C++ gcc：未定义的引用
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gcc: undefined reference to
提问by jamie_y
I would like to compile this.
我想编译这个。
program.c
程序.c
#include <libavcodec/avcodec.h>
int main(){
int i = avpicture_get_size(AV_PIX_FMT_RGB24,300,300);
}
Running this
运行这个
gcc -I$HOME/ffmpeg/include program.c
gives error
给出错误
/tmp/ccxMLBme.o: In function `main':
program.c:(.text+0x18): undefined reference to `avpicture_get_size'
collect2: ld returned 1 exit status
However, avpicture_get_size is defined. Why is this happening?
但是，定义了 avpicture_get_size。为什么会这样？
回答by alk
However, avpicture_get_size is defined.
但是，定义了 avpicture_get_size。
No, as the header (<libavcodec/avcodec.h>) just declaresit.
不，因为标题 ( <libavcodec/avcodec.h>) 只是声明了它。
The definitionis in the library itself.
该定义是库本身。
So you might like to add the linker option to link libavcodecwhen invoking gcc:