Decode Audio from Memory - C++

I have two functions:

• a internet-socket function which gets mp3-data and writes it to file ,
• a function which decodes mp3-files.

However, I would rather decode the data, which is currently written to disk, be decoded in-memory by the decode function.

My decode function looks like this, and it is all initialized via

    avformat_open_input(AVCodecContext, filename, NULL, NULL) 

How can I read in the AVCodecContext without a filename, and instead using only the in-memory buffer?