who命令的實現

[cpp] #include <stdio.h>  
#include <stdlib.h>  
#include <utmp.h>  
#include <fcntl.h>  
#include <unistd.h>  
#include <time.h>  
 
void show_info(struct utmp *); 
void showtime(long); 
 
int main() 
{ 
    int         fd; 
    struct utmp current_record; 
    int         reclen = sizeof(struct utmp); 
 
    /* UTMP_FILE就是/var/run/utmp.在/usr/include/paths.h下 */ 
    fd = open(UTMP_FILE, O_RDONLY); 
    if (fd == -1) 
    { 
        perror(UTMP_FILE); 
        exit(1); 
    }    
 
    while (read(fd, ¤t_record, reclen)) 
        show_info(¤t_record); 
     
    close(fd); 
    return 0; 
} 
 
/* displays the contents of the utmp struct, display nothing if utmp has no user name */ 
void show_info(struct utmp *utbufp) 
{ 
    if (utbufp->ut_type != USER_PROCESS) 
        return; 
 
    printf("%-8.8s", utbufp->ut_user);  /* The user name */ 
    printf(" "); 
    printf("%-8.8s", utbufp->ut_line);  /* The tty name */ 
    printf(" "); 
    showtime(utbufp->ut_time);          /* utime */ 
#ifdef SHOWHOST  
    if (utbufp->ut_host[0] != '\0') 
        printf(" (%s)", utbufp->ut_host);   /* the host */ 
#endif  
        printf("\n"); 
} 
 
/* displays time in a format */ 
void showtime(long timeval) 
{ 
    char *cp; 
    cp = ctime(&timeval);       /* convert time to string */ 
    printf("%20.20s", cp+4);    /* %12.12s prints a string 20 chars wide and LIMITS it to 20 chars. cp+4是為了把最前面的星期去掉 */ 
} 

#include <stdio.h>
#include <stdlib.h>
#include <utmp.h>
#include <fcntl.h>
#include <unistd.h>
#include <time.h>

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

int main()
{
    int         fd;
    struct utmp current_record;
    int         reclen = sizeof(struct utmp);

    /* UTMP_FILE就是/var/run/utmp.在/usr/include/paths.h下 */
    fd = open(UTMP_FILE, O_RDONLY);
    if (fd == -1)
    {
        perror(UTMP_FILE);
        exit(1);
    }  

    while (read(fd, ¤t_record, reclen))
        show_info(¤t_record);
   
    close(fd);
    return 0;
}

/* displays the contents of the utmp struct, display nothing if utmp has no user name */
void show_info(struct utmp *utbufp)
{
    if (utbufp->ut_type != USER_PROCESS)
        return;

    printf("%-8.8s", utbufp->ut_user);  /* The user name */
    printf(" ");
    printf("%-8.8s", utbufp->ut_line);  /* The tty name */
    printf(" ");
    showtime(utbufp->ut_time);          /* utime */
#ifdef SHOWHOST
    if (utbufp->ut_host[0] != '\0')
        printf(" (%s)", utbufp->ut_host);   /* the host */
#endif
        printf("\n");
}

/* displays time in a format */
void showtime(long timeval)
{
    char *cp;
    cp = ctime(&timeval);       /* convert time to string */
    printf("%20.20s", cp+4);    /* %12.12s prints a string 20 chars wide and LIMITS it to 20 chars. cp+4是為了把最前面的星期去掉 */
}

增加了緩沖機制的who命令實現：這樣會減少read()函數調用的次數，提高程序效率和性能。

主函數為：who1.c

[cpp]  #include <stdio.h>  
#include <stdlib.h>  
#include <utmp.h>  
#include <fcntl.h>  
#include <unistd.h>  
#include <time.h>  
#include </root/utmplib.c>  /* 包含實現緩沖的文件 */  
 
void show_info(struct utmp *); 
void showtime(long); 
 
int main() 
{ 
    int         fd; 
    struct utmp *utbufp; 
    int         reclen = sizeof(struct utmp); 
 
    /* UTMP_FILE就是/var/run/utmp.在/usr/include/paths.h下 */ 
    if (utmp_open(UTMP_FILE) == -1) 
    { 
        perror(UTMP_FILE); 
        exit(1); 
    } 
 
    while((utbufp = utmp_next()) != ((struct utmp *)NULL)) 
        show_info(utbufp); 
    utmp_close(); 
    return 0; 
} 
 
