# CLOCKS_PER_SEC

## A peculiar way to get the biggest (max/maximum) value between two variables using bitwise operations

Recently, we wanted to make a test and see how we could find the maximum value between two variables using bitwise operations.

We ended up with the following peculiar way to get the biggest value between two variables using bitwise operations

```r = a ^ ((a ^ b) & -(a < b));

```

The above formula has two modes:

1. When `a < b`
2. When `a >= b`

### When `a < b` then the formula will change as follows:

```r = a ^ ((a ^ b) & 0xFFFFFFFF);

```

As we all (should) know, when one of the operators on a bitwise `AND` operation is composed only from `1`s, then the result is whatever value the other operator was holding.
So, the formula then simplifies as follows:

```r = a ^ (a ^ b);

```

which is equal to

```r = b;

```

because we when we apply twice the same value using `XOR` on another value, we revert back to the original value (so the second `^a` nullifies the first `^a`)

### When `a >= b` then the formula will change as follows:

```r = a ^ ((a ^ b) & 0x00000000);

```

When one of the operators on a bitwise `AND` operation is composed only from `0`s, then the result is always `0` no matter what value the other operator was holding.
So, the formula then simplifies as follows:

```r = a ^ (0x00000000);

```

which is equal to

```r = a;

```

because when one of the operators in a `XOR` operation is only composed from `0`s then the result will be the value of the other operator, no matter what it was.

### Full example

Below you will find a full example that compares the execution speed of the two methods by executing each several thousands of time on the same random data.

```#include <stdio.h>
#include <time.h>
#include <stdlib.h>

int main() {
{
const clock_t start = clock();

srand(10);
unsigned long int i;
unsigned int max = 0;
for (i = 0; i < 1000000000; i++) {
const int a = rand();
max = max < a ? a : max;
}
const clock_t end = clock();
const float seconds = (float) (end - start) / CLOCKS_PER_SEC;
printf("Seconds elapsed %f\tIf statement. Overall max value = %u\n", seconds, max);
}

{
const clock_t start = clock();

srand(10);
unsigned long int i;
unsigned int max = 0;
for (i = 0; i < 1000000000; i++) {
const int a = rand();
max = a ^ ((a ^ max) & -(a < max));
}
const clock_t end = clock();
const float seconds = (float) (end - start) / CLOCKS_PER_SEC;
printf("Seconds elapsed %f\tBitwise operation. Overall max value = %u\n", seconds, max);
}
return 0;
}

```

### Results

Our results show that using the traditional if statement with assignment is faster than using our formula as expected.
Which makes sense as there is an if statement in the formula as well and then additional operations to get the result, instead of just the assignment.

```Seconds elapsed 5.770000 If statement. Overall max value = 2147483647
Seconds elapsed 6.180000 Bitwise operation. Overall max value = 2147483647```

10 times bigger input

```Seconds elapsed 57.450001 If statement. Overall max value = 2147483647
Seconds elapsed 63.869999 Bitwise operation. Overall max value = 2147483647```

## C/C++: Comparing the performance of syslog vs printf

The following code tries to compare the performance of `syslog()` with the `printf()` command. printf_vs_syslog.c (compressed) (299 downloads)
On our machine, it appears that `syslog()` is faster than `printf()`.

To be as fair as possible, when the application was executing, we were monitoring the system logs as well, so that they will be printed on screen.
On `CentOS 7`, you can see the `syslog` in the file `/var/log/messages`.
The command we used was: `sudo tail -f /var/log/messages`

### Results:

```printf: Seconds elapsed 0.480000
syslog: Seconds elapsed 0.180000```

### Full source code for test:

```#include <stdio.h>
#include <syslog.h>
// #include <stdlib.h> is needed for the resolution of EXIT_SUCCESS
#include <stdlib.h>
// #include <time.h> is needed for the clock() function and the macro CLOCKS_PER_SEC
#include <time.h>
// #include <unistd.h> and #include <sys/types.h> are needed for the functions uid_t getuid(void); and uid_t geteuid(void);
//getuid() returns the real user ID of the calling process.
//geteuid() returns the effective user ID of the calling process.
//These functions are always successful.
#include <unistd.h>
#include <sys/types.h>

#define RANGE (100000)

int main()
{
{
const clock_t start = clock();

unsigned int i;
for (i = 0; i < RANGE; i++){
printf ("Program started by Real User %u (Effective User %u)\n", getuid(), geteuid());
}
printf("\n");

const clock_t end = clock();
const float seconds = (float) (end - start) / CLOCKS_PER_SEC;
printf("printf: Seconds elapsed %f\n", seconds);
}
{
const clock_t start = clock();

openlog ("bytefreaks", LOG_CONS | LOG_PID | LOG_NDELAY, LOG_LOCAL1);
unsigned int i;
for (i = 0; i < RANGE; i++){
syslog (LOG_NOTICE, "Program started by Real User %u (Effective User %u)", getuid(), geteuid());
}
closelog ();

const clock_t end = clock();
const float seconds = (float) (end - start) / CLOCKS_PER_SEC;
printf("syslog: Seconds elapsed %f\n", seconds);

}
return EXIT_SUCCESS;
}

```