编译FFMPEG时，Mac上的g ++链接错误 [英] g++ Linking Error on Mac while compiling FFMPEG

问题描述

test.cpp

  #include< iostream> 
使用命名空间std; 
 #include< libavcodec / avcodec.h> // required headers 
 #include< libavformat / avformat.h> 
 int main（int argc，char ** argv）{
 av_register_all（）; //违规图书馆调用
返回0; 
} 
  

当我尝试使用以下命令编译这个命令

  g ++ test.cpp -I / usr / local / include -L / usr / local / lib \ 
 -lavcodec -lavformat -lavutil -lz -lm -o test 
  

我收到错误
未定义的符号：
av_register_all（），引用来自：
_main in ccUD1ueX.o
ld：未找到符号
collect2：ld返回1退出状态

有趣的是，如果我有一个等价的c代码，
test.c

  #include < stdio.h中> 
 #include< libavcodec / avcodec.h> 
 #include< libavformat / avformat.h> 
 int main（int argc，char ** argv）{
 av_register_all（）; 
 return 0; 
} 
  

gcc编译它很好

  gcc test.c -I / usr / local / include -L / usr / local / lib \ 
 -lavcodec -lavformat -lavutil -lz -lm  - o test 
  

我正在使用Mac OS X 10.6.5

  $ g ++ --version 
 i686-apple-darwin10-g ++  -  4.2.1（GCC）4.2.1（Apple Inc. build 5664）
 $ gcc --version 
 i686-apple-darwin10-gcc-4.2.1（GCC）4.2.1（Apple Inc. build 5664）
  

FFMPEG的libavcodec，libavformat等是C库，我已经在我的机器上构建了它们，如：

  ./ configure --enable-gpl --enable-pthreads --enable-shared \ 
 --disable-doc --enable-libx264 
 make&& sudo make install 
  

正如所料，libavformat确实包含符号av_register_all

  $ nm /usr/local/lib/libavformat.a | grep av_register_all 
 0000000000000000 T _av_register_all 
 00000000000089b0 S _av_register_all.eh 
  

我很倾向相信g ++和gcc对我机器上的库有不同的看法。 g ++不能拿起正确的库。任何线索？

解决方案

这可能是因为 av_register_all 在 externC块中，因此当前向声明由C ++编译器解释时，它的名称被破坏。尝试将您的代码更改为：

  #include< iostream> 
使用命名空间std; 
 externC{
 #include  #include< libavformat / avformat.h> 
} 
 int main（int argc，char ** argv）{
 av_register_all（）; //违规图书馆调用
返回0; 
} 
  

C ++编译器使用的名称允许覆盖相同的函数使用不同的参数，但不由C编译器（不提供函数覆盖）执行。

通常，用C编写的标题可以包含在C ++中文件应具有以下结构，以防止发生此类错误。您应该通知ffmpeg开发人员修改其代码：

  //标准包括保护
 #ifndef INCLUDED_AVCODEC_H 
 #define INCLUDED_AVCODEC_H 
 
 //防止包含C ++文件
 #ifdef __cplusplus 
 externC{
 #endif 
 
 // C代码
 // .... 
 
 //关闭防止包含C ++文件的保护
 #ifdef __cplusplus 
} 
 #endif 
 #endif 
  

：我刚刚发现在 FFmpeg wiki中提到了这一点。

g++ on Snow Leopard is throwing linking errors on the following piece of code

test.cpp

#include <iostream>
using namespace std;
#include <libavcodec/avcodec.h>    // required headers
#include <libavformat/avformat.h>
int main(int argc, char**argv) {
    av_register_all();             // offending library call
    return 0;
}

When I try to compile this using the following command

g++ test.cpp -I/usr/local/include -L/usr/local/lib \
-lavcodec -lavformat -lavutil -lz -lm -o test

I get the error Undefined symbols: "av_register_all()", referenced from: _main in ccUD1ueX.o ld: symbol(s) not found collect2: ld returned 1 exit status

Interestingly, if I have an equivalent c code, test.c

#include <stdio.h>
#include <libavcodec/avcodec.h>
#include <libavformat/avformat.h>
int main(int argc, char**argv) {
    av_register_all();
    return 0;
}

gcc compiles it just fine

gcc test.c -I/usr/local/include -L/usr/local/lib \
-lavcodec -lavformat -lavutil -lz -lm -o test

I am using Mac OS X 10.6.5

$ g++ --version
i686-apple-darwin10-g++-4.2.1 (GCC) 4.2.1 (Apple Inc. build 5664)
$ gcc --version
i686-apple-darwin10-gcc-4.2.1 (GCC) 4.2.1 (Apple Inc. build 5664)

FFMPEG's libavcodec, libavformat etc. are C libraries and I have built them on my machine like thus:

./configure --enable-gpl --enable-pthreads --enable-shared \
--disable-doc --enable-libx264
make && sudo make install

As one would expect, libavformat indeed contains the symbol av_register_all

$ nm /usr/local/lib/libavformat.a | grep av_register_all
0000000000000000 T _av_register_all
00000000000089b0 S _av_register_all.eh

I am inclined to believe g++ and gcc have different views of the libraries on my machine. g++ is not able to pick up the right libraries. Any clue?

解决方案

This is probably because the av_register_all function is not in an extern "C" block and thus when the forward declaration is interpreted by the C++ compiler, it's name is mangled. Try to change your code to:

#include <iostream>
using namespace std;
extern "C" {
#include <libavcodec/avcodec.h>    // required headers
#include <libavformat/avformat.h>
}
int main(int argc, char**argv) {
    av_register_all();             // offending library call
    return 0;
}

The name mangling is used by the C++ compilers to allow override of the same function with differents arguments, but is not performed by C compilers (which do not offer function overriding).

Generally, the headers that are written in C and can be included in C++ file should have the following structure to prevent such bugs to occurs. You should probably inform the ffmpeg developpers to have their code modified:

// Standard includes guards
#ifndef INCLUDED_AVCODEC_H
#define INCLUDED_AVCODEC_H

// Protection against inclusion by a C++ file
#ifdef __cplusplus
extern "C" {
#endif

// C code
// ....

// Closing the protection against inclusion by a C++ file
#ifdef __cplusplus
}
#endif
#endif

[Edit]: I just found out that this is mentioned on FFmpeg wiki.

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