/* displays the contents of the utmp struct, display nothing if utmp has no user name */ 
void show_info(struct utmp *utbufp) 
{ 
    if (utbufp->ut_type != USER_PROCESS) 
        return; 
 
    printf("%-8.8s", utbufp->ut_user);  /* The user name */ 
    printf(" "); 
    printf("%-8.8s", utbufp->ut_line);  /* The tty name */ 
    printf(" "); 
    showtime(utbufp->ut_time);          /* utime */ 
#ifdef SHOWHOST  
    if (utbufp->ut_host[0] != '\0') 
        printf(" (%s)", utbufp->ut_host);   /* the host */ 
#endif  
        printf("\n"); 
} 
/* displays time in a format */ 
void showtime(long timeval) 
{ 
    char *cp; 
    cp = ctime(&timeval);       /* convert time to string */ 
    printf("%20.20s", cp+4);    /* %12.12s prints a string 20 chars wide and LIMITS it to 20 chars. cp+4是為了把最前面的星期去掉 */ 
} 

#include <stdio.h>
#include <stdlib.h>
#include <utmp.h>
#include <fcntl.h>
#include <unistd.h>
#include <time.h>
#include </root/utmplib.c>  /* 包含實現緩沖的文件 */

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

int main()
{
    int         fd;
    struct utmp *utbufp;
    int         reclen = sizeof(struct utmp);

    /* UTMP_FILE就是/var/run/utmp.在/usr/include/paths.h下 */
    if (utmp_open(UTMP_FILE) == -1)
    {
        perror(UTMP_FILE);
        exit(1);
    }

    while((utbufp = utmp_next()) != ((struct utmp *)NULL))
        show_info(utbufp);
    utmp_close();
    return 0;
}

/* displays the contents of the utmp struct, display nothing if utmp has no user name */
void show_info(struct utmp *utbufp)
{
    if (utbufp->ut_type != USER_PROCESS)
        return;

    printf("%-8.8s", utbufp->ut_user);  /* The user name */
    printf(" ");
    printf("%-8.8s", utbufp->ut_line);  /* The tty name */
    printf(" ");
    showtime(utbufp->ut_time);          /* utime */
#ifdef SHOWHOST
    if (utbufp->ut_host[0] != '\0')
        printf(" (%s)", utbufp->ut_host);   /* the host */
#endif
        printf("\n");
}
/* displays time in a format */
void showtime(long timeval)
{
    char *cp;
    cp = ctime(&timeval);       /* convert time to string */
    printf("%20.20s", cp+4);    /* %12.12s prints a string 20 chars wide and LIMITS it to 20 chars. cp+4是為了把最前面的星期去掉 */
}實現緩沖的文件：utmplib.c

[cpp]  #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];    /* storage */ 
static int  num_recs;                   /* num stored */ 
static int  cur_rec;                    /* next to go */ 
static int  fd_utmp = -1;               /* read from */ 
 
utmp_open(char *filename) 
{ 
    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; 
         
    recp = (struct utmp *)&utmpbuf[cur_rec * UTSIZE];   /* 依次獲取下一個記錄的地址 */ 
    cur_rec++;       
    return recp;         
}                            
                                 
                                     
int utmp_reload()                        
{ 
    int     amt_read;                        
                                                         
    /* 不是第一次讀時都是從上次讀到的位置繼續讀 */ 
    amt_read = read(fd_utmp, utmpbuf, NRECS * UTSIZE);           
  
    num_recs = amt_read/UTSIZE;     /* 讀到的真實個數 */ 
 
    cur_rec = 0;                    /* 重置為0 */ 
    return num_recs; 
} 
utmp_close() 
{ 
    if (fd_utmp != -1)  /* 只有在打開之後才關閉 */ 
        close(fd_utmp); 
} 

#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];    /* storage */
static int  num_recs;                   /* num stored */
static int  cur_rec;                    /* next to go */
static int  fd_utmp = -1;               /* read from */

utmp_open(char *filename)
{
    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;
       
    recp = (struct utmp *)&utmpbuf[cur_rec * UTSIZE];   /* 依次獲取下一個記錄的地址 */
    cur_rec++;     
    return recp;       
}                          
                               
                                   
int utmp_reload()                      
{
    int     amt_read;                      
                                                       
    /* 不是第一次讀時都是從上次讀到的位置繼續讀 */
    amt_read = read(fd_utmp, utmpbuf, NRECS * UTSIZE);         
 
    num_recs = amt_read/UTSIZE;     /* 讀到的真實個數 */

    cur_rec = 0;                    /* 重置為0 */
    return num_recs;
}
utmp_close()
{
    if (fd_utmp != -1)  /* 只有在打開之後才關閉 */
        close(fd_utmp);
}