未定义的引用，在 Ubuntu，64 位系统上使用 FFMpeg-library (AvCodec)

Question

我正在运行最新的 FFMpeg 库的示例代码。 我已将示例代码插入到文件videofecencoder.c：

/*
* copyright (c) 2001 Fabrice Bellard
*
* This file is part of Libav.
*
* Libav is free software; you can redistribute it and/or
* modify it under the terms of the GNU Lesser General Public
* License as published by the Free Software Foundation; either
* version 2.1 of the License, or (at your option) any later version.
*
* Libav is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
* Lesser General Public License for more details.
*
* You should have received a copy of the GNU Lesser General Public
* License along with Libav; if not, write to the Free Software
* Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
*/
#pragma GCC diagnostic ignored "-Wdeprecated-declarations"


#include <stdlib.h>
#include <stdio.h>
#include <string.h>

#ifdef HAVE_AV_CONFIG_H
#undef HAVE_AV_CONFIG_H
#endif

#include "libavutil/imgutils.h"
#include "libavutil/opt.h"
#include "libavcodec/avcodec.h"
#include "libavutil/mathematics.h"
#include "libavutil/samplefmt.h"

#define INBUF_SIZE 4096
#define AUDIO_INBUF_SIZE 20480
#define AUDIO_REFILL_THRESH 4096

/*
* Video encoding example
*/
static void video_encode_example(const char *filename, int codec_id)
{
   AVCodec *codec;
   AVCodecContext *c= NULL;
   int i, out_size, size, x, y, outbuf_size;
   FILE *f;
   AVFrame *picture;
   uint8_t *outbuf;
   int nrOfFramesPerSecond  =25;
   int nrOfSeconds =1;


   printf("Video encoding\n");

   /* find the mpeg1 video encoder */
   codec = avcodec_find_encoder((CodecID) codec_id);
   if (!codec) {
       fprintf(stderr, "codec not found\n");
       exit(1);
   }

   c = avcodec_alloc_context3(codec);
   picture= avcodec_alloc_frame();

   /* put sample parameters */
   c->bit_rate = 400000;
   /* resolution must be a multiple of two */
   c->width = 352;
   c->height = 288;
   /* frames per second */
   c->time_base= (AVRational){1,25};
   c->gop_size = 10; /* emit one intra frame every ten frames */
   c->max_b_frames=1;
   c->pix_fmt = PIX_FMT_YUV420P;

   if(codec_id == CODEC_ID_H264)
       av_opt_set(c->priv_data, "preset", "slow", 0);

   /* open it */
   if (avcodec_open2(c, codec, NULL) < 0) {
       fprintf(stderr, "could not open codec\n");
       exit(1);
   }

   f = fopen(filename, "wb");
   if (!f) {
       fprintf(stderr, "could not open %s\n", filename);
       exit(1);
   }

   /* alloc image and output buffer */
   outbuf_size = 100000;
   outbuf = (uint8_t*) malloc(outbuf_size);

   /* the image can be allocated by any means and av_image_alloc() is
    * just the most convenient way if av_malloc() is to be used */
   av_image_alloc(picture->data, picture->linesize,
                  c->width, c->height, c->pix_fmt, 1);

   /* encode 1 second of video */
   int nrOfFramesTotal = nrOfFramesPerSecond * nrOfSeconds;

   /* encode 1 second of video */
   for(i=0;i < nrOfFramesTotal; i++) {
       fflush(stdout);
       /* prepare a dummy image */
       /* Y */
       for(y=0;y<c->height;y++) {
           for(x=0;x<c->width;x++) {
               picture->data[0][y * picture->linesize[0] + x] = x + y + i * 3;
           }
       }

       /* Cb and Cr */
       for(y=0;y<c->height/2;y++) {
           for(x=0;x<c->width/2;x++) {
               picture->data[1][y * picture->linesize[1] + x] = 128 + y + i * 2;
               picture->data[2][y * picture->linesize[2] + x] = 64 + x + i * 5;
           }
       }

       /* encode the image */
       out_size = avcodec_encode_video(c, outbuf, outbuf_size, picture);
       printf("encoding frame %3d (size=%5d)\n", i, out_size);
       fwrite(outbuf, 1, out_size, f);
   }

