A utility to display QR codes in the CLI/terminal. To build, run `gcc showqrcode.c -ldl -o showqrcode`

showqrcode.c

// showqrcode: A utility to display QR codes in the terminal.
// Copyright © 2019 Kevin Gallagher <kevingallagher@gmail.com>
// Modified and based upon original code from libpam-google-authenticator:
// https://github.com/google/google-authenticator-libpam
// The original license is printed below.
//
// Helper program to generate a new secret for use in two-factor
// authentication.
//
// Copyright 2010 Google Inc.
// Author: Markus Gutschke
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//      http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

#include <dlfcn.h>
#include <getopt.h>
#include <stdio.h>
#include <stdio_ext.h>
#include <stdlib.h>
#include <string.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <unistd.h>
#include <regex.h>
#include <locale.h>

#define ANSI_RESET        "\x1B[0m"
#define ANSI_BLACKONGREY  "\x1B[30;47;27m"
#define ANSI_WHITE        "\x1B[27m"
#define ANSI_BLACK        "\x1B[7m"
#define UTF8_BOTH         "\xE2\x96\x88"
#define UTF8_TOPHALF      "\xE2\x96\x80"
#define UTF8_BOTTOMHALF   "\xE2\x96\x84"

static enum { QR_UNSET=0, QR_NONE, QR_ANSI, QR_UTF8 } qr_mode = QR_UTF8;

static int displayQRCode(const char* url) {
  FILE * out = stdout;
  setbuf(out, NULL);
  void *qrencode = dlopen("libqrencode.so.4", RTLD_NOW | RTLD_LOCAL);
  if (!qrencode) {
    qrencode = dlopen("libqrencode.so.3", RTLD_NOW | RTLD_LOCAL);
  }
  if (!qrencode) {
    qrencode = dlopen("libqrencode.3.dylib", RTLD_NOW | RTLD_LOCAL);
  }
  if (!qrencode) {
    qrencode = dlopen("libqrencode.so.2", RTLD_NOW | RTLD_LOCAL);
  }
  if (!qrencode) {
    return 0;
  }
  typedef struct {
    int version;
    int width;
    unsigned char *data;
  } QRcode;
  QRcode *(*QRcode_encodeString8bit)(const char *, int, int) =
      (QRcode *(*)(const char *, int, int))
      dlsym(qrencode, "QRcode_encodeString8bit");
  void (*QRcode_free)(QRcode *qrcode) =
      (void (*)(QRcode *))dlsym(qrencode, "QRcode_free");
  if (!QRcode_encodeString8bit || !QRcode_free) {
    dlclose(qrencode);
    return 0;
  }
  QRcode *qrcode = QRcode_encodeString8bit(url, 0, 1);
  const char *ptr = (char *)qrcode->data;
  if (qr_mode == QR_UTF8) {
    fprintf(out, "%s", ANSI_BLACKONGREY);
    for (int i = 0; i < qrcode->width + 4; ++i) {
      printf(" ");
    }
    puts(ANSI_RESET);
    for (int y = 0; y < qrcode->width; y += 2) {
      fprintf(out, "%s", ANSI_BLACKONGREY"  ");
      for (int x = 0; x < qrcode->width; ++x) {
        const int top = qrcode->data[y*qrcode->width + x] & 1;
        int bottom = 0;
        if (y+1 < qrcode->width) {
          bottom = qrcode->data[(y+1)*qrcode->width + x] & 1;
        }
        if (top) {
          if (bottom) {
            fprintf(out, "%s", UTF8_BOTH);
          } else {
            fprintf(out, "%s", UTF8_TOPHALF);
          }
        } else {
          if (bottom) {
            fprintf(out, "%s", UTF8_BOTTOMHALF);
          } else {
            printf(" ");
          }
        }
      }
      puts("  "ANSI_RESET);
    }
    fprintf(out, "%s", ANSI_BLACKONGREY);
    for (int i = 0; i < qrcode->width + 4; ++i) {
      printf(" ");
    }
    puts(ANSI_RESET);
    fflush(out);
  }
  QRcode_free(qrcode);
  dlclose(qrencode);
  
  return 1;
}

static void usage(void) {
  puts(
    "showqrcode [<options>]\n"
    " -h, --help                    Print this message\n"
    " -d, --data=<string>           Encode arbitrary strings\n"
    " -p, --pipe                    Accept input from STDIN\n"
    " -u, --url=<otpauth://>        Supply the \"otpauth://\" URL to encode\n");
}

int main(int argc, char *argv[]) {
  int opt;
  int idx = -1;
  char *url = NULL;
  char *data = NULL;
  char str[256];
  int otp_regex;
  int d = 0, p = 0, u = 0;

  regex_t regex;
  regmatch_t rematch[2];
  size_t nmatch = 2;
  setlocale(LC_ALL, "en_US.UTF-8");

  if(argc <= 1) {
    fprintf(stderr, "Error: Expected at least one argument.\n");
    usage();
    _exit(1);
  }

  otp_regex = regcomp(&regex, "^otpauth://.*", 0);

  static struct option options[] = {
    { "help",             0, 0, 'h' },
    { "data",           1, 0, 'd' },
    { "pipe",           0, 0, 'p' },
    { "url",            1, 0, 'u' },
    { 0,                  0, 0,  0  }
  };
  static const char optstring[] = "+h:d:pu:";

  while (opt = getopt_long(argc, argv, "+h:d:pu:", options, NULL), opt != -1) {
    switch (opt) {     
      case '0':
        if (options[opt].val != 0) {
          break;
        }
        if (optarg) {
          break;
        }
      case '-':
         break;
      case 'h':
        usage();
        exit(0);
        break;
      case 'd':
        data = strdup(optarg);
        d = 1;
        break;
      case 'p':
        data = fgets(str, 256, stdin);
        p = 1;
        break;
      case 'u':
        url = strdup(optarg);
        u = 1;
        otp_regex = regexec(&regex, url, 0, NULL, 0);
        if (otp_regex) {
          fprintf(stderr, "The input URL %s is not in the correct format.\n", url);
          _exit(1);
        }
        if (strlen(url) < 16) {
          fprintf(stderr, "The input URL %s does not contain enough data.\n", url);
          _exit(1);
        }
        break;
      case '?':
        usage();
        _exit(1);
        break;
      default:
        fprintf(stderr, "Error: getopt returned unexpected value.\n");
    }
    if ((d && u) || (d && p) || (p && u)) {
      fprintf(stderr, "Error: you may only specify one mode at a time.\n");
      usage();
      _exit(1);
    }
  }
  if (data) {
    url = strdup(data);
  } else if ((!url) && (!data)) {
    fprintf(stderr, "Error: No input to encode.\n");
    _exit(1);
  }
  //if (isatty(1)) {
    if (!displayQRCode(url)) {
      printf(
        "Failed to use libqrencode to show QR code visually for scanning.\n"
        "Consider typing the OTP secret into your app manually.\n");
        _exit(1);
    }
  //} else {
  //    _exit(1);
  //}
  fclose(stdout);
  return 0;
}