Get metadata from pdf via Mupdf library in Android

In this link, one of my previous posts, I have explained how to add mupdf library as a module into an Android Studio project. You can either start with this guide or directly continue to this post.

The target file to edit is  mupdf.c. This file contains ndk functions in order to be called from java part. We will just add a ndk function to fetch metadata of a pdf file. Because mupdf library has a functionality to provide it, we don't need any further coding.

JNIEXPORT jobjectArray JNICALL
JNI_FN(MuPDFCore_metadataInternal)(JNIEnv * env, jobject thiz)
{
 char info[64];
 globals *glo = get_globals(env, thiz);
 jobjectArray arr;
 jclass stringClass;
 const int nkeys = 4;
 const char *keys[nkeys];
 int i;
 keys[0] = "Title";
 keys[1] = "Author";
 keys[2] = "Subject";
 keys[3] = "Keywords";

 stringClass = (*env)->FindClass(env, "java/lang/String");
 arr = (*env)->NewObjectArray(env, nkeys, stringClass, NULL);


 LOGI("Getting metadata");
 for(i=0; idoc, FZ_META_INFO, info, sizeof(info));

  LOGI("%s : %s", keys[i], info);
  jstring s = (*env)->NewStringUTF(env, info);
  if (s != NULL) {
   (*env)->SetObjectArrayElement(env, arr, i, s);
  }
  (*env)->DeleteLocalRef(env, s);
 }

 return arr;
}

This ndk function returns a string array which contains metadata information, respectively to keys in keys array. If there is no such info for a key, it returns an empty string. As seen, just 4 of keys are retrieved. There are more in pdf, you can use them if you want too:

Title
Author
Subject
Keywords
Creator
Producer
CreationDate
ModDate
Trapped

Further explanation, refer to pdf reference guide.

I am working with mupdf with version: 1.6. When I use the function above, I have get an error. After some debugging, I recognized that the problem is on following line in pdf-xref.c file.

info = pdf_dict_gets(info, *(char **)ptr);

ptr is a void array and passed as parameter to the function. What I have passed in ndk function above is a char array. It works well until this line, but here it results a fatal error:

(Fatal signal 11 (SIGSEGV), code 1, fault addr 0x6c746954 in tid 29833)

I have done following fix on same line:

info = pdf_dict_gets(info, (char *)ptr);

Then, the problem has been solved and this functionality has begun to work smoothly.

Mupdf is a great library for pdf rendering. It contains many functionalities to perform on pdf files. Beyond all, it can be integrated to mobile platforms. These characteristics easily show why people tend to prefer this library. Even my post about it takes great hits. All these strongly point out that these guys deserve appreciation. Good job!