Overview
I'm trying to build one of the example files from the ffmpeg src directory on Windows using mingw, but I get a ton of undefined references to a bunch of the libav* functions.
I've already looked at this question and several others, but none of them resolved my issue. Specifically, I am not building a C++ program, and I am not using g++, so the whole extern "C" thing does not apply (in fact I already tried it, and it didn't work).
Details
I've downloaded Zeranoe's 64-bit dev package, and unpacked it to C:\apps\pf\ffmpeg. Now I'm just trying to build the decoding_encoding.c file in doc/examples. My CWD is C:\apps\pf\ffmpeg\doc\examples, and my commands look like this (line-continuation and comments added):
#Compile
gcc -o decoding_encoding.obj -c -IC:\apps\pf\ffmpeg\include decoding_encoding.c
#Link
gcc -o decoding_encoding.exe decoding_encoding.obj \
-LC:\apps\pf\ffmpeg\lib -lavformat -lavcodec -lavdevice \
-lavfilter -lavutil -lswscale -lm
The compile works fine, but the link spits out these errors:
decoding_encoding.obj:decoding_encoding.c:(.text+0xca): undefined reference to `av_get_channel_layout_nb_channels'
decoding_encoding.obj:decoding_encoding.c:(.text+0x131): undefined reference to `avcodec_find_encoder'
decoding_encoding.obj:decoding_encoding.c:(.text+0x179): undefined reference to `avcodec_alloc_context3'
decoding_encoding.obj:decoding_encoding.c:(.text+0x1fa): undefined reference to `av_get_sample_fmt_name'
decoding_encoding.obj:decoding_encoding.c:(.text+0x26c): undefined reference to `av_get_channel_layout_nb_channels'
decoding_encoding.obj:decoding_encoding.c:(.text+0x28f): undefined reference to `avcodec_open2'
decoding_encoding.obj:decoding_encoding.c:(.text+0x313): undefined reference to `avcodec_alloc_frame'
decoding_encoding.obj:decoding_encoding.c:(.text+0x3c8): undefined reference to `av_samples_get_buffer_size'
decoding_encoding.obj:decoding_encoding.c:(.text+0x3d6): undefined reference to `av_malloc'
decoding_encoding.obj:decoding_encoding.c:(.text+0x44f): undefined reference to `avcodec_fill_audio_frame'
decoding_encoding.obj:decoding_encoding.c:(.text+0x4ce): undefined reference to `av_init_packet'
decoding_encoding.obj:decoding_encoding.c:(.text+0x5c2): undefined reference to `avcodec_encode_audio2'
decoding_encoding.obj:decoding_encoding.c:(.text+0x63a): undefined reference to `av_free_packet'
decoding_encoding.obj:decoding_encoding.c:(.text+0x680): undefined reference to `avcodec_encode_audio2'
decoding_encoding.obj:decoding_encoding.c:(.text+0x6f8): undefined reference to `av_free_packet'
decoding_encoding.obj:decoding_encoding.c:(.text+0x722): undefined reference to `av_freep'
decoding_encoding.obj:decoding_encoding.c:(.text+0x72d): undefined reference to `avcodec_free_frame'
decoding_encoding.obj:decoding_encoding.c:(.text+0x738): undefined reference to `avcodec_close'
decoding_encoding.obj:decoding_encoding.c:(.text+0x743): undefined reference to `av_free'
decoding_encoding.obj:decoding_encoding.c:(.text+0x77b): undefined reference to `av_init_packet'
decoding_encoding.obj:decoding_encoding.c:(.text+0x7a1): undefined reference to `avcodec_find_decoder'
decoding_encoding.obj:decoding_encoding.c:(.text+0x7e9): undefined reference to `avcodec_alloc_context3'
decoding_encoding.obj:decoding_encoding.c:(.text+0x840): undefined reference to `avcodec_open2'
decoding_encoding.obj:decoding_encoding.c:(.text+0x8e6): undefined reference to `av_free'
decoding_encoding.obj:decoding_encoding.c:(.text+0x947): undefined reference to `avcodec_alloc_frame'
decoding_encoding.obj:decoding_encoding.c:(.text+0x999): undefined reference to `avcodec_get_frame_defaults'
