Android 在引用 FFmpeg 静态库时，出现 libavformat/hls.c:881: error: undefined reference to ‘atof’

错误信息如下:

  libavformat/hls.c:881: error: undefined reference to 'atof'
  libavformat/hlsenc.c:1233: error: undefined reference to 'atof'
  libavformat/http.c:1758: error: undefined reference to 'inflateEnd'
  libavformat/http.c:773: error: undefined reference to 'inflateEnd'
  libavformat/http.c:774: error: undefined reference to 'inflateInit2_'
  libavformat/http.c:779: error: undefined reference to 'zlibCompileFlags'
  libavformat/http.c:1535: error: undefined reference to 'inflate'
  libavformat/id3v2.c:1031: error: undefined reference to 'uncompress'
  libavformat/img2dec.c:266: error: undefined reference to 'glob'
  ...

造成这种问题的原因有三个：

1. 有可能是在编译 FFmpeg 库时 Android API 版本太高导致，具体出错原因可以参考原文解释：

   Google have moved some of the C standard library functions like atof() from being inline functions in header files to normal functions. The latest NDKs will default to building a .so that is only compatible with the latest Android devices that have the atof() function in the device’s standard C library (libc.so). This means if you run a library on an older device that has an older version of the C library, you will get an error loading the dll as the expected atof() function will not exist.

   解决方案： 将 FFmpeg 编译脚本 Android API 版本改为 17 ，重新编译

2. FFmpeg 库的依赖顺序错误，导致定义的符号找不到，造成编译错误。
   简单说一下编译链接工作原理：

• 链接器从左往右扫描目标文件和静态库

• 扫描发现未解析的符号会先记住这个未解析的符号

• 当扫描到后面的静态库时，找到了前面未解析的符号，会提取相关代码

• 最终没有任何未解析的符号，编译链接工作完成

  既然知道工作原理了，只需要把依赖顺序写对就好：

avfilter avformat avcodec avutil swresample swscale

3. 因为是引入静态库，FFmpeg 需要依赖 zlib 库，所以在链接 FFmpeg 静态库时，需要加入 zlib 库

target_link_libraries(...
        avfilter avformat avcodec avutil swresample swscale
                      z)