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• Go / Cgo - How to access a field of a Cstruct ?

  17 août 2021, par ChrisG

  I develope an application in Go for transcode an audio file from one format to another one :

  


  I use the goav library that use Cgo to bind the FFmpeg C-libs :
https://github.com/giorgisio/goav/

  


  ---

  


  The goav library ; package avformat has a typedef that cast the original FFmpeg lib C-Struct AVOutputFormat :

  


  type ( 
   OutputFormat               C.struct_AVOutputFormat
)


  


  In my code i have a variable called outputF of the type OutputFormat that is a C.struct_AVOutputFormat.

  


  The C real AVOutputFormat struct has fields :

  


  name, long_name, mime_type, extensions, audio_codec, video_codec, subtitle_codec,..


  


  and many fields more.

  


  See : https://ffmpeg.org/doxygen/2.6/structAVOutputFormat.html

  


  ---

  


  I verified the situation by fmt.Println(outputF) and reached :

  


  {0x7ffff7f23383 0x7ffff7f23907 0x7ffff7f13c33 0x7ffff7f23383 86017 61 0 128 <nil> 0x7ffff7f8cfa0 <nil> 3344 0x7ffff7e3ec10 0x7ffff7e3f410 0x7ffff7e3ecc0 <nil> 0x7ffff7e3dfc0 <nil> <nil> <nil> <nil> <nil> <nil> 0 0x7ffff7e3e070 0x7ffff7e3e020 <nil>}&#xA;</nil></nil></nil></nil></nil></nil></nil></nil></nil></nil>

  


  The audio codec field is on position 5 and contains 86017

  


  I verified the field name by using the package reflect :

  


  val := reflect.Indirect(reflect.ValueOf(outputF))
fmt.Println(val)
fmt.Println("Fieldname: ", val.Type().Field(4).Name)

Output:
Fieldname:  audio_codec


  


  ---

  


  I try to access the field audio_codec of the original AVOutputFormat using :

  


  fmt.Println(outputF.audio_codec)
ERROR: outputF.audio_codec undefined (cannot refer to unexported field or method audio_codec)


fmt.Println(outputF._audio_codec)
ERROR: outputF._audio_codec undefined (type *avformat.OutputFormat has no field or method _audio_codec)


  


  


  As i read in the Cgo documentation :
Within the Go file, C's struct field names that are keywords in Go can be accessed by prefixing them with an underscore : if x points at a C struct with a field named "type", x._type accesses the field. C struct fields that cannot be expressed in Go, such as bit fields or misaligned data, are omitted in the Go struct, replaced by appropriate padding to reach the next field or the end of the struct.

  


  


  But I have no idea what im doing wrong.

  


  Edit :
Okay for sure no underscore is required as audio_codec is not a keyword in Go. This i understood for now. But still there is the question why im not able to access the CStruct field "audio_codec".

  


• Go / Cgo - How to access a field of a Cstruct - could not make it

  15 août 2021, par ChrisG

  I develope an application in Go for transcode an audio file from one format to another one :

  


  I use the goav library that use Cgo to bind the FFmpeg C-libs :
https://github.com/giorgisio/goav/

  


  ---

  


  The goav library ; package avformat has a typedef that cast the original FFmpeg lib C-Struct AVOutputFormat :

  


  type ( 
   OutputFormat               C.struct_AVOutputFormat
)


  


  In my code i have a variable called outputF of the type OutputFormat that is a C.struct_AVOutputFormat.

  


  The C real AVOutputFormat struct has fields :

  


  name, long_name, mime_type, extensions, audio_codec, video_codec, subtitle_codec,..


  


  and many fields more.

  


  See : https://ffmpeg.org/doxygen/2.6/structAVOutputFormat.html

  


  ---

  


  I verified the situation by fmt.Println(outputF) and reached :

  


  {0x7ffff7f23383 0x7ffff7f23907 0x7ffff7f13c33 0x7ffff7f23383 86017 61 0 128 <nil> 0x7ffff7f8cfa0 <nil> 3344 0x7ffff7e3ec10 0x7ffff7e3f410 0x7ffff7e3ecc0 <nil> 0x7ffff7e3dfc0 <nil> <nil> <nil> <nil> <nil> <nil> 0 0x7ffff7e3e070 0x7ffff7e3e020 <nil>}&#xA;</nil></nil></nil></nil></nil></nil></nil></nil></nil></nil>

  


  The audio codec field is on position 5 and contains 86017

  


  I verified the field name by using the package reflect :

  


  val := reflect.Indirect(reflect.ValueOf(outputF))
fmt.Println(val)
fmt.Println("Fieldname: ", val.Type().Field(4).Name)

Output:
Fieldname:  audio_codec


  


  ---

  


  I try to access the field audio_codec of the original AVOutputFormat using :

  


  fmt.Println(outputF.audio_codec)
ERROR: outputF.audio_codec undefined (cannot refer to unexported field or method audio_codec)


fmt.Println(outputF._audio_codec)
ERROR: outputF._audio_codec undefined (type *avformat.OutputFormat has no field or method _audio_codec)


  


  


  As i read in the Cgo documentation :
Within the Go file, C's struct field names that are keywords in Go can be accessed by prefixing them with an underscore : if x points at a C struct with a field named "type", x._type accesses the field. C struct fields that cannot be expressed in Go, such as bit fields or misaligned data, are omitted in the Go struct, replaced by appropriate padding to reach the next field or the end of the struct.

