[Libav-user] alternative to file inputs

Thu Jan 23 22:41:11 EET 2020

avio_alloc_context() is the right direction.  Here is my implementation 
of your "read_fun" which uses a QIODevice 
<https://doc.qt.io/qt-5/qiodevice.html> instead of std::fstream.   libav 
calls read_fun when it wants data.  opaque will point to your ifs.  You 
need to read from ifs and fill libav's buffer (*buf) and then return how 
many bytes you put into the buffer.

static int read_fun(void *opaque, uint8_t *buf, int bufSize)
{
     QIODevice *dataStream = (QIODevice *) opaque;
     bufSize = FFMIN(bufSize, dataStream->bytesAvailable());
     dataStream->read((char*)buf, bufSize);

     return bufSize;
}

On 2020-01-23 1:22 p.m., Ed Rupp wrote:
> Hi,
>
> TL;DR; Any direction on how to feed avformat a buffer of bytes (the 
> contents of which are a complete file) instead of a file from disk is 
> all I really need.
> Thanks for making this lib it's pretty amazing and I'm just getting 
> started...
>
> I'm working within a framework in which I receive complete files in 
> the form of a uint8_t* and length.
> At first it seemed like the way to go would be custom IO...
> Here's some of the code
> //////////////////////
> AVFormatContext *fmt = avformat_alloc_context();
> std::fstream ifs;
> ifs.read(reinterpret_cast<char*>(my_data_from_framework), length);
> uint8_t *buf = (unsigned char*)av_malloc(8192); // magic number from 
> web examples
> //The read functions below us fstream operations tellg and seekp.
> AVIOContext *avio = avio_alloc_context(buf, 8192, 0, (void*)&ifs, 
> read_fun, NULL, seek_fun);
> fmt->pb = avio;
>
> //Some of the docs I read made it seem like passing a NULL for 
> filename would work.
> int status = avformat_open_input(&fmt, NULL, NULL,NULL)
> //avformat_open_input is returning -1 and yes the seg fault happens at 
> the next call below
>
> ///////////////
>
> After doing the above I follow up with the following call and get a 
> segfault
>
> avformat_find_stream_info(fmt, NULL);
>
>
> The docs and the code in libformat utils.c: init_input (see below) 
> make it seem like I should be able to do this i.e. set pb on 
> AVFormatContext but its not doing what I hoped:
> 325 /* Open input file and probe the format if necessary. */
> 326 
> <https://www.ffmpeg.org/doxygen/2.5/libavformat_2utils_8c.html#a18b3e6a0f95cd0b00f35765781f19d36> 
> static int init_input 
> <https://www.ffmpeg.org/doxygen/2.5/libavformat_2utils_8c.html#a18b3e6a0f95cd0b00f35765781f19d36>(AVFormatContext 
> <https://www.ffmpeg.org/doxygen/2.5/structAVFormatContext.html> *s 
> <https://www.ffmpeg.org/doxygen/2.5/avisynth__c_8h.html#ab87f55bd0280d90925050a4188c14ab5>, 
> const char *filename,
> 327 AVDictionary 
> <https://www.ffmpeg.org/doxygen/2.5/structAVDictionary.html> **options 
> <https://www.ffmpeg.org/doxygen/2.5/ffmpeg_8h.html#abdbaa7127ef32aa4c435f3c609b4c2cd>)
> 328 {
> 329 int ret 
> <https://www.ffmpeg.org/doxygen/2.5/avfilter_8c.html#a339672ff94e6199019102f50d317c3d7>;
> 330 AVProbeData 
> <https://www.ffmpeg.org/doxygen/2.5/structAVProbeData.html> pd = { 
> filename, NULL, 0 };
> 331 int score = AVPROBE_SCORE_RETRY 
> <https://www.ffmpeg.org/doxygen/2.5/avformat_8h.html#adeacd8263046043734062588db39bce9>;
> 332
> 333 if (s->pb 
> <https://www.ffmpeg.org/doxygen/2.5/structAVFormatContext.html#a1e7324262b6b78522e52064daaa7bc87>) 
> {
> 334  s->flags 
> <https://www.ffmpeg.org/doxygen/2.5/structAVFormatContext.html#a32379cc371463b235d54235d4af06a15> 
> |= AVFMT_FLAG_CUSTOM_IO 
> <https://www.ffmpeg.org/doxygen/2.5/avformat_8h.html#ac5fdea71141dcc30346a57df75cf408e>;
