Parsing libv4l2.h and videodev2.h

[mihetare]

Hi, I am trying to make a javacpp preset for V4L2. At the moment, I can compile everything successfully, though with a bit of modifications in the source files. In videodev2.h, I have to get manually rid of the attribute ((packed)) -keyword that is used in conjunction of some Structs in the file. Otherwise these cause NullPointerExceptions during the mvn install.

In addition, in libv4l2.h functions such as v4l2_open are declared as a variadic function and I have not yet found a way to deal with this properly. While the wrapping works, I seem to be losing functionality as the ... -part inf the function declaration is ignored.

The draft code can be found in https://github.com/Viritystila/V4L2JB

Best regards, Mikael Reponen

[saudet]

Thanks! This guide should help you with these issues: https://github.com/bytedeco/javacpp/wiki/Mapping-Recipes Let me know if there is anything you're not able to figure out though.

[mihetare]

I have managed to map most of the problem points, but I have been unable to cleanly parse the structs with the __attribute__ ((packed)) attribute. I can set infoMap.put(new Info("__attribute__").skip()), which removes the NullPointerException but causes the structs, defined for example as

struct v4l2_rds_data { __u8 lsb; __u8 msb; __u8 block; } __attribute__ ((packed));

, to no be parsed. Similar method does not seem to affect the "((packed))" -part at all.

Concerning the varargs, I have tried such mappings as

        infoMap.put(new Info ("...").valueTypes("int").pointerTypes("IntPointer", "IntBuffer", "int[]", "Pointer"));   
        infoMap.put(new Info("...").valueTypes("@Cast(\"unsigned long int\") long"));

with no success. The C function is declared as

v4l2_ioctl(int fd, unsigned long int request, ...);

but the last vararg argument is still omitted from the Java method.

Thanks in advance!

[saudet]

For the attribute, try instead infoMap.put(new Info("attribute").annotation())

[saudet]

As for varargs, we can declare the actual parameters we need to use with Info.javaText(): https://github.com/bytedeco/javacpp/wiki/Mapping-Recipes#mapping-a-declaration-to-custom-code

[mihetare]

Thanks! I got the wrapper to work at least partially, I can read an image using v4l2_read, though not consistently. Also, I still need to manually remove the __attribute__((packed)) from the source file but that is ok as I need to change some other lines there too.

The c -function v4l2_mmap returns a mmap(), I suppose I have to modify the return value to BytePointer or similar in Java?

[saudet]

You mean you need to call the system mmap() function? We could map it manually like I did here with poll() for libdc1394: https://github.com/bytedeco/javacpp-presets/blob/master/libdc1394/src/main/java/org/bytedeco/javacpp/presets/dc1394.java#L87

Though, it sounds like something that we could add to the systems presets: https://github.com/bytedeco/javacpp-presets/tree/master/systems

[mihetare]

The v4l2_mmap -function calls mmap() and returns a pointer to the mapped buffer.

One separate issue is that the C-function v4l2_ioctl(int fd, unsigned long int request, ...); accepts both Struct and int as the vargarg. While I got the Struct sorted with a pointer, I have not yet figured how to manage the int arg simultaneously.

[saudet]

Java supports method overloading, that's what it's for. :)

[saudet]

As for the issue with attribute declarations, I've just fixed this in commit https://github.com/bytedeco/javacpp/commit/e435a018957143bcbaef022212cae785249eb7e7.

Let me know if you encounter any other issues, and thanks for reporting!

[mihetare]

Thanks, I'll put this into use. So far I have gotten V4L2 input/output working using the read and write functions. The supposedly better method of streaming with memory mapping crashes the JVM, but am still looking into why it happens.

[barrypitman]

@mihetare did you ever get your preset for V4L2 working?

[mihetare]

Sorry for the delay! I managed to get something working, but it would need a bit more work to make it good for general use. Please check it out in https://github.com/Viritystila/V4L2JB/tree/master