Passing a native fd int to FFMPEG from openable URI

Question

I am trying to open a file descriptor from a CATEGORY_OPENABLE URI from the Storage Access Framework. I am first trying with a file on the sdcard, which I can already resolve to a file path using the _data column and open (I am trying to get away from doing this, and use the file descriptor instead).

I get the the native int fd like this:

int fd = getContentResolver().openFileDescriptor(data.getData(), "r").detachFd();

Then in C++, I am trying to open it like this, the idea taken from How to properly pass an asset FileDescriptor to FFmpeg using JNI in Android:

pFormatCtx = avformat_alloc_context();
pFormatCtx->iformat = av_find_input_format("mp3");

char path[50];
sprintf(path, "pipe:%d", fd);

int e;
if(e=(avformat_open_input(&pFormatCtx,path,NULL,NULL)!=0)){
    av_strerror(e, path, 50);
    return error;
}

This yields an "Unknown error" from avformat_open_input. The same thing happens if I use the jni method jniGetFDFromFileDescriptor from the above linked on a FileDescriptor object to get the int fd instead. How can I open an openable URI with FFMPEG correctly without using the file path?

http://stackoverflow.com/questions/24701029/how-to-properly-pass-an-asset-filedescriptor-to-ffmpeg-using-jni-in-android looks similar — Robert Rowntree, May 15 '17 at 17:50
@RobertRowntree I linked it in my question already, that's what I based what I tried to do on. — Steve M, May 15 '17 at 17:51
@SteveM as per https://ffmpeg.org/doxygen/2.8/group__lavu__error.html#ga4ebaa288c7684a26a591c501208910e4 the error is generated from supporting libraries. You might wanna take a look at the source code to figure out which library is causing the error — Dipen Shah, May 18 '17 at 20:04
@DipenShah This is the only useful information so far - I think the error is because I built it without the pipe protocol. — Steve M, May 22 '17 at 16:52
Build without `pipe` is definitely a good cause for the problem. But using `pipe:` for many formats is not good enough, because ffmpeg does not enable seek for this protocol, and some formats need seek for correct read and/or write. To work correctly with files that come from [SAF](https://stackoverflow.com/questions/tagged/storage-access-framework), I [have defined](https://github.com/tanersener/mobile-ffmpeg/pull/440) a custom `saf:` protocol. — Alex Cohn, May 30 '20 at 19:30

William Seemann · Accepted Answer · 2017-05-23T14:32:54.173

3

My project (FFmpegMediaMetadataRetriever) accomplishes this. See this gist or the full file here.

Note: make sure you have built FFmpeg with the pipe protocol enabled!

I used avformat_open_input, dup() the file descriptor and made sure to set the offset via:

 if (state->offset > 0) {
        state->pFormatCtx = avformat_alloc_context();
        state->pFormatCtx->skip_initial_bytes = state->offset;
 }

edited May 23 '17 at 14:32

answered May 22 '17 at 20:49

William Seemann

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score 2 · Answer 2 · edited May 23 '17 at 12:34

2

@Steve M, may be you got your answer from these posts: such as : 1. ffmpeg encoding sample wanted?

and finally you explore this project . https://github.com/illuusio/ffmpeg-example

Best of luck! @Stev M

edited May 23 '17 at 12:34

Community

1
1

answered May 20 '17 at 06:09

@Steve M, you can follow these links, please :) – May 20 '17 at 06:10

score 0 · Answer 3 · answered May 22 '17 at 16:45

0

Maybe it is a silly observation, but did you try to replace:

avformat_open_input(&avFormatPtr, "dummyFilename", nullptr, nullptr);

(avformat_open_input(&pFormatCtx,path,NULL,NULL)

Replace for:

(avformat_open_input(&pFormatCtx,path,nullptr,nullptr)

?

answered May 22 '17 at 16:45

developer_hatch

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ffmpeg libraries are pure C, and calling them from pure C (which does not have a notion of **nullptr**) is entirely legitimate. – Alex Cohn May 30 '20 at 18:12

Passing a native fd int to FFMPEG from openable URI

3 Answers3

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