I want to write a Python C extension that includes a function convertVideo() that converts a video from one format to another making use of FFmpeg 3.4.8 (the libav* libaries). The code of the extension is at the end of the question. The extension compiles successfully but whenever I open Python and want to call it (using a simple Python wrapper code that I don't include here), I get
Python 3.7.10 (default, May 2 2021, 18:28:10)
[GCC 9.1.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import myModule
>>> myModule.convert_video("/home/admin/video.h264", "/home/admin/video.mp4")
convert 0
convert 1
Format raw H.264 video, duration -9223372036854775808 us
convert 2
Segmentation fault (core dumped)
The interesting thing is, I wrote a simple helper program test_convert.cc that calls convertVideo() like so
#include <stdlib.h>
#include <stdio.h>
int convertVideo(const char *in_filename, const char *out_filename);
int main() {
int res = convertVideo("/home/admin/video.h264", "/home/admin/video.mp4");
return 0;
}
and I compiled this program making use of the shared library that Python generates when building the C extension like so
gcc test_convert.cc /usr/lib/python3.7/site-packages/_myModule.cpython-37m-aarch64-linux-gnu.so -o test_convert
And it works! The output is
root# ./test_convert
convert 0
convert 1
Format raw H.264 video, duration -9223372036854775808 us
convert 2
convert 3
convert 4
convert 5
convert 6
Output #0, mp4, to '/home/admin/video.mp4':
Stream #0:0: Video: h264 (High), yuv420p(tv, bt470bg, progressive), 1280x720 [SAR 1:1 DAR 16:9], q=2-31
convert 7
The extension code looks like this
#include <Python.h>
#include <stdio.h>
#include <stdbool.h>
#include <unistd.h>
#include <pthread.h>
#include <stdlib.h>
#include <assert.h>
#include <string.h>
extern "C"
{
#include "libavformat/avformat.h"
#include "libavutil/imgutils.h"
}
int convertVideo(const char *in_filename, const char *out_filename)
{
// Input AVFormatContext and Output AVFormatContext
AVFormatContext *input_format_context = avformat_alloc_context();
AVPacket pkt;
int ret, i;
int frame_index = 0;
printf("convert 0\n");
av_register_all();
printf("convert 1\n");
// Input
if ((ret = avformat_open_input(&input_format_context, in_filename, NULL,
NULL)) < 0)
{
printf("Could not open input file.");
return 1;
}
else
{
printf("Format %s, duration %lld us\n",
input_format_context->iformat->long_name,
input_format_context->duration);
}
printf("convert 2\n");
if ((ret = avformat_find_stream_info(input_format_context, 0)) < 0)
{
printf("Failed to retrieve input stream information");
return 1;
}
printf("convert 3\n");
AVFormatContext *output_format_context = avformat_alloc_context();
AVPacket packet;
int stream_index = 0;
int *streams_list = NULL;
int number_of_streams = 0;
int fragmented_mp4_options = 0;
printf("convert 4\n");
avformat_alloc_output_context2(&output_format_context, NULL, NULL,
out_filename);
if (!output_format_context)
{
fprintf(stderr, "Could not create output context\n");
ret = AVERROR_UNKNOWN;
return 1;
}
printf("convert 5\n");
AVOutputFormat *fmt = av_guess_format(0, out_filename, 0);
output_format_context->oformat = fmt;
number_of_streams = input_format_context->nb_streams;
streams_list =
(int *)av_mallocz_array(number_of_streams, sizeof(*streams_list));
if (!streams_list)
{
ret = AVERROR(ENOMEM);
return 1;
}
for (i = 0; i < input_format_context->nb_streams; i++)
{
AVStream *out_stream;
AVStream *in_stream = input_format_context->streams[i];
AVCodecParameters *in_codecpar = in_stream->codecpar;
if (in_codecpar->codec_type != AVMEDIA_TYPE_AUDIO &&
in_codecpar->codec_type != AVMEDIA_TYPE_VIDEO &&
in_codecpar->codec_type != AVMEDIA_TYPE_SUBTITLE)
{
streams_list[i] = -1;
continue;
}
streams_list[i] = stream_index++;
out_stream = avformat_new_stream(output_format_context, NULL);
if (!out_stream)
{
fprintf(stderr, "Failed allocating output stream\n");
ret = AVERROR_UNKNOWN;
return 1;
}
ret = avcodec_parameters_copy(out_stream->codecpar, in_codecpar);
if (ret < 0)
{
fprintf(stderr, "Failed to copy codec parameters\n");
return 1;
}
}
printf("convert 6\n");
av_dump_format(output_format_context, 0, out_filename, 1);
if (!(output_format_context->oformat->flags & AVFMT_NOFILE))
{
ret = avio_open(&output_format_context->pb, out_filename, AVIO_FLAG_WRITE);
if (ret < 0)
{
fprintf(stderr, "Could not open output file '%s'", out_filename);
return 1;
}
}
AVDictionary *opts = NULL;
printf("convert 7\n");
ret = avformat_write_header(output_format_context, &opts);
if (ret < 0)
{
fprintf(stderr, "Error occurred when opening output file\n");
return 1;
}
int n = 0;
while (1)
{
AVStream *in_stream, *out_stream;
ret = av_read_frame(input_format_context, &packet);
if (ret < 0)
break;
in_stream = input_format_context->streams[packet.stream_index];
if (packet.stream_index >= number_of_streams ||
streams_list[packet.stream_index] < 0)
{
av_packet_unref(&packet);
continue;
}
packet.stream_index = streams_list[packet.stream_index];
out_stream = output_format_context->streams[packet.stream_index];
out_stream->codec->time_base.num = 1;
out_stream->codec->time_base.den = 30;
packet.pts = n * 3000;
packet.dts = n * 3000;
packet.duration = 3000;
packet.pos = -1;
ret = av_interleaved_write_frame(output_format_context, &packet);
if (ret < 0)
{
fprintf(stderr, "Error muxing packet\n");
break;
}
av_packet_unref(&packet);
n++;
}
av_write_trailer(output_format_context);
avformat_close_input(&input_format_context);
if (output_format_context &&
!(output_format_context->oformat->flags & AVFMT_NOFILE))
avio_closep(&output_format_context->pb);
avformat_free_context(output_format_context);
av_freep(&streams_list);
if (ret < 0 && ret != AVERROR_EOF)
{
fprintf(stderr, "Error occurred\n");
return 1;
}
return 0;
}
// PyMethodDef and other orchestration code is skipped
What is the reason that the code works as expected in my test_convert but not within Python?
