Random Numbers

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Test Environments

Overview

This example demonstrates how to set up and use pseudo-random numbers with your Nut/OS application.

Source Code

#include <dev/board.h>
#include <stdio.h>
#include <io.h>
#include <time.h>
#include <sys/timer.h>
 
int main(void)
{
    unsigned long baud = 115200;
 
    NutRegisterDevice(&DEV_DEBUG, 0, 0);
    freopen(DEV_DEBUG_NAME, "w", stdout);
    _ioctl(_fileno(stdout), UART_SETSPEED, &baud);
 
    puts("\nNut/OS Random Numbers Demo");
 
    /* initialize the random seed */
    srand(time(NULL));
 
    puts("We will now be generating a pseudo-random number each second...");
 
    /* calculate "random" numbers */
    for (;;) {
        printf("%d... ", rand());
        NutSleep(1000);
    }
    return 0;
}

Details

unsigned long baud = 115200;
 
NutRegisterDevice(&DEV_DEBUG, 0, 0);
freopen(DEV_DEBUG_NAME, "w", stdout);
_ioctl(_fileno(stdout), UART_SETSPEED, &baud);
 
puts("\nNut/OS Random Numbers Demo");

In order to get started we initialize the debug device and tell the user the application name.

/* initialize the random seed */
srand(time(NULL));

Although this part is incredibly short, it requires some explanation. First of all: The rand() function does not return random numbers. What it does return is a pseudo-random number that is generated by a convoluted algorithm. Now the problem with any algorithm is that given the same input values it will always output the same output values. This is valid for the numbers that are generated by rand() as well.

Since that is obviously not very useful and nobody needs random numbers that aren't, you know, random, there is the idea of a "random seed". This number decides which sequences of numbers rand() will output.

In order to make the random numbers as "random-y" as possible, we set the random seed to a value that is likely to be different every time our program runs: The current time.

So in short: srand(time(NULL)); -> We set the random seed to our current time.

/* calculate "random" numbers */
for (;;) {
    printf("%d... ", rand());
    NutSleep(1000);
}

With the random seed set, obtaining a random number is as simple as calling rand(). This is exactly what we do here: Output a random number, then wait a second. That's all there is to random numbers.

One thing to note: Nut/OS doesn't have its own rand() and srand(), it reuses the ones from the runtime libs (avr-libc on AVR/newlib on ARM and AVR32 respectively) so there may be slight differences with the numbers a given random seed outputs.

And one final hint: If you want to limit the maximum value of the random number you get, simply do something like

myvar = rand() % MAXIMUM_VALUE;

where MAXIMUM_VALUE can be a number, numeric variable or define. Note that the resulting number will always be below the maximum value.

Output

Nut/OS Random Numbers Demo
We will now be generating a pseudo-random number each second...
20034... 24315... 12703... 22240... 10073... 18561... 20058... 5761... 8550... 1
2705... 17166... 2396... 12622...

See also

• More Nut/OS Examples