It may be bug in avformat.avio_open2 JNI wrapper

[grimaz]

Hello,
I can not find from which source the FFMPEG JNI came to your project, but it seems bug in
public static native int avio_open2(@ByPtrPtr avformat.AVIOContext var0, String var1, int var2, @const avformat.AVIOInterruptCB var3, @ByPtrPtr AVDictionary var4);
when call as
avformat.avio_open2(lDstContext.pb(), destination, avformat.AVIO_FLAG_WRITE, null, null);
APK crashes inside avio_open2/sub_... because storing value to null.

If you point me where you get JNI wrappers, I will check and send fix there.

Regards.

[grimaz]

Seems it is not a bug, but @ByPtrPtr requires workaround.
Function call can not be used as
avformat.avio_open2(lDstContext.pb(), ...);
But only as
avformat.AVIOContext lAvioContext = new avformat.AVIOContext();
avformat.avio_open2(lAvioContext, ...);
lDstContext.pb(lAvioContext);

[saudet]

The JNI code is generated at build time, when executing mvn install or something. For that particular function we get this code:

JNIEXPORT jint JNICALL Java_org_bytedeco_javacpp_avformat_avio_1open2__Lorg_bytedeco_javacpp_avformat_00024AVIOContext_2Ljava_lang_String_2ILorg_bytedeco_javacpp_avformat_00024AVIOInterruptCB_2Lorg_bytedeco_javacpp_avutil_00024AVDictionary_2(JNIEnv* env, jclass cls, jobject arg0, jstring arg1, jint arg2, jobject arg3, jobject arg4) {
    ::AVIOContext* ptr0 = arg0 == NULL ? NULL : (::AVIOContext*)jlong_to_ptr(env->GetLongField(arg0, JavaCPP_addressFID));
    jint position0 = arg0 == NULL ? 0 : env->GetIntField(arg0, JavaCPP_positionFID);
    ptr0 += position0;
    const char* ptr1 = arg1 == NULL ? NULL : env->GetStringUTFChars(arg1, NULL);
    ::AVIOInterruptCB* ptr3 = arg3 == NULL ? NULL : (::AVIOInterruptCB*)jlong_to_ptr(env->GetLongField(arg3, JavaCPP_addressFID));
    jint position3 = arg3 == NULL ? 0 : env->GetIntField(arg3, JavaCPP_positionFID);
    ptr3 += position3;
    ::AVDictionary* ptr4 = arg4 == NULL ? NULL : (::AVDictionary*)jlong_to_ptr(env->GetLongField(arg4, JavaCPP_addressFID));
    jint rarg = 0;
    jthrowable exc = NULL;
    try {
        int rvalue = avio_open2((arg0 == NULL ? NULL : &ptr0), ptr1, arg2, (const ::AVIOInterruptCB*)ptr3, (arg4 == NULL ? NULL : &ptr4));
        rarg = (jint)rvalue;
    } catch (...) {
        exc = JavaCPP_handleException(env, 17);
    }

    ptr0 -= position0;
    if (arg0 != NULL) env->SetLongField(arg0, JavaCPP_addressFID, ptr_to_jlong(ptr0));
    if (arg1 != NULL) env->ReleaseStringUTFChars(arg1, ptr1);
    if (arg4 != NULL) env->SetLongField(arg4, JavaCPP_addressFID, ptr_to_jlong(ptr4));
    if (exc != NULL) {
        env->Throw(exc);
    }
    return rarg;
}

So, as you can see, when arg4 == NULL, it passes NULL to avio_open2(), and I'm afraid it looks like there is a bug in FFmpeg. Could you report that upstream and post a link here for reference? Thanks!

[saudet]

Let's see, I think you are asking about the first parameter? Yes, I see how this can be problematic... As you found out, and as stated in the README.md file, allocating a temporary output object is the way to go, but to prevent allocating native memory needlessly, we can call the constructor with a null value:

AVIOContext ioctx = new AVIOContext(null);
avformat.avio_open2(ioctx, ...);
fmtctx.pb(ioctx);

Unfortunately, I don't know of a nicer way to make this work, but if you have any ideas, please do share them! Thanks for reporting