE];        //下一个记录
}

int utmp_reload(int fd_utmp)
{    //缓冲区数据读取完,重新放数据到缓冲区utmp_buf
    cur_rec = num_recs = 0;
    int bytes_r = read(fd_utmp,utmp_buf,UTMP_SIZE*UTMP_NRECS);
    if(bytes_r == -1)
    {
        perror("read failed");
        return 0;
    }
    num_recs = bytes_r / UTMP_SIZE;

    return num_recs;
}

char * whats_my_line(int fd)
{    //获取当前终端名称
    char * tty_name;
    if( tty_name = ttyname(fd) )
    {
        printf("%s\n",tty_name);
        if( strncmp("/dev/",tty_name,5) == 0 )
            tty_name += 5;
    }
    return tty_name;
}

void show_info(struct utmp * utbuf_fp)
{
    if(utbuf_fp->ut_type != USER_PROCESS)//没有登录的用户
        return;
    printf("%-8.8s ",utbuf_fp->ut_name);
    printf("%-8.8s ",utbuf_fp->ut_line);
    printf("%s",ctime(&utbuf_fp->ut_time));//ctime转换后带\n

}