LinuxQuestions.org - ctime() on ut_tv.tv

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- - ctime() on ut_tv.tv_sec of utmp structure? (https://www.linuxquestions.org/questions/programming-9/ctime-on-ut_tv-tv_sec-of-utmp-structure-823035/)

ctime() on ut_tv.tv_sec of utmp structure?

I'm using ubuntu9.10 (x86_64) and have some code,

Code:

void show_utmp(struct utmp* ent)

{

        time_t tmp;

        if  (ent->ut_type != USER_PROCESS)

                return;

        printf("%10s ", ent->ut_user);

        printf("%10s ", ent->ut_line);

        

        tmp = ent->ut_tv.tv_sec;

        printf("%25s ", ctime(&tmp)); // display ok

        printf("\n");

        printf("%25s", ctime((time_t *)&(ent->ut_tv.tv_sec))); // display wrong

        printf("\n");

}

the output is like the following:
guest tty7 Sat Jul 31 04:37:29 2010

Thu Feb 16 20:35:21 28472119

why?

and in /usr/include/bits/utmp.h, i found sippet:

Code:

#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

I want to know, where __WORDSIZE would be defined?

can you make a compilable example?

sorry for late reply.

the following is the example.

Code:

#include <stdio.h>

#include <utmp.h>

#include <fcntl.h>

#include <time.h>

#include <limits.h>



#define SHOWHOST



#define UTMP "/var/run/utmp"

#define CNT 12

#define UTMPSIZE (sizeof(struct utmp))





void show_utmp(struct utmp* ent);

int open_utmp();

struct utmp* read_utmp(int fd);

int reload_utmp(int fd);





char utmpbuf[CNT * UTMPSIZE];

int total, curent;



int main(int argc, char **argv) {

        int fd;

        struct utmp *entry;

        fd = open_utmp();

        while ( (entry = read_utmp(fd)) != NULL )

                show_utmp(entry);

}



int open_utmp()

{

        int fd;

        if ((fd = open(UTMP, O_RDONLY)) == -1)

        {

                perror("open error");

                return -1;

        }

        curent = 0;

        return fd;

}



struct utmp* read_utmp(int fd)

{

        struct utmp *ut;

        if (curent == total && reload_utmp(fd) <= 0)

                return (struct utmp*)NULL;

        ut = &(utmpbuf[curent * UTMPSIZE]);  

        curent++;

        return ut;

}



int reload_utmp(int fd)

{

        int nsize;

        nsize = read(fd , utmpbuf, CNT*UTMPSIZE);

        if (nsize < 0) {

                perror("reaload utmp error");

                return 0;

        }

        total = nsize / UTMPSIZE;

        curent = 0;

        return total;

}



void show_utmp(struct utmp* ent)

{

        time_t tmp;

        if  (ent->ut_type != USER_PROCESS)

                return;

        printf("%10s ", ent->ut_user);

        printf("%10s ", ent->ut_line);

        tmp = ent->ut_tv.tv_sec;

        printf("%25s ", ctime(&tmp)); 

        printf("\n");

        printf("%25s", ctime((time_t *)&(ent->ut_tv.tv_sec)));

#ifdef SHOWHOST

        printf("%10s", ent->ut_host);

#endif

        printf("\n");

}

ctime() creates a printable date-time string which includes a newline character.

Code:

    The ctime() function adjusts the time value for the current time zone, in the same

    manner as localtime().  It returns a pointer to a 26-character string of the form:



          Thu Nov 24 18:22:48 1986\n\0

Oh wait, you are probably concerned about the formatting itself. My bad.

When compiling with -Wall, your code gives warnings

Code:

fortyfourgalena@desktop1:~/progs/c/lq823035$ gcc -Wall lq823035.c -o lq823035

lq823035.c: In function ‘main’:

lq823035.c:29: warning: control reaches end of non-void function

lq823035.c: In function ‘read_utmp’:

lq823035.c:48: warning: assignment from incompatible pointer type

lq823035.c: In function ‘reload_utmp’:

lq823035.c:56: warning: implicit declaration of function ‘read’

fortyfourgalena@desktop1:~/progs/c/lq823035$

Not said that this is the problem, but the incompatible pointer type is worrying.

And this is the output

Code:

fortyfourgalena@desktop1:~/progs/c/lq823035$ ./lq823035 

fortyfourgalena      tty9 Mon Aug  2 16:59:04 2010

 

Mon Aug  2 16:59:04 2010

        :0

fortyfourgalena      pts/0 Mon Aug  2 17:15:53 2010

 

Mon Aug  2 17:15:53 2010

      :0.0

fortyfourgalena      pts/1 Mon Aug  2 17:17:20 2010

 

Mon Aug  2 17:17:20 2010

      :0.0

fortyfourgalena@desktop1:~/progs/c/lq823035$

the following is my output:
plz put you uname output...I think the problem only appears in 64-bit OS. for ctime() expects a pointer to 64bit, but i give it a pointer to 32bit, so the following 32bit is included in, so produce the wrong result.

guest@bequiet:~/workspaceCpp/PGMPractise$ uname -a

Code:

Linux bequiet 2.6.31-14-generic #48-Ubuntu SMP Fri Oct 16 14:05:01 UTC 2009 x86_64 GNU/Linux

guest@bequiet:~/workspaceCpp/PGMPractise$ make iwho

Code:

cc    iwho.c  -o iwho

iwho.c: In function ‘read_utmp’:

iwho.c:59: warning: assignment from incompatible pointer type

guest@bequiet:~/workspaceCpp/PGMPractise$ ./iwho

Code:

    guest      tty7 Mon Aug  2 22:52:51 2010

 

Fri Jul 16 16:22:27 11813086

        :0

    guest      pts/0 Mon Aug  2 22:55:20 2010

 

Wed Feb 18 04:40:56 81151660

      :0.0

I indeed use a 32-bit version of Ubuntu 8.04

Code:

fortyfourgalena@desktop1:~$ uname -a

Linux desktop1 2.6.24-26-386 #1 Tue Dec 1 17:56:13 UTC 2009 i686 GNU/Linux

fortyfourgalena@desktop1:~$

so, how to write portable program...?