> 335 if (!s->iformat 
> <https://www.ffmpeg.org/doxygen/2.5/structAVFormatContext.html#a78efc5a53c21c8d81197445207ac4374>)
> 336 return av_probe_input_buffer2 
> <https://www.ffmpeg.org/doxygen/2.5/group__lavf__decoding.html#ga69e44bd9ade0160a8cd6196aa37505ea>(s->pb 
> <https://www.ffmpeg.org/doxygen/2.5/structAVFormatContext.html#a1e7324262b6b78522e52064daaa7bc87>, 
> &s->iformat 
> <https://www.ffmpeg.org/doxygen/2.5/structAVFormatContext.html#a78efc5a53c21c8d81197445207ac4374>, 
> filename,
> 337  s, 0, s->format_probesize 
> <https://www.ffmpeg.org/doxygen/2.5/structAVFormatContext.html#a7d1f665ecabe03e3d18accbb1ebb32b1>);
> 338 else if (s->iformat 
> <https://www.ffmpeg.org/doxygen/2.5/structAVFormatContext.html#a78efc5a53c21c8d81197445207ac4374>->flags 
> <https://www.ffmpeg.org/doxygen/2.5/structAVInputFormat.html#a1b30f6647d0c2faf38ba8786d7c3a838> 
> & AVFMT_NOFILE 
> <https://www.ffmpeg.org/doxygen/2.5/avformat_8h.html#a752cce390d480521919aa5d8be24ac0b>)
> 339 av_log 
> <https://www.ffmpeg.org/doxygen/2.5/log_8c.html#a33c4311b86f031152c5a23b91c4fe862>(s, 
> AV_LOG_WARNING 
> <https://www.ffmpeg.org/doxygen/2.5/group__lavu__log__constants.html#ga85b57516ca703cc47d9bbe5f4658c716>, 
> "Custom AVIOContext makes no sense and "
> 340 "will be ignored with AVFMT_NOFILE format.\n");
> 341 return 0;
> 342  }
> 343
> 344 if ((s->iformat 
> <https://www.ffmpeg.org/doxygen/2.5/structAVFormatContext.html#a78efc5a53c21c8d81197445207ac4374> 
> && s->iformat 
> <https://www.ffmpeg.org/doxygen/2.5/structAVFormatContext.html#a78efc5a53c21c8d81197445207ac4374>->flags 
> <https://www.ffmpeg.org/doxygen/2.5/structAVInputFormat.html#a1b30f6647d0c2faf38ba8786d7c3a838> 
> & AVFMT_NOFILE 
> <https://www.ffmpeg.org/doxygen/2.5/avformat_8h.html#a752cce390d480521919aa5d8be24ac0b>) 
> ||
> 345  (!s->iformat 
> <https://www.ffmpeg.org/doxygen/2.5/structAVFormatContext.html#a78efc5a53c21c8d81197445207ac4374> 
> && (s->iformat 
> <https://www.ffmpeg.org/doxygen/2.5/structAVFormatContext.html#a78efc5a53c21c8d81197445207ac4374> 
> = av_probe_input_format2 
> <https://www.ffmpeg.org/doxygen/2.5/group__lavf__decoding.html#gae8b938f6e7c3741dd27a6c171e72f33d>(&pd, 
> 0, &score))))
> 346 return score;
> 347
> 348 if ((ret = avio_open2 
> <https://www.ffmpeg.org/doxygen/2.5/avio_8h.html#ade8a63980569494c99593ebf0d1e891b>(&s->pb 
> <https://www.ffmpeg.org/doxygen/2.5/structAVFormatContext.html#a1e7324262b6b78522e52064daaa7bc87>, 
> filename, AVIO_FLAG_READ 
> <https://www.ffmpeg.org/doxygen/2.5/avio_8h.html#a21e61cb486bd1588eb7f775998cf8c77> 
> | s->avio_flags 
> <https://www.ffmpeg.org/doxygen/2.5/structAVFormatContext.html#a5e6814c9de3c272396f07e2ff18c7b27>,
> 349  &s->interrupt_callback 
> <https://www.ffmpeg.org/doxygen/2.5/structAVFormatContext.html#a5b37acfe4024d92ee510064e80920b40>, 
> options)) < 0)
> 350 return ret 
> <https://www.ffmpeg.org/doxygen/2.5/avfilter_8c.html#a339672ff94e6199019102f50d317c3d7>;
> 351 if (s->iformat 
> <https://www.ffmpeg.org/doxygen/2.5/structAVFormatContext.html#a78efc5a53c21c8d81197445207ac4374>)
> 352 return 0;
> 353 return av_probe_input_buffer2 
> <https://www.ffmpeg.org/doxygen/2.5/group__lavf__decoding.html#ga69e44bd9ade0160a8cd6196aa37505ea>(s->pb 
> <https://www.ffmpeg.org/doxygen/2.5/structAVFormatContext.html#a1e7324262b6b78522e52064daaa7bc87>, 
> &s->iformat 
> <https://www.ffmpeg.org/doxygen/2.5/structAVFormatContext.html#a78efc5a53c21c8d81197445207ac4374>, 
> filename,
> 354
> Thanks
> Ed
>
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