#include <stdlib.h>
#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>

const int default_chars = 10;

const char CHARS[] = 
	"0123456789"
	"ABCDEFGHIJKLMNOPQRSTUVWXYZ"
	"abcdefghijklmnopqrstuvwxyz"
	".,-+_/";

#define RNDFILE "/dev/random"

void help() {
	fprintf(stderr, "Usage: passgen [number of chars]\n");
}

int main(int argc, char * argv[])
{
	int fd, chars = default_chars;
	if (argc > 1 && *argv[1] != 0) {
		char *endptr;
		chars = strtol(argv[1], &endptr, 10);
		if (*endptr != 0) {
			fprintf(stderr, "invalid char '%c' in argument '%s'\n", *endptr, argv[1]);
			help();
			return 255;
		}
		if (chars <= 0) {
			fprintf(stderr, "invalid number %i, must be > 0\n", chars);
			help();
			return 255;
		}
	}
	if ((fd = open(RNDFILE, O_RDONLY)) < 0) {
		perror("open "RNDFILE);
		return 255;
	}
	for (; chars > 0; --chars) {
		char idx;
		if (read(fd, &idx, 1) < 1) {
			perror("read from "RNDFILE);
			return 255;
		}
		printf("%c", CHARS[idx % sizeof(CHARS)]);
	}
	printf("\n");
	return 0;
}