Need Help Programming in C comparing times together - c

Question

i need some advice and help with a plugin i am writing for Nagios.
i am writing the plugin in C, but have no previous experience in the language except for minor amounts whilst trying to get this plugin to work.
basically what i am trying to-do is the following.
read a text file that is generated on a remote PC by an application i have written, this program writes nothing more than 5 characters into the file, the first 4 chars are the time in 24 hour format. e.g. 22:30 > 10:30pm
it then needs to take these 4 characters convert them into a time and compare it to the current system time (if there is a difference of 5 mins then it generates a reply to nagios to flag a warning).
I have tried many different ways of doing this, my first attempt was to convert the characters into an integer, then convert the time into an integer and compare the difference .. failed at doing this.
my second attempt is to generate two Time Structs one with the current time in and the other with my "homemade" time in and compare them but this is not working either.
heres my code, no matter what i try the date from the file is always the same as the current system time, i know its something to-do with having to set the time at the top.
t_of_file = time(NULL);
time_from_file = localtime(&t_of_file);
but if i do not do this i get a segmentation fault.
heres the code.
#include <fcntl.h>
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <string.h>
#define COPYMODE 0644
int main(int argc, char *argv[])
{
struct tm *time_from_file;
struct tm *the_system_time;
time_t t_of_file;
time_t t_of_sys;
t_of_sys = time(NULL);
the_system_time = localtime(&t_of_sys);
t_of_file = time(NULL);
time_from_file = localtime(&t_of_file);
time_from_file->tm_year = the_system_time->tm_year;
time_from_file->tm_mon = the_system_time->tm_mon;
time_from_file->tm_mday = the_system_time->tm_mday;
time_from_file->tm_hour = 10; //should be read in from file
time_from_file->tm_min = 30; //should be read in from file
time_from_file->tm_sec = the_system_time->tm_sec;
time_from_file->tm_isdst = the_system_time->tm_isdst;
t_of_file = mktime(time_from_file);
printf("%s\n",ctime(&t_of_file));
t_of_sys = mktime(the_system_time);
printf("%s\n",ctime(&t_of_sys));
double difference = difftime(t_of_file, t_of_sys );
printf("%lf\n",(double)t_of_file);
printf("%lf\n",(double)t_of_sys);
if (difference >= 0.0) { //this should be 5 mins, not quite sure what to put here yet
// second is later than first
printf("later\n");
}
else if (difference < 0.0) {
// second is earlier than first
printf("earlier\n");
}
printf("%lf\n", difference);
return 0;//STATE_OK;
}
any help you can offer would be appreciated on this.
Following the answers i got, PACE's answer was spot on for what i wanted to-do and now i have a simpler bit of code that works perfectly for what i am trying to-do and is easier to under stand. below is the code in its modified form (it compiles perfectly on Linux btw).
#include <stdio.h>
#include <time.h>
int main ()
{
time_t filetime;
time_t presenttime;
struct tm * timeinfo;
time ( &filetime );
time ( &presenttime);
timeinfo = localtime ( &filetime );
timeinfo->tm_hour = 14; //this should be entered from file
timeinfo->tm_min = 15; //this should be entered from file
filetime = mktime ( timeinfo );
printf("my time %s\n",ctime(&filetime));
printf("pc time %s\n",ctime(&presenttime));
double difference = difftime(filetime, presenttime );
printf("%lf\n",(double)filetime);
printf("%lf\n",(double)presenttime);
if (difference > 300.0) {
// second is later than first
printf("later\n");
}
else if (difference < 0.0) {
// second is earlier than first
printf("earlier\n");
}
printf("%lf\n", difference);
return 0;
cheers for the help guys.

Answer 1

There is an example of making a time here. You can then compare the times with the difftime method.

Answer 2

If it were me, I'd be tempted to just save the result of time() to the file. That would save a whole lot of string parsing work on the other side.

Answer 3

I modified your code as follows and it does now what you were expecting:
struct tm time_from_file;
struct tm the_system_time;
time_t t_of_file;
time_t t_of_sys;
t_of_sys = time(NULL);
memcpy(&the_system_time,localtime(&t_of_sys),sizeof(struct tm));
t_of_file = time(NULL);
memcpy(&time_from_file,localtime(&t_of_sys),sizeof(struct tm));
time_from_file.tm_hour = 10; //should be read in from file
time_from_file.tm_min = 30; //should be read in from file
t_of_file = mktime(&time_from_file);
printf("%s\n",ctime(&t_of_file));
printf("%s\n",ctime(&t_of_sys));
double difference = difftime(t_of_file, t_of_sys );
printf("%lf\n",(double)t_of_file);
printf("%lf\n",(double)t_of_sys);
printf("%lf\n", difference);
What was wrong with your code:
You were using and modifying the pointer returned by localtime which happens to be same in both cases.
Consequently your pointers time_from_file and the_system_time were infact pointing to the same location.Any modification of one modifies the other as well and finally you have a difference of zero.
(Thanks gdb for helping me figure out that!!:D)

Answer 4

Paring your format is easy in C. Use this to read the time components from the file:
int hr, min;
fscanf (file, "%d:%d", &hr, &min);
Here is a complete solution:
#include <stdio.h>
#include <time.h>
#include <stdlib.h>
struct HrMin_ {
int hr;
int min;
};
typedef struct HrMin_ HrMin;
static const double TIME_DIFFERENCE = (double) (5 * 1000);
static HrMin read_time (FILE* file);
int
main (int argc, char **argv)
{
time_t sys_time_t = 0;
struct tm *sys_time = NULL;
struct tm *file_time = NULL;
double d = 0.0f;
if (argc != 2)
{
printf ("Usage: time_cmp <time_file>\n");
return 1;
}
time (&sys_time_t);
sys_time = localtime (&sys_time_t);
file_time = malloc (sizeof (struct tm));
file_time->tm_sec = sys_time->tm_sec;
file_time->tm_min = sys_time->tm_min;
file_time->tm_hour = sys_time->tm_hour;
file_time->tm_mday = sys_time->tm_mday;
file_time->tm_mon = sys_time->tm_mon;
file_time->tm_year = sys_time->tm_year;
file_time->tm_wday = sys_time->tm_wday;
file_time->tm_yday = sys_time->tm_yday;
file_time->tm_isdst = sys_time->tm_isdst;
FILE *file = fopen (argv[1], "r");
if (file == NULL)
{
printf ("Failed to open file: %s\n", argv[1]);
return 1;
}
HrMin hr_min = read_time (file);
fclose (file);
file_time->tm_hour = hr_min.hr;
file_time->tm_min = hr_min.min;
d = difftime (sys_time_t, mktime (file_time));
free (file_time);
if (d < 0) d *= -1;
printf ("Diff: %f\n", d);
if (d >= TIME_DIFFERENCE)
printf ("WARN!\n");
return 0;
}
static HrMin
read_time (FILE *file)
{
HrMin hr_min;
hr_min.hr = 0;
hr_min.min = 0;
fscanf (file, "%d:%d", &hr_min.hr, &hr_min.min);
return hr_min;
}