gcc -o test test.c -lavformat && ./test ~/Music/eddie-money-home-tonight.m4a

test.c

#include <stdio.h>
#include <stdlib.h>
#include <libavcodec/avcodec.h>
#include <libavfilter/avfilter.h>
#include <libavfilter/buffersink.h>
#include <libavfilter/buffersrc.h>
#include <libavformat/avformat.h>
#include <libavformat/avio.h>
#include <libavutil/channel_layout.h>
#include <libavutil/dict.h>
#include <libavutil/frame.h>
#include <libavutil/log.h>
#include <libavutil/mem.h>
#include <libavutil/opt.h>

struct Context {
    FILE *file;
};

static void panic(char *msg) {
    fprintf(stderr, "%s\n", msg);
    abort();
}

static int read_packet(void *opaque, uint8_t *buf, int buf_size) {
    struct Context *context = opaque;
    return fread(buf, 1, buf_size, context->file);
}

static int write_packet(void *opaque, uint8_t *buf, int buf_size) {
    struct Context *context = opaque;
    return fwrite(buf, 1, buf_size, context->file);
}

static int64_t seek(void *opaque, int64_t offset, int whence) {
    struct Context *context = opaque;
    return fseek(context->file, offset, whence);
}

static int decode_interrupt_cb(void *ctx) {
    return 0;
}

int main(int argc, char **argv) {
    struct Context context;

    avcodec_register_all();
    av_register_all();

    char *filename = argv[1];

    context.file = fopen(filename, "rb");
    if (!context.file)
        panic("unable to open file");

    AVFormatContext *ic = avformat_alloc_context();
    if (!ic)
        panic("unable to alloc ic");

    ic->interrupt_callback.callback = decode_interrupt_cb;
    ic->interrupt_callback.opaque = &context;

    static const int buffer_size = 8 * 1024;
    unsigned char *avio_buf = av_malloc(buffer_size);
    if (!avio_buf)
        panic("unable to alloc avio_buf");

    AVIOContext *avio = avio_alloc_context(avio_buf, buffer_size, 0, &context,
            read_packet, write_packet, seek);
    if (!avio)
        panic("unable to alloc avio context");


    // comment this line out and it will work
    ic->pb = avio;

    int err = avformat_open_input(&ic, filename, NULL, NULL);

    if (err < 0) {
        char buf[512];
        av_strerror(err, buf, sizeof(buf));
        fprintf(stderr, "open error: '%s'\n", buf);
        panic("it failed");
    }

    fprintf(stderr, "OK\n");

    return 0;
}