   /* get the delayed frames */
   for(; out_size; i++) {
       fflush(stdout);

       out_size = avcodec_encode_video(c, outbuf, outbuf_size, NULL);
       printf("write frame %3d (size=%5d)\n", i, out_size);
       fwrite(outbuf, 1, out_size, f);
   }

   /* add sequence end code to have a real mpeg file */
   outbuf[0] = 0x00;
   outbuf[1] = 0x00;
   outbuf[2] = 0x01;
   outbuf[3] = 0xb7;
   fwrite(outbuf, 1, 4, f);
   fclose(f);
   free(outbuf);

   avcodec_close(c);
   av_free(c);
   av_free(picture->data[0]);
   av_free(picture);
   printf("\n");
}

int main(int argc, char **argv)
{
   const char *filename;

   /* register all the codecs */
   avcodec_register_all();

   if (argc <= 1) {

       video_encode_example("/grb_1.mpg", CODEC_ID_MPEG1VIDEO);
   } else {
       filename = argv[1];
   }


   return 0;
}

当我运行 gcc videofecencoder.cc -lavcodec 时，我收到以下错误消息：

/tmp/ccJg8IDy.o: In function `video_encode_example(char const*, int)':
videofecencoder.cc:(.text+0x35): undefined reference to `avcodec_find_encoder(CodecID)'
videofecencoder.cc:(.text+0x74): undefined reference to `avcodec_alloc_context3(AVCodec*)'
videofecencoder.cc:(.text+0x7d): undefined reference to `avcodec_alloc_frame()'
videofecencoder.cc:(.text+0x113): undefined reference to `av_opt_set(void*, char const*, char const*, int)'
videofecencoder.cc:(.text+0x12b): undefined reference to `avcodec_open2(AVCodecContext*, AVCodec*, AVDictionary**)'
videofecencoder.cc:(.text+0x1f0): undefined reference to `av_image_alloc(unsigned char**, int*, int, int, PixelFormat, int)'
videofecencoder.cc:(.text+0x35c): undefined reference to `avcodec_encode_video(AVCodecContext*, unsigned char*, int, AVFrame const*)'
videofecencoder.cc:(.text+0x3cf): undefined reference to `avcodec_encode_video(AVCodecContext*, unsigned char*, int, AVFrame const*)'
videofecencoder.cc:(.text+0x47c): undefined reference to `avcodec_close(AVCodecContext*)'
videofecencoder.cc:(.text+0x488): undefined reference to `av_free(void*)'
videofecencoder.cc:(.text+0x497): undefined reference to `av_free(void*)'
videofecencoder.cc:(.text+0x4a3): undefined reference to `av_free(void*)'
/tmp/ccJg8IDy.o: In function `main':
videofecencoder.cc:(.text+0x4c3): undefined reference to `avcodec_register_all()'
collect2: ld returnerade avslutningsstatus 1

nm libavcodec.a | grep avcodec_find 命令的结果是：

00000000000008e0 T avcodec_find_best_pix_fmt
0000000000000740 T avcodec_find_best_pix_fmt2
                 U avcodec_find_encoder
0000000000002ca0 T avcodec_find_decoder
0000000000002cf0 T avcodec_find_decoder_by_name
0000000000002bd0 T avcodec_find_encoder
0000000000002c30 T avcodec_find_encoder_by_name

我在另一个库中也有另一个类似的错误： Undefined reference despite linking in OpenFEC-library

我的系统：Ubuntu 11，64位机器

我的下一步是尝试在带有 Ubuntu 32 位（在 Windows 操作系统上运行）的 VirtualBox 上编译它。

Answer 1

我（终于！！）提到了 FFMpeg 邮件列表中的解决方案： http://ffmpeg.org/faq.html#I_0027m-using-FFmpeg-from-within-my-C_002b_002b-application-but-the-linker-complains-about-missing-symbols-which-seem-to-be-available_002e

引用：“FFmpeg 是一个纯 C 项目，因此要在 C++ 应用程序中使用库，您需要明确声明您正在使用 C 库。您可以通过使用 extern "C" 包含您的 FFmpeg 来做到这一点。 "