MyIOContext

MyIOContext.cpp

#include <cstdio>
#include <string>

class MyIOContext {
public:
	std::string datafile;
	AVIOContext *ioCtx;
	uint8_t *buffer; // internal buffer for ffmpeg
	int bufferSize;
	FILE *fh;

public:
	MyIOContext(const std::string &datafile);
	~MyIOContext();

	void initAVFormatContext(AVFormatContext *);
};

static int IOReadFunc(void *data, uint8_t *buf, int buf_size) {
	MyIOContext *hctx = (MyIOContext*)data;
	size_t len = fread(buf, 1, buf_size, hctx->fh);
	if (len == 0) {
		// Let FFmpeg know that we have reached EOF, or do something else
		return AVERROR_EOF;
	}
	return (int)len;
}

// whence: SEEK_SET, SEEK_CUR, SEEK_END (like fseek) and AVSEEK_SIZE
static int64_t IOSeekFunc(void *data, int64_t pos, int whence) {
	if (whence == AVSEEK_SIZE) {
		// return the file size if you wish to
	}
	
	MyIOContext *hctx = (MyIOContext*)data;
	int rs = fseek(hctx->fh, (long)pos, whence);
	if (rs != 0) {
		return -1;
	}
	long fpos = ftell(hctx->fh); // int64_t is usually long long
	return (int64_t)fpos;
}

MyIOContext::MyIOContext(const std::string &s) {
	datafile.assign(s);
	
	// allocate buffer
	bufferSize = 4096;
	buffer = (uint8_t *)av_malloc(bufferSize); // see destructor for details
	
	// open file
	fh = fopen(datafile.c_str(), "rb");
	if (!fh) {
		fprintf(stderr, "MyIOContext: failed to open file %s\n",
			datafile.c_str());
	}
	
	// allocate the AVIOContext
	ioCtx = avio_alloc_context(
				buffer, bufferSize, // internal buffer and its size
				0,            // write flag (1=true, 0=false) 
				(void*)this,  // user data, will be passed to our callback functions
				IOReadFunc, 
				0,            // no writing
				IOSeekFunc
	);
}

MyIOContext::~MyIOContext() {
	if (fh) fclose(fh);
	
	av_free(buffer);
	av_free(ioCtx);
}

void MyIOContext::initAVFormatContext(AVFormatContext *pCtx) {
	pCtx->pb = ioCtx;
	pCtx->flags |= AVFMT_FLAG_CUSTOM_IO;
	
	// you can specify a format directly
	//pCtx->iformat = av_find_input_format("h264");
	
	// or read some of the file and let ffmpeg do the guessing
	size_t len = fread(buffer, 1, bufferSize, fh);
	if (len == 0) return;
	fseek(fh, 0, SEEK_SET); // reset to beginning of file
	
	AVProbeData probeData;
	probeData.buf = buffer;
	probeData.buf_size = bufferSize - 1;
	probeData.filename = "";
	pCtx->iformat = av_probe_input_format(&probeData, 1);
}

Hello Marika, great job with this sample. Saved me a lot of time figuring out the interface for the callback functions.

If I may suggest an improvement, consider following changes:
line 49:

buffer = (uint8_t *)av_malloc(bufferSize + AVPROBE_PADDING_SIZE); // see destructor for details

line 90:

probeData.buf_size = bufferSize;
probeData.mime_type = "";

Setting the padding to 1 instead of the defined value in the library can cause invalid read outside of the buffer, which leads to segfault.
And apparently not setting the mime_type can cause trouble as well. Empty string should work fine, as avformat tries to match it against its known types, before using it.
Source:
https://ffmpeg.org/doxygen/2.8/structAVProbeData.html#a814cca49dda3f578ebb32d4b2f74368a
Cheers!