... and the idea gave birth to a fruit

pedometer.c

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/io.h>

#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>

#include <signal.h>


#define INTERFACE "wlan0"
#define IOBASE 0x0378 /* printer port base address */
#define PWM_PERIOD 1000 /* micro seconds */
#define MAX_DUTY_CYCLE 650 /* micro seconds */
#define MAX_RX_RATE (3500000 / 8) /* 4 mbit/s */
#define MAX_TX_RATE (760000 / 8) /* 500kbit/s */
#define REFRESH_INTERVAL 0.5 /* 0.5 seconds */
#define MICROSECONDS 1000000
#define LINE_BUFFER 4096 /* Size of buffer for reading /proc/net/dev */


int sendport(int i)
{
    if (!ioperm(IOBASE, 1, 1)) {
        outb((unsigned char)i, IOBASE);
        return 1;
    }
    else { return 0; }
}

/*
duty_cycle in percent 0-100%
always sleeps PWM_PERIOD uS
*/
void pwm(int percent)
{
    unsigned int duty_cycle;
    if (percent > 100)
        percent = 100;
    duty_cycle = MAX_DUTY_CYCLE * percent / 100;
    if (duty_cycle > 0) {
        sendport(1);
        usleep(duty_cycle);
    }
    sendport(0);
    usleep(PWM_PERIOD - duty_cycle);
}



void get_bytes(unsigned long *receive, unsigned long *transmit)
{
    char line[LINE_BUFFER];
    unsigned long a[16];
    int k;
    FILE *fd = fopen("/proc/net/dev", "r");
    while (fgets(line, sizeof(line), fd)) {
        k = sscanf(line, " " INTERFACE ": %lu %lu %lu %lu %lu %lu %lu %lu %lu %lu %lu %lu %lu %lu %lu %lu",
            &a[0], &a[1], &a[2], &a[3], &a[4], &a[5], &a[6], &a[7],
            &a[8], &a[9], &a[10], &a[11], &a[12], &a[13], &a[14], &a[15]);
        if (k)
            break;
    }
    fclose(fd);
    *receive = a[0];
    *transmit = a[8];
}

void sig_handler(int signum) {
    printf("Reiceived %d signal\nShutting down...", signum);
    pwm(0);
    exit(0);
}


int main(int argc, char* argv[])
{
    unsigned long receive, prev_receive, transmit, prev_transmit;
    unsigned long tx_rate, rx_rate;
    unsigned int usec;

    get_bytes(&prev_receive, &prev_transmit);
    usleep((int)(REFRESH_INTERVAL*MICROSECONDS));
    signal(SIGINT, sig_handler);
    while (1) {
        get_bytes(&receive, &transmit);
        rx_rate = (receive - prev_receive) / REFRESH_INTERVAL;
        rx_rate = 100 * rx_rate / MAX_RX_RATE; // percent of MAX
        tx_rate = (transmit - prev_transmit) / REFRESH_INTERVAL;
        tx_rate = 100 * tx_rate / MAX_TX_RATE; // percent of MAX
        printf("%lu%% %lu%%\n", rx_rate, tx_rate);
        prev_transmit = transmit;
        prev_receive = receive;
        usec = 0;
        while (usec < REFRESH_INTERVAL * MICROSECONDS) {
            pwm(rx_rate);
            usec += PWM_PERIOD;
        }
    }
}

pwm((tx_rate > rx_rate) ? tx_rate : rx_rate)