Java 22 Foreign Function & Memory API: traversing structs

Question 1

I'm trying to use the Foreign Function & Memory API in Java 22 to call into FFmpeg's libavformat. First, I used jextract to generate a binding:

jextract --target-package org.ffmpeg.avformat \    --output /path/to/src/main/java \    -I <base include dir> libavformat/avformat.h

My initial goal is to open a video file and get its dimensions. Many FFmpeg functions make use of a struct called AVFormatContext, which is a main context struct that refers to other structs, which refer to other structs, etc. So I need to walk through a few different structs to get the width & height:

struct AVFormatContext {    unsigned int nb_streams;    AVStream **streams;    // etc.}struct AVStream {    AVCodecParameters *codecpar;    // etc.}struct AVCodecParameters {    int width;    int height;    // etc.}

In C, this would involve something like:

int width  = formatCtx->streams[i]->codecpar->width;int height = formatCtx->streams[i]->codecpar->height;

Here is my attempt in Java:

try (Arena arena = Arena.ofConfined()) {    ///////////////////// begin setup ////////////////////////    MemorySegment formatCtxPtr  = arena.allocate(C_POINTER);    MemorySegment inputFormat   = MemorySegment.NULL;    MemorySegment formatDictPtr = MemorySegment.NULL;    MemorySegment url = arena.allocateFrom("file:/path/to/video.mp4");    int result = avformat_open_input(formatCtxPtr, url, inputFormat, formatDictPtr);    if (result != 0) {        throw new IOException("Couldn't open file");    }    MemorySegment formatCtx = formatCtxPtr.get(C_POINTER, 0);    // Verify the AVFormatContext    av_dump_format(formatCtx, 0, url, 0);    ///////////////////// end setup ////////////////////////    StructLayout formatCtxLayout = (StructLayout) AVFormatContext.layout();    VarHandle streamsHandle  = formatCtxLayout.varHandle(            MemoryLayout.PathElement.groupElement("streams"));    MemorySegment streamsPtr = (MemorySegment) streamsHandle.get(formatCtx, 0);    MemorySegment stream     = streamsPtr.getAtIndex(C_POINTER, 0);    StructLayout avStreamLayout  = (StructLayout) AVStream.layout();    VarHandle codecParamsHandle  = avStreamLayout.varHandle(            MemoryLayout.PathElement.groupElement("codecpar"));    MemorySegment codecParamsPtr = (MemorySegment) codecParamsHandle.get(stream, 0);    MemorySegment codecParams    = codecParamsPtr.get(C_POINTER, 0);    StructLayout codecParamsLayout = (StructLayout) AVCodecParameters.layout();    VarHandle widthHandle          = codecParamsLayout.varHandle(            MemoryLayout.PathElement.groupElement("width"));    int width                      = (int) widthHandle.get(codecParams, 0);}

Sadly that last call towidthHandle.get() causes:java.lang.InternalError: a fault occurred in an unsafe memory access operation

Question 2

I'm not familiar with libavformat but jextract should have generated getter (and indexed) getter calls to retrieve each member field of each of those structures, avoiding you need to use Layout/VarHandle. eg see if you haveAVFormatContext.streams(formatCtx) etc

Question 3

It looks like you have one too many dereferences:

MemorySegment codecParams    = codecParamsPtr.get(C_POINTER, 0);

ThecodecParamsPtr is aAVCodecParameters*, which you then try to load a pointer from. That will however just read the first 8 bytes of the struct as a pointer.

Also, like @DuncG says in the comments, you can just use the getters that jextract generates. This should work:

// AVStream**MemorySegment streams = AVFormatContext.streams(formatCtx);// AVStream*MemorySegment stream = streams.getAtIndex(C_POINTER, 0);// AVCodecParameters*MemorySegment codecParams = AVStream.codecpar(stream);int width = AVCodecParameters.width(codecParams);

Question 4

Works like magic! Thanks!

Jorn Vernee 34.3k5 gold badges84 silver badges99 bronze badges · Accepted Answer · 2024-05-23 15:42:30Z

It looks like you have one too many dereferences:

MemorySegment codecParams    = codecParamsPtr.get(C_POINTER, 0);

ThecodecParamsPtr is aAVCodecParameters*, which you then try to load a pointer from. That will however just read the first 8 bytes of the struct as a pointer.

Also, like @DuncG says in the comments, you can just use the getters that jextract generates. This should work:

// AVStream**MemorySegment streams = AVFormatContext.streams(formatCtx);// AVStream*MemorySegment stream = streams.getAtIndex(C_POINTER, 0);// AVCodecParameters*MemorySegment codecParams = AVStream.codecpar(stream);int width = AVCodecParameters.width(codecParams);

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Java 22 Foreign Function & Memory API: traversing structs

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