getting system date(int & char mixed) using sscanf - c

Question

I am trying to include system date in a program, which would calculate the total value of days since 2000. So that I could compare values of various dates (like- Antivirus update date, OS updation date) with it in order to find what is the delay in system updation. The procedure of finding date in C was killing me so I used system command to find out the current date.
system("date/t > system_date.txt");
I want to read the date from this file. There was no problem if the date was all in digits(ex.- 03/09/13). But the date displayed from DOS is of type : 03-Sep-13 . If I use sscanf as follows:
FILE *fp_date=fopen("system_date.txt","r");
char buffer[11]; int day,yr; char yr[3];
fgets(buffer,11,fp_date);
sscanf("%d-??????-%d",&day,&mon,&yr);
Now how do I scan these separately with their data types? I don't know what to use in place of ??????. Or the other method which I have been using is to store whole line in an array, then run for loops to get elements separated by '-'. But I just wanted to be a bit more sophisticated. Thanks...

Answer 1

You don't need system (and not even the date command). BTW, using popen("date","r") might be simpler but is still not the right way.
Read first the time(7) man page.
Then use code like
time_t now=0;
time (&now);
struct tm* ltm = localtime(&now);
The time(2) syscall gives the time from the Unix epoch (start of 1970 year) in seconds. Use clock_gettime(2) if you want more precision (or measure CPU, or thread, time). gettimeofday(2) is becoming obsolete. localtime(3) converts a Unix time to a struct tm containing seconds, minutes, ... day of week, day of month, month, .... fields. Use mktime for the opposite conversion of a struct tm to a time_t. And time_t is also given by other functions or syscalls (like stat(2) giving metadata about a file, including its modification time).
You can use fields from ltm programmatically like ltm->tm_sec for seconds, ltm->tm_min for minutes, .... ltm->tm_monfor month (0 for January, etc...), etc....
If you want a string, use strftime(3) like
char timebuf[32];
strftime (timebuf, sizeof(timbuf), "%d %b %Y", ltm);
The opposite function is strptime(3) which parses a string into a struct tm.
Notice that strftime use localization (see locale(7) first). So your user could get a french abbreviated month from %b (if his system is localized for French).
If you want universal (so called GMT) time, use gmtime instead of localtime. In multithreaded programs use localtime_r a,d gmtime_r and give them your own struct tm.

Answer 2

Don't do this. Obtaining the current date/time in C is trivial:
#include <time.h>
/* ... */
struct tm *tm = localtime(time(0));
The way you're trying to do it is much harder, not to mention error-prone. (For example, what happens if two instances run at the same time and both try to write to the same file?) In general, system is always the wrong way to do something.

Answer 3

Don't do that. Just use time(2) (C89) or gettimeofday(3) (POSIX) to get the current system time in Unix time format. If you need to convert that to other forms like a string representation or a struct tm object, then you can use various conversion functions such as ctime(3), gtime(3), and localtime(3).
#include <time.h>
time_t now = time(NULL);
printf("The current time is: %s\n", ctime(&now)); // Warning: not thread-safe
struct tm *now_tm = localtime(now); // Warning: not thread-safe
printf("Year=%d month=%d day=%d\n hour=%d minute=%d second=%d",
1900 + now_tm->tm_year,
1 + now_tm->tm_mon,
now_tm->tm_mday,
now_tm->tm_hour,
now_tm->tm_min,
now_tm->tm_sec);