Undefined references to swscale functions

Question

My app fails to link to swscale library at all. I mean that any swscale method I'm trying to use in my code becomes an undefined reference. I know what this error means - a linker couldn't find any definition of used functions.

But the problem is that I made sure that:

1. -L/usr/local/Cellar/ffmpeg/2.8.2/lib -lswscale is in linker output, and the linker actually finds the lib, otherwise it would complain about missing swscale

2. The library is really there:

   $ll /usr/local/Cellar/ffmpeg/2.8.2/lib/libswscale*

output:

-r--r--r--  1 myuser  admin   519K Nov 20 19:33 /usr/local/Cellar/ffmpeg/2.8.2/lib/libswscale.3.1.101.dylib
lrwxr-xr-x  1 myuser  admin    24B Nov 20 19:32 /usr/local/Cellar/ffmpeg/2.8.2/lib/libswscale.3.dylib -> libswscale.3.1.101.dylib
-r--r--r--  1 myuser  admin   607K Nov 20 19:32 /usr/local/Cellar/ffmpeg/2.8.2/lib/libswscale.a
lrwxr-xr-x  1 myuser  admin    24B Nov 20 19:32 /usr/local/Cellar/ffmpeg/2.8.2/lib/libswscale.dylib -> libswscale.3.1.101.dylib

3. The library is of the correct architecture:

   $lipo -info /usr/local/Cellar/ffmpeg/2.8.2/lib/libswscale.3.dylib

output:

 Non-fat file: /usr/local/Cellar/ffmpeg/2.8.2/lib/libswscale.3.dylib is architecture: x86_64

My machine architecture:

$uname -m

output:

x86_64

4. The symbols exist in the binary, for example:

   $nm /usr/local/Cellar/ffmpeg/2.8.2/lib/libswscale.3.1.101.dylib | grep sws_alloc_context

output:

000000000004e3b4 T _sws_alloc_context

5. I also tried a couple of different swscale binaries, not only the "brew" ones, but nothing changed. I also tried to do the same on my Ubuntu laptop, but it is still the same.

This is actually a QT app I'm trying to build. I should mention, that I don't have issues with linking other libraries the same way. I also compiled successfully swscale-test.c (which is in libwscale sources) using:

clang swscale-test.c -lswscale -lavutil

Do you have any ideas what is wrong or what else can I try to find out the reason?

Answer 1

You're not showing your code, but typically the problem is that you need extern "C" around ffmpeg #include statements when compiling c++ code (see FAQ ):

extern "C" {
#include "libswscale/swscale.h"
}

The same goes for all other ffmpeg includes.