decoding_encoding.obj:decoding_encoding.c:(.text+0x9c2): undefined reference to `avcodec_decode_audio4'
decoding_encoding.obj:decoding_encoding.c:(.text+0xa44): undefined reference to `av_samples_get_buffer_size'
decoding_encoding.obj:decoding_encoding.c:(.text+0xb7d): undefined reference to `avcodec_close'
decoding_encoding.obj:decoding_encoding.c:(.text+0xb88): undefined reference to `av_free'
decoding_encoding.obj:decoding_encoding.c:(.text+0xb96): undefined reference to `avcodec_free_frame'
decoding_encoding.obj:decoding_encoding.c:(.text+0xbe4): undefined reference to `avcodec_find_encoder'
decoding_encoding.obj:decoding_encoding.c:(.text+0xc2c): undefined reference to `avcodec_alloc_context3'
decoding_encoding.obj:decoding_encoding.c:(.text+0xcee): undefined reference to `av_opt_set'
decoding_encoding.obj:decoding_encoding.c:(.text+0xd08): undefined reference to `avcodec_open2'
decoding_encoding.obj:decoding_encoding.c:(.text+0xd8c): undefined reference to `avcodec_alloc_frame'
decoding_encoding.obj:decoding_encoding.c:(.text+0xe2f): undefined reference to `av_image_alloc'
decoding_encoding.obj:decoding_encoding.c:(.text+0xe83): undefined reference to `av_init_packet'
decoding_encoding.obj:decoding_encoding.c:(.text+0xfd3): undefined reference to `avcodec_encode_video2'
decoding_encoding.obj:decoding_encoding.c:(.text+0x105f): undefined reference to `av_free_packet'
decoding_encoding.obj:decoding_encoding.c:(.text+0x10a9): undefined reference to `avcodec_encode_video2'
decoding_encoding.obj:decoding_encoding.c:(.text+0x1135): undefined reference to `av_free_packet'
decoding_encoding.obj:decoding_encoding.c:(.text+0x117e): undefined reference to `avcodec_close'
decoding_encoding.obj:decoding_encoding.c:(.text+0x1189): undefined reference to `av_free'
decoding_encoding.obj:decoding_encoding.c:(.text+0x1194): undefined reference to `av_freep'
decoding_encoding.obj:decoding_encoding.c:(.text+0x119f): undefined reference to `avcodec_free_frame'
decoding_encoding.obj:decoding_encoding.c:(.text+0x1272): undefined reference to `avcodec_decode_video2'
decoding_encoding.obj:decoding_encoding.c:(.text+0x13b1): undefined reference to `av_init_packet'
decoding_encoding.obj:decoding_encoding.c:(.text+0x13fa): undefined reference to `avcodec_find_decoder'
decoding_encoding.obj:decoding_encoding.c:(.text+0x1442): undefined reference to `avcodec_alloc_context3'
decoding_encoding.obj:decoding_encoding.c:(.text+0x14ba): undefined reference to `avcodec_open2'
decoding_encoding.obj:decoding_encoding.c:(.text+0x153e): undefined reference to `avcodec_alloc_frame'
decoding_encoding.obj:decoding_encoding.c:(.text+0x1674): undefined reference to `avcodec_close'
decoding_encoding.obj:decoding_encoding.c:(.text+0x167f): undefined reference to `av_free'
decoding_encoding.obj:decoding_encoding.c:(.text+0x168a): undefined reference to `avcodec_free_frame'
decoding_encoding.obj:decoding_encoding.c:(.text+0x16ab): undefined reference to `avcodec_register_all'
collect2.exe: error: ld returned 1 exit status
I can already see in the README that this library was built with MinGW-w64 and is targeting 64-bit. Plain old MinGW does not support 64-bit targets. Therefore, if you are using MinGW, then of course it cannot link against 64-bit libraries, hence you experience these errors.
To solve your problem, simply install MinGW-w64. I highly recommend to download its distribution from here. There is no installation nightmare: just unpack, add bin to the PATH environment variable and you are good to go.
Finally, take a look at this post, it might be useful for you to understand who is who.