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Full Discussion: C Help; generating a random number.
Top Forums Programming C Help; generating a random number. Post 302360583 by Zykl0n-B on Friday 9th of October 2009 11:08:02 AM
Old 10-09-2009
As Corona688 says,
If you use the same value to generate a "random" value, the algorithm is always going to perform the same operation with the same number, so, you need to parse values which are always going to be different, and yes, with gettimeofday() you can parse system time in nanoseconds to get a different number every nanosecond.

Try this:

Code:
#include <stdio.h>
#include <time.h>
#include <sys/time.h>

int main(void){
int i;
unsigned int value;
struct timeval tv;
struct timezone tz;
struct tm *tm;
tm = localtime(&tv.tv_sec);

for(i = 0;i < 30; i++){
gettimeofday(&tv, &tz);
value = rand_r(&tv.tv_usec);
printf("%i\n", value);
}
return(0);
}

Zykl0n-B
 

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LEARN ABOUT FREEBSD

timerclear

TIMERADD(3)						   BSD Library Functions Manual 					       TIMERADD(3)

NAME

     timeradd, timersub, timerclear, timerisset, timercmp -- operations on timevals

SYNOPSIS

     #include <sys/time.h>

     void
     timeradd(struct timeval *a, struct timeval *b, struct timeval *res);

     void
     timersub(struct timeval *a, struct timeval *b, struct timeval *res);

     void
     timerclear(struct timeval *tvp);

     int
     timerisset(struct timeval *tvp);

     int
     timercmp(struct timeval *a, struct timeval *b, CMP);

DESCRIPTION

     These macros are provided for manipulating timeval structures for use with the gettimeofday(2) and settimeofday(2) calls.	The structure is
     defined in <sys/time.h> as:

     struct timeval {
	     long    tv_sec;	     /* seconds since Jan. 1, 1970 */
	     long    tv_usec;	     /* and microseconds */
     };

     timeradd() adds the time information stored in a to b and stores the resulting timeval in res.  The results are simplified such that the
     value of res->tv_usec is always less than 1,000,000 (1 second).

     timersub() subtracts the time information stored in b from a and stores the resulting timeval in res.

     timerclear() initializes tvp to midnight (0 hour) January 1st, 1970 (the Epoch).

     timerisset() returns true if tvp is set to any time value other than the Epoch.

     timercmp() compares a to b using the comparison operator given in CMP, and returns the result of that comparison.

SEE ALSO

     gettimeofday(2)

HISTORY

     The timeradd() family of macros were imported from NetBSD 1.1, and appeared in FreeBSD 2.2.6.

BSD
								  August 11, 1999							       BSD
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