Mark this as setuid and put in your path, call with 'up' or 'down' to adjust your laptop's brightness.

brightness.c

#include <stdio.h>

const char *path = "/sys/devices/LNXSYSTM:00/LNXSYBUS:00/TOS6208:00/backlight/toshiba/brightness";

int main(int argc, char *argv[])
{
        if (argc != 2) {
                fputs("Need exactly one argument", stderr);
                return 1;
        }

        FILE *f = fopen(path, "r+");
        if (f == NULL) {
                perror("Couldn't open the backlight special file");
                return 1;
        }

        // On this particular laptop, valid values are a single ASCII digit between
        // 0 and 7, inclusive. So we just read one character. And by the magic of
        // ASCII, incrementing the character ASCII code increments the digit.
        int current = fgetc(f);
        if (current == EOF) {
                fputs("Got EOF when reading from the backlight special file", stderr);
                goto err;
        }

        if (strcmp(argv[1], "up") == 0) {
                current++;
        } else if (strcmp(argv[1], "down") == 0) {
                current--;
        } else {
                fputs("Invalid argument, need 'up' or 'down'", stderr);
                goto err;
        }

        if (fputc(current, f) == EOF) {
                fputs("Got EOF when writing to the backlight special file", stderr);
                goto err;
        }

        fclose(f);
        return 0;

        err:
                fclose(f);
                return 1;
}

xbindkeysrc

"brightness up"
        XF86MonBrightnessUp

"brightness down"
        XF86MonBrightnessDown