因为同uptime命令获得的时间没有秒，所以只能自己写一个了；

具体的实现可参见：

http://src.opensolaris.org/source/xref/onnv/onnv-gate/usr/src/cmd/w/w.c

总的思路就是读取UTMPX_FILE，取出其中ut_type为BOOT_TIME用户的ut_xtime，

然后用now减去ut_xtime，就是系统启动时间了；

static long getSysUpTime(void)
{
struct stat sbuf;
size_t size = 0;

struct utmpx ut;
struct utmpx utmpbegin;
struct utmpx utmpend;
struct utmpx utp;

time_t now = 0;
time_t uptime = 0;

//read the UTMP_FILE (contains information about each logged in user)
if (stat(UTMPX_FILE, &sbuf) == -1) {
printf(“read /etc/utmpx file error.”);
exit(1);
}

//get the entry number in UTMPX_FILE;
int entries = sbuf.st_size / sizeof (struct futmpx);
size = sizeof (struct utmpx) * entries;
//malloc memory to store info from UTMPX_FILE;
if ((ut = malloc(size)) == NULL) {
printf(“malloc error when create ut info;”);
exit(1);
}

//prepare to iterator UTMPX_FILE;
(void) utmpxname(UTMPX_FILE);

//iterator UTMPX_FILE

utmpbegin = ut;
utmpend = (struct utmpx )((char )utmpbegin + size);

setutxent();
while ((ut < utmpend) && ((utp = getutxent()) != NULL))
(void) memcpy(ut++, utp, sizeof (*ut));
endutxent();

//get current time;
(void) time(&now);
//iterator utmp entry in memory;
for (ut = utmpbegin; ut < utmpend; ut++)
{
if (ut->ut_type == BOOT_TIME)
{
//this entry is boot_time, so get uptime with now;
uptime = now - ut->ut_xtime;
}
}

printf(“%f”, uptime);
//time is sencond 100;
return uptime100;
}