  


  


  But I have no idea what im doing wrong.

  


  Edit :
Okay for sure no underscore is required as audio_codec is not a keyword in Go. This i understood for now. But still there is the question why im not able to access the CStruct field "audio_codec".

  


• Duration of wav file saved in S3 using AWS Lambda

  3 juin 2021, par Salim Shamim

  Objective

  


  To calculate the duration of a wav file which is saved in S3 by AWS Lambda using node.js. I had to add ffmpeg and ffprobe executable inside a lambda layer (Downloaded linux-64 version from here). These files could be found in /opt folder on lambda file system.

  


  What I have tried

  


  I have been trying using ffprobe in numerous ways, but I get Invalid Data as error.
Here's one example

  


  const AWS = require('aws-sdk');
const s3 = new AWS.S3();
const fs = require('fs');
const ffmpeg = require('fluent-ffmpeg');

exports.handler = async function(event) {
    let path = await load();
    console.log(`Saved Path ${path}`);

    ffmpeg.setFfmpegPath('/opt/ffmpeg');
    ffmpeg.setFfprobePath("/opt/ffprobe");

    let dur = await duration(path).catch(err => {
        console.log(err);
    })
    console.log(dur);
}


function duration(path) {
    return new Promise((resolve, reject) => {
        ffmpeg(path).ffprobe(path, function(err, metadata) {
            //console.dir(metadata); // all metadata
            if (err) {
                reject(err);
            }
            else {
                resolve(metadata.format.duration);

            }
        });
    })
}

async function listFiles(path) {
    console.log('list files');
    return new Promise((resolve, reject) => {
        fs.readdir(path, (err, files) => {
            if (err) {
                console.error('Error in readdir');
                reject(err);
            }
            else {
                console.log('recieved files');
                resolve(files);
            }

        });

    });

}

async function load() {
    return new Promise((resolve, reject) => {
        let params = {
            Key: 'Fanfare60.wav',
            Bucket: 'samplevideosshamim'
        };
        console.log(`Getting s3 object : ${JSON.stringify(params)}`);
        s3.getObject(params, (err, data) => {
            if (err) {
                console.error(err);
                reject(err);
            }
            else if (data) {
                console.log('Recieved Data');
                let path = `/tmp/${params.Key}`;
                console.log('Path: ' + path);
                fs.writeFileSync(path, data.body);
                resolve(path);
            }
        });
    });

}


  


  Error :

  


  Error: ffprobe exited with code 1&#xA;ffprobe version 4.2.1-static https://johnvansickle.com/ffmpeg/  Copyright (c) 2007-2019 the FFmpeg developers&#xA;  built with gcc 6.3.0 (Debian 6.3.0-18&#x2B;deb9u1) 20170516&#xA;  configuration: --enable-gpl --enable-version3 --enable-static --disable-debug --disable-ffplay --disable-indev=sndio --disable-outdev=sndio --cc=gcc-6 --enable-fontconfig --enable-frei0r --enable-gnutls --enable-gmp --enable-libgme --enable-gray --enable-libaom --enable-libfribidi --enable-libass --enable-libvmaf --enable-libfreetype --enable-libmp3lame --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libopenjpeg --enable-librubberband --enable-libsoxr --enable-libspeex --enable-libsrt --enable-libvorbis --enable-libopus --enable-libtheora --enable-libvidstab --enable-libvo-amrwbenc --enable-libvpx --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxml2 --enable-libdav1d --enable-libxvid --enable-libzvbi --enable-libzimg&#xA;  libavutil      56. 31.100 / 56. 31.100&#xA;  libavcodec     58. 54.100 / 58. 54.100&#xA;  libavformat    58. 29.100 / 58. 29.100&#xA;  libavdevice    58.  8.100 / 58.  8.100&#xA;  libavfilter     7. 57.100 /  7. 57.100&#xA;  libswscale      5.  5.100 /  5.  5.100&#xA;  libswresample   3.  5.100 /  3.  5.100&#xA;  libpostproc    55.  5.100 / 55.  5.100&#xA;/tmp/Fanfare60.wav: Invalid data found when processing input&#xA;&#xA;    at ChildProcess.<anonymous> (/var/task/node_modules/fluent-ffmpeg/lib/ffprobe.js:233:22)&#xA;    at ChildProcess.emit (events.js:314:20)&#xA;    at ChildProcess.EventEmitter.emit (domain.js:483:12)&#xA;    at Process.ChildProcess._handle.onexit (internal/child_process.js:276:12)&#xA;</anonymous>

  


  I am guessing it doesn't support wav format, but internet searches provide no proof of that.

  


  A point to note here is, I was able to get the duration of a local file when I ran this code on my local machine, but I have a windows machine, so perhaps only linux executable of ffprobe has issue ?

  


  Possible Solutions I am looking for

  


  

  1. Is there a way to specify format ?

  


  2. Can I use a different library (code example for the same) ?

  


  3. Any possible way to get duration of a wav file in the mentioned scenario (AWS Lambda NodeJS and S3 file (private file) ?