wireless-wifi/led-blinker.c

#include <string.h>
#include <stdio.h>
#include <stdint.h>
#include <errno.h>
#include <unistd.h>
#define BUFFER_SIZE 64

// use sysfs to control the LED on connected device
int main(int argc, char *argv[]) {
    FILE *f;
    char buf[BUFFER_SIZE];
    char *sysfs_LEDS = "/sys/class/leds/";

    // go to sysfs node, this preempts the need to concatenate user data (argv)
    if(0 != chdir(sysfs_LEDS)) {
        fprintf(stderr, "failed to change directory to %s\n", sysfs_LEDS);
        return 1;
    }

    // [?] argc is one when no parameters supplied and argv[0] is program name
    if(/* less than two parameters, terminate with error */ 2 > argc) {
        fprintf(stderr, "must provide sysfs device name\n");
        // short when we cannot provide user with list of device names
        if(NULL == (f = popen("ls", "r"))) return 1;
        // provide user with list of possible device names
        fprintf(stderr, "you might want to try:\n");
        while(NULL != fgets(buf, BUFFER_SIZE, f)) fprintf(stderr, "%s", buf);
        pclose(f);
        return 1;
    }

    // make sure supplied device name is short and is mostly alpha-numeric
    for(uint8_t i = 0; i < BUFFER_SIZE; i++) {
        char comp = argv[1][i];
        uint8_t okay = 0;
        if('-' == comp || '_' == comp || '\0' == comp) okay |= 1;
        if('a' <= comp && 'z' >= comp) okay |= 1;
        if('A' <= comp && 'Z' >= comp) okay |= 1;
        if('0' <= comp && '9' >= comp) okay |= 1;
        // terminate if device name is not okay
        if(! okay) fprintf(stderr, "Can not deal with this device name.\n");
        // terminate if device name is really long
        if(BUFFER_SIZE - 1 == i && '\0' != comp) {
            fprintf(stderr, "Device name might be too long.\n");
            // clear okay flag so we can fail out
            okay = 0;
        }
        if(! okay) return 1;
        // short if we are done before BUFFER_SIZE characters
        if('\0' == comp) break;
    }

    // [!] argv contains at most 64 of: a..z A..Z 0..9 hyphen or underscore
    if(0 != chdir(argv[1])) {
        fprintf(stderr, "failed to change directory to %s\n", argv[1]);
        return 1;
    }

    // make sure we are in control of the led, not some other source
    {   if(NULL == (f = fopen("trigger", "r+"))) {
            fprintf(stderr, "error when opening the trigger: %s\n", strerror(errno));
            return 1;
        }
        if(-1 == fwrite("none\n", 1, 5, f)) {
            fprintf(stderr, "couldn't write \"none\" to trigger: %s\n", strerror(errno));
            return 1;
        }
        fclose(f);
    }

    // blink the leds, on for a second, off for a second
    uint8_t state = 0;
    while(1) {
        // on every loop, reopen the file to make sure it is still there
        {   if(NULL == (f = fopen("brightness", "r+"))) {
                fprintf(stderr, "oh no: %s\n", strerror(errno));
                return 1;
            }
            if(-1 == fwrite(state ?"1\n" :"0\n", 1, 2, f)) {
                fprintf(stderr, "uh oh setting \"%d\" led: %s\n", state, strerror(errno));
                return 1;
            }
            fclose(f);
        }
        // one second on, one second off
        sleep(1);
        // toggle state on ... off ... on ... off ...
        state = !state;
    }

}