Dates difference in C (seconds) - c

Question

I have never programmed in C and therefore it is being a real challenge to write a small piece of software which returns the difference between two dates in seconds.
Just as background information, I implemented a kind of heart beat in a Python application with updates a txt file every 15 seconds. Now I would like to create a C application that checks this txt constantly and in case the difference is greater than 30 seconds, it restarts my computer.
That is how far I went so far:
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <sys/stat.h>
#include <sys/types.h>
//Returns the number seconds since the last time the files has been updated
int lastUpdate() {
struct stat attr;
stat("watcher.txt", &attr);
//Gets the last update in the following format: Mon Aug 13 08:23:14 2012
//ctime(&attr.st_mtime);
time_t lastChange = &attr.st_mtime
time_t now = time(0)
//No idea how to implement it :-(
//int seconds = timediff(now, lastChange)
return seconds;
}
//Constantly checks if application is sending heart beats in a 30 seconds time frame
main()
{
while(1 == 1)
{
if(lastUpdate() > 30)
{
sprintf(cmd, "sudo reboot -i -p");
system(cmd);
}
}
}
Would anyone be so nice and give some hints on how to get this working?
Thank you very much!
EDITED:
Final code working without issues:
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <sys/stat.h>
#include <sys/types.h>
//Returns the number seconds since the last time the files has been updated
int lastUpdate() {
struct stat attr;
stat("/home/pi/watcher.txt", &attr);
time_t lastChange = attr.st_mtime;
time_t now = time(0);
int seconds = now - lastChange;
return seconds;
}
//Constantly checks if application is sending heart beats in a 30 seconds time frame
main()
{
sleep(120);
while(1 == 1)
{
sleep(1);
if(lastUpdate() > 30)
{
system("sudo reboot -i -p");
}
}
}

Answer 1

You're in luck! time_t is a number, and the timestamps are the time since the beginning of 1970 in seconds. So:
int seconds = now - lastChange;
Oh, and
time_t lastChange = &attr.st_mtime
should be
time_t lastChange = attr.st_mtime

Answer 2

time_t is typically the number of seconds since Jan 1, 1970 0:00:00 UTC (GMT ). So a simply subtraction will work.
time_t lastChange = &attr.st_mtime
time_t now = time(0)
time_t seconds = now - lastChange;
printf("%ll\n", (long long) seconds);
Stricly speaking, time_t may be some other scalar.
A portable solution is to use double difftime(time_t time1, time_t time0);
#include <time.h>
double seconds = difftime(now, lastChange);
printf("%f\n", seconds);

Answer 3

You just need to subtract the time_t's but you have a problem in this line
time_t lastChange = &attr.st_mtime
because stat.st_mtime is of type time_t not time_t *, so you should change it to
time_t lastChange = attr.st_mtime
and then
return new - lastChange;
and also, check stat("watcher.txt", &attr) != -1