利用ffmpeg實現一個播放器,ffmpeg提供動態庫,但是編譯鏈接的時候遇到下面的問題:
../ffmpegWidgetPlayer/videoplayerwidget.cpp:23: error: undefined reference to 'sws_freeContext(SwsContext*)'
../ffmpegWidgetPlayer/videoplayerwidget.cpp:28: error: undefined reference to 'av_frame_free(AVFrame**)'
../ffmpegWidgetPlayer/videoplayerwidget.cpp:33: error: undefined reference to 'av_frame_free(AVFrame**)'
../ffmpegWidgetPlayer/videoplayerwidget.cpp:38: error: undefined reference to 'avcodec_close(AVCodecContext*)'
../ffmpegWidgetPlayer/videoplayerwidget.cpp:39: error: undefined reference to 'avcodec_free_context(AVCodecContext**)'
../ffmpegWidgetPlayer/videoplayerwidget.cpp:44: error: undefined reference to 'avformat_close_input(AVFormatContext**)'
../ffmpegWidgetPlayer/videoplayerwidget.cpp:55: error: undefined reference to 'avformat_network_init()'
../ffmpegWidgetPlayer/videoplayerwidget.cpp:58: error: undefined reference to 'avformat_open_input(AVFormatContext**, char const*, AVInputFormat const*, AVDictionary**)'
../ffmpegWidgetPlayer/videoplayerwidget.cpp:64: error: undefined reference to 'avformat_find_stream_info(AVFormatContext*, AVDictionary**)'
../ffmpegWidgetPlayer/videoplayerwidget.cpp:84: error: undefined reference to 'avcodec_find_decoder(AVCodecID)'
../ffmpegWidgetPlayer/videoplayerwidget.cpp:91: error: undefined reference to 'avcodec_alloc_context3(AVCodec const*)'
../ffmpegWidgetPlayer/videoplayerwidget.cpp:98: error: undefined reference to 'avcodec_parameters_to_context(AVCodecContext*, AVCodecParameters const*)'
../ffmpegWidgetPlayer/videoplayerwidget.cpp:104: error: undefined reference to 'avcodec_open2(AVCodecContext*, AVCodec const*, AVDictionary**)'
../ffmpegWidgetPlayer/videoplayerwidget.cpp:110: error: undefined reference to 'av_frame_alloc()'
../ffmpegWidgetPlayer/videoplayerwidget.cpp:117: error: undefined reference to 'av_frame_alloc()'
../ffmpegWidgetPlayer/videoplayerwidget.cpp:124: error: undefined reference to 'av_image_get_buffer_size(AVPixelFormat, int, int, int)'
../ffmpegWidgetPlayer/videoplayerwidget.cpp:125: error: undefined reference to 'av_malloc(unsigned long)'
../ffmpegWidgetPlayer/videoplayerwidget.cpp:128: error: undefined reference to 'av_image_fill_arrays(unsigned char**, int*, unsigned char const*, AVPixelFormat, int, int, int)'
../ffmpegWidgetPlayer/videoplayerwidget.cpp:131: error: undefined reference to 'sws_getContext(int, int, AVPixelFormat, int, int, AVPixelFormat, int, SwsFilter*, SwsFilter*, double const*)'
../ffmpegWidgetPlayer/videoplayerwidget.cpp:148: error: undefined reference to 'av_read_frame(AVFormatContext*, AVPacket*)'
../ffmpegWidgetPlayer/videoplayerwidget.cpp:151: error: undefined reference to 'avcodec_send_packet(AVCodecContext*, AVPacket const*)'
../ffmpegWidgetPlayer/videoplayerwidget.cpp:157: error: undefined reference to 'avcodec_receive_frame(AVCodecContext*, AVFrame*)'
../ffmpegWidgetPlayer/videoplayerwidget.cpp:164: error: undefined reference to 'sws_scale(SwsContext*, unsigned char const* const*, int const*, int, int, unsigned char* const*, int const*)'
../ffmpegWidgetPlayer/videoplayerwidget.cpp:170: error: undefined reference to 'av_frame_unref(AVFrame*)'
../ffmpegWidgetPlayer/videoplayerwidget.cpp:174: error: undefined reference to 'av_packet_unref(AVPacket*)'
../ffmpegWidgetPlayer/videoplayerwidget.cpp:187: error: undefined reference to 'sws_freeContext(SwsContext*)'
../ffmpegWidgetPlayer/videoplayerwidget.cpp:188: error: undefined reference to 'sws_getContext(int, int, AVPixelFormat, int, int, AVPixelFormat, int, SwsFilter*, SwsFilter*, double const*)'
collect2: error: ld returned 1 exit status
make: *** [Makefile:288: ffmpegWidgetPlayer] Error 1
10:29:50: 進程"/usr/bin/make"退出,退出代碼 2 。
Error while building/deploying project ffmpegWidgetPlayer (kit: Desktop Qt 5.15.2 GCC 64bit)
When executing step "Make"
10:29:50: Elapsed time: 00:03.
?通過分析報錯信息可以看到在鏈接階段提示ffmpeg相關的api找不到符號,看到這個問題百思不得其解,該工程我是利用qt編譯鏈接,可以肯定的是pro文件中已經通過LIBS關鍵字指明了依賴庫的路徑和依賴庫的名稱,但是仍然提示找不到,最后通過查閱資料,才了解問題產生的原因:qt工程是c++代碼,ffmpeg是c原因編寫的,當c++代碼調用C庫時對c++代碼中調用的頭文件需要使用extern "C"包含起來,以此讓編譯器按照C的方式編譯ffmpeg相關的頭文件。
解決方法如下:
#include <QWidget>
#include <QImage>
#include <QTimer>
#include <QPainter>
#include <QCoreApplication>
#include <QResizeEvent>
extern "C" //按照C語言方式編譯api接口
{
#include <libavformat/avformat.h>
#include <libavcodec/avcodec.h>
#include <libswscale/swscale.h>
#include <libavutil/avutil.h>
#include <libpostproc/postprocess.h>
#include <libavutil/ffversion.h>
#include <libavutil/imgutils.h>
}
總結
下面對編譯階段“undefined reference to”的報錯原因進行總結,希望能夠大家有所幫助:
- 產生這個報錯簡單的是就是依賴庫未包含,就是說未指定依賴庫的路徑和和依賴庫名稱,這個通過LIBS關鍵字就可以解決
- 如果確定LIBS等類似的方式已經指明依賴庫的路徑和依賴庫名稱,還是有這個報錯,那么確認下是否是C++代碼調用了C語言的庫,如果屬實,那么就是用extern "C"關鍵字包含C庫的頭文件