Time as filename

Hi guys, this is my first post here Hope you can help me out with the problem I have...

I'm trying to create a text file with a variabe filename. My idea was to use the time from this function:

Code:

/* time example */
#include <stdio.h>
#include <time.h>

int main ()
{
  time_t seconds;

  seconds = time (NULL);
  printf ("%ld hours since January 1, 1970", seconds/3600);
  
  return 0;
}

as the filename, according to this function:

Code:

#include "stdio.h"
File Input/Output C Tutorial 10-5
main( )
{
FILE *fp1;
char oneword[100],filename[25];
char *c;
printf("enter filename -> ");
scanf("%s",filename); /* read the desired filename */
fp1 = fopen(filename,"r");
do {
c = fgets(oneword,100,fp1); /* get one line from the file */
if (c != NULL)
printf("%s",oneword); /* display it on the monitor */
} while (c != NULL); /* repeat until NULL */
fclose(fp1);
}

Now this is what I got so far:

Code:

    FILE *stream;
    
    time_t seconds;

    seconds = time (NULL);

    stream = fopen(seconds, "w");

But when I compile this, I get the error that Im doing an "Invalid conversion from time_t to const_char.


Has anyone got an idea how to solve this? Any help is highly appreciated, thanks in advance.

René

Hint: the first argument of fopen() is not supposed to be a time_t. Construct a filename using a char array. snprintf() may help you.

Originally Posted by cwr

Hint: the first argument of fopen() is not supposed to be a time_t. Construct a filename using a char array. snprintf() may help you.

Thanks for your quick reply. I've been searching for about an hour but can't find anything useful about snprintf... at least not useful enough to make me understand the working principle, let alone how to use it in my code...

Have you maybe got a link to a website that explains how snprintf works?

Thanks again, René

I think you need to use strftime() if you want to format your time into a nice text string you can use as a filename.

Originally Posted by Salem

I think you need to use strftime() if you want to format your time into a nice text string you can use as a filename.

Thank you very much for your hint Salem, that works perfectly!

Code:

time_t now;
            
    /* Time Function for filename definition */    
    now = time(NULL);
    struct tm *t = localtime(&now);
    char date_time[30];
    strftime( date_time, sizeof(date_time), "%d%m%y_%H%M%S.txt", t );
    /*****************************************/

    /* Open text file for writing */
    stream = fopen(date_time, "w");

\o/

PS Congratulations with your 10000th post!

you could have done the same thing with simple sprintf()

Code:

time_t now;
            
    /* Time Function for filename definition */    
    now = time(NULL);
    struct tm *t = localtime(&now);
    char date_time[30];
    sprintf(date_time,"%02d/%02d/%04d %02d:%02d:%03d",
 
    strftime( date_time, sizeof(date_time), "%d%m%y_%H%M%S.txt", t );
    /*****************************************/

Originally Posted by Ancient Dragon

you could have done the same thing with simple sprintf()

Code:

time_t now;
            
    /* Time Function for filename definition */    
    now = time(NULL);
    struct tm *t = localtime(&now);
    char date_time[30];
    sprintf(date_time,"%02d/%02d/%04d %02d:%02d:%03d",
 
    strftime( date_time, sizeof(date_time), "%d%m%y_%H%M%S.txt", t );
    /*****************************************/

Doesn't look simpler to me, in fact it's even got an extra line. I'll stick with what I had, thanks anyway!

Originally Posted by rkooij

Doesn't look simpler to me, in fact it's even got an extra line. I'll stick with what I had, thanks anyway!

ignore my post -- I did something (don't know what) and the browser posted before I was done thying. I thought I deleted my post, but guess I couldn't even do that right But yes, stay with what you had. I was just suggesting an alternative.

It was supposed to look like below.

Code:

sprintf(date_time,"%02d/%02d/%04d %02d:%02d:%03d",
   t->tm_day,t->tm_mon+1,t->tm_year+1900,
  t->tm_hour,t->tm_minute,t->tm_second);

Originally Posted by Ancient Dragon

ignore my post -- I did something (don't know what) and the browser posted before I was done thying. I thought I deleted my post, but guess I couldn't even do that right But yes, stay with what you had. I was just suggesting an alternative.

It was supposed to look like below.

Code:

sprintf(date_time,"%02d/%02d/%04d %02d:%02d:%03d",
   t->tm_day,t->tm_mon+1,t->tm_year+1900,
  t->tm_hour,t->tm_minute,t->tm_second);

Hehehe, ok that explains! Well, I understand less of your solution in comparison to the one further above. So in this case I'll still stay with the original solution because that's easier when there's changes needed in the future . I do thank you for the effort!