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# difftime(S)

difftime -- computes the difference between time values

## Syntax

cc . . . -lc

```#include <time.h>

double difftime(time2, time1)
time_t time2;
time_t time1;
```

## Description

The difftime function computes the difference of time2 time1.

See the example which follows.

## Return value

The difftime function returns the elapsed time in seconds from time1 to time2 as a double-precision number.

## See also

ctime(S), mktime(S), nl_cxtime(S), stime(S), time(S)

## Standards conformance

difftime is conformant with:

ANSI X3.159-1989 Programming Language -- C .

## Example

```   #include <time.h>

int mark;

main()
{
time_t  start, finish;
register int i, loop, n, num, step;
printf("This program takes about 3 minutes ");
printf("on an AT and 8 on a PC.\n Working...\n");
time(&start);

for (loop = 0; loop < 1000; ++loop)
for (num = 0,n = 3; n < 10000; n += 2)
if (!mark[n])
{
/* printf("%d\n",n); */
step = 2*n;
for (i = 3*n; i < 10000; i += step)
mark[i] = -1;
++num;
}
time(&finish);

/* Prints average of 1000 loops through "sieve": */
printf("\nProgram takes %f seconds to find %d primes.\n",
difftime(finish, start), num);
}
```

Output:

```   Program takes 0.482000 seconds to find 1228 primes.
```
This program calculates the amount of time needed to find the prime numbers between 3 and 10,000. To display the prime numbers, delete the outermost loop and the comment delimiters around the expression `printf("%d",n);`.
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