utmplib.c

#include<stdio.h>
#include<fcntl.h>
#include<sys/types.h>
#include<utmp.h>

#define NRECS 16
#define NULLUT ((struct utmp *)NULL)
#define UTSIZE (sizeof(struct utmp))

static char utmpbuf[NRECS*UTSIZE];
static int num_recs;
static int cur_rec;
static int fd_utmp = -1;

int utmp_open(char *filename)
{
/* 打开文件
 */
	fd_utmp = open(filename,O_RDONLY);
	cur_rec = num_recs = 0;
	return fd_utmp;
}


struct utmp * utmp_next()
{
	struct utmp *recp;
	if(fd_utmp == -1)
		return NULLUT;
	if(cur_rec == num_recs && utmp_reload() ==0)
		return NULLUT;
//一次读取一个结构体
	recp = (struct utmp *)&utmpbuf[cur_rec * UTSIZE];
//当前记录的位置
	cur_rec++;
//结构体内容
	return recp;
}

int utmp_reload()
{
	int amt_read;
	//一次性读取16倍的utmp结构体
	amt_read = read(fd_utmp,utmpbuf,NRECS * UTSIZE);
	
	num_recs = amt_read/UTSIZE;
	cur_rec = 0;
	return num_recs;
}

void utmp_close()
{	
	if(fd_utmp != -1)
		close(fd_utmp);
}