[Techtalk] getting the time as a string in c

[Almut Behrens]

On Wed, Sep 28, 2005 at 09:07:47PM +0100, Dan wrote:
> So what you want to do is:
> 
>    time_t timeInSeconds;
>    const struct tm *timeptr;
> 
>    timeInSeconds = time(NULL);
>    timeptr = localtime(&timeInSeconds);   /* "&" means "pointer to" */
> 
>    /* Note: a second call to localtime() 
>     * might overwrite timeptr's memory.
>     * that's not a concern here, though.    */
>    strftime(timestring, timestrsize, timeformat, timeptr);
> 
>    printf("%s\n", timestring);

In case the timeformat you want is just "%c", you can also use ctime()
(as Dan has mentioned in his other reply), which is somewhat shorter,
as ctime() directly returns the desired string (i.e. a pointer to a
char buffer, which you don't even need to allocate yourself).

A fully working program to print the current time would be no longer
than this

  #include <stdio.h>
  #include <time.h>
  
  int main(void) {
      
      time_t t = time(NULL);
      printf("%s", ctime(&t));
      
      return 0;
  }

compared to using strftime():

  #include <stdio.h>
  #include <time.h>
  
  #define TSTR_MAXSIZE 40
  
  int main(void) {
      
      char timestring[TSTR_MAXSIZE];

      time_t t = time(NULL);
      const struct tm *timeptr = localtime(&t);
      strftime(timestring, TSTR_MAXSIZE, "%c", timeptr);
      
      printf("%s\n", timestring);
      
      return 0;
  }

Cheers,
Almut

P.S.: according to my experiences from giving introductory courses,
most problems learners of C are facing revolve around memory and
pointers:
* where is data stored
* who is resposible for the associated memory/buffers, i.e.
  * who allocates it
  * who needs to free it (if it's dynamic)
* how to address it, i.e. when to use *foo, foo or &foo

So, if you don't want your program to segfault, it's essential to get a
good grasp if this stuff... ;)