Loop termination question. I hope this is the right forum

[grocery_stocker]

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

static int user_info(struct utmp *user){

setutent();

printf("%12s %8s %22s %14s",
user->ut_user,
user->ut_line,
user->ut_host,
ctime(&user->ut_time)
);
endutent();

return 0;
}

int main(void) {
struct utmp log_file;

FILE *fp;

print_banner();

for((fp =
fopen("/var/log/wtmp","r"))!=0;fread(&log_file,sizeof(struct
utmp),1,fp) !=0 {

user_info(&log_file);


/*if(!feof(fp)){
perror( "End of File \n" );
exit(EXIT_SUCCESS);
}

if(fp == NULL) {
fprintf(stderr,"Can't open file\n");
exit(EXIT_SUCCESS);
}*/
fclose(fp);
}

}


Why does this only output one line from the following:
$./emulate
Name Terminal Hostname Time
reboot ~ 2.6.4-52-default Tue Jun 21 07:38:30 2005
End of File
: Permission denied

Why doesn't the loop go through the entire file?

 

[Chris Dollin]

grocery_stocker wrote:

for((fp =
fopen("/var/log/wtmp","r"))!=0;fread(&log_file,sizeof(struct
utmp),1,fp) !=0 {
(snip)

fclose(fp);

You close `fp`, then you try and `fread` from it.

Oops. All bets are off.

 

[grocery_stocker]

Okay, then one last question.

How come when I change the path from

"/var/log/wtmp" to something like "/home/cdalten/non-existant-file.txt"
I get Segmentation fault? I thought the error would be stored in 'fp'.

Chad

 

[Johan Borkhuis]

Okay, then one last question.

How come when I change the path from

"/var/log/wtmp" to something like "/home/cdalten/non-existant-file.txt"
I get Segmentation fault? I thought the error would be stored in 'fp'.

Because you always do a read, even if the open fails. Just take a good
look at the first part of your for statement.

(SPOILER: your compare doesn't do anything......)

Kind regards,
Johan

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o _____ || http://www.borkhuis.com |
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>(________|__|_[_________]_|________________________________|

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== VxWorks-page: http://www.xs4all.nl/~borkhuis/vxworks/vxworks.html ==

 

[Chris Dollin]

Okay, then one last question.

How come when I change the path from

"/var/log/wtmp" to something like "/home/cdalten/non-existant-file.txt"
I get Segmentation fault? I thought the error would be stored in 'fp'.

Did you read the spec for `fopen`?

 

[Lawrence Kirby]

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

static int user_info(struct utmp *user){

setutent();

This is very Unix specific, a good place to ask would be
comp.unix.programmer.

Lawrence

 

[Tommi Johnsson]

grocery_stocker wrote:





You close `fp`, then you try and `fread` from it.

Oops. All bets are off.

.... you should also open binary file as binary file.

- jt