用I/O标准库实现who命令

linux的who命令实际上是能通过读取utmp文件来获得相关的信息

然后按格式输出来，很简单，主要用来了utmp结构，下面是他的描述

struct utmp {
short ut_type; /* type of login */
pid_t ut_pid; /* PID of login process */
char ut_line[UT_LINESIZE]; /* device name of tty - "/dev/" */
char ut_id[4]; /* init id or abbrev. ttyname */
char ut_user[UT_NAMESIZE]; /* user name */
char ut_host[UT_HOSTSIZE]; /* hostname for remote login */
struct exit_status ut_exit; /* The exit status of a process
marked as DEAD_PROCESS */

/* The ut_session and ut_tv fields must be the same size when
compiled 32- and 64-bit. This allows data files and shared
memory to be shared between 32- and 64-bit applications */
#if __WORDSIZE == 64 && defined __WORDSIZE_COMPAT32
int32_t ut_session; /* Session ID, used for windowing */
struct {
int32_t tv_sec; /* Seconds */
int32_t tv_usec; /* Microseconds */
} ut_tv; /* Time entry was made */
#else
long int ut_session; /* Session ID, used for windowing */
struct timeval ut_tv; /* Time entry was made */
#endif

int32_t ut_addr_v6[4]; /* IP address of remote host */
char __unused[20]; /* Reserved for future use */
};

具体的可以查看utmp.h文件，实现就比较简单了，我就贴个源码算了，呵呵

#include <stdio.h>
#include <utmp.h>
#include <time.h>

void showtime(long);
void show_info(struct utmp*);

int
main ( )
{
struct utmp utbuf;
FILE* fd;
int rdnumber;

if ((fd=fopen(UTMP_FILE,"r"))==NULL )
{
perror(UTMP_FILE);
}

while (fread(&utbuf,sizeof(utbuf),1,fd)==1 )
show_info(&utbuf);
fclose(fd);
return 0;
} /* ----- end of function main ----- */

void show_info ( struct utmp* utbufp )
{
if ( utbufp->ut_type!=USER_PROCESS )
return;

printf("% -8.8s",utbufp->ut_name);
printf(" ");
printf("% -8.8s",utbufp->ut_line);
printf(" ");
showtime(utbufp->ut_time);
#ifdef SHOWHOST
if(utbufp->ut_host[0]!='/0')
printf("(%s)",utbufp->ut_host);
#endif
printf("/n");
} /* ----- end of function show_info ----- */

void showtime ( long timeval )
{
char* cp;
cp=ctime(&timeeval);
printf("%12.12s",cp+4);
} /* ----- end of function showtime ----- */