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1、问题描述

昨天使用ffmpeg库编译demo一切正常，今天再次链接ffmpeg库时报了一堆错误：

error: undefined reference to `av_frame_alloc()'error: undefined reference to `avio_close(AVIOContext*)'error: undefined reference to `avcodec_find_encoder(AVCodecID)'error: undefined reference to `avcodec_alloc_context3(AVCodec const*)'error: undefined reference to `av_opt_set(void*, char const*, char const*, int)'error: undefined reference to `avcodec_open2(AVCodecContext*, AVCodec const*, AVDictionary**)'error: undefined reference to `av_frame_alloc()'error: undefined reference to `av_image_alloc(unsigned char**, int*, int, int, AVPixelFormat, int)'error: undefined reference to `avformat_alloc_context()'error: undefined reference to `av_malloc(unsigned long)'error: undefined reference to `avio_alloc_context(unsigned char*, int, int, void*, int (*)(void*, unsigned char*, int), int (*)(void*, unsigned char*, int), long (*)(void*, long, int))'error: undefined reference to `avformat_alloc_output_context2(AVFormatContext**, AVOutputFormat*, char const*, char const*)'error: undefined reference to `avformat_free_context(AVFormatContext*)'

2、原因查找

1> 确认过ffmpeg库链接路径、库名字都对

-L[安装ffmpeg路径]/lib -lavcodec -lavformat -lavutil -lswscale -lpthread -lm -lswresample -lx264

2> 以为是系统以前安装的旧版本ffmpeg库造成干扰，将/usb/lib/x86_64-linux-gnu/下的ffmpeg库删除，还是不行。

3、解决方法

在c++中使用ffmpeg时，需要用extern “C” 声明ffmpeg的头文件。

#ifdef __cplusplusextern "C" {
   #endif#include 
   
    #include 
    
     #include 
     
      #include 
      
       #include 
       
        #include 
        
         #include 
         
          #include 
          
           #ifdef __cplusplus} // endof extern "C"#endif
          
         
        
       
      
     
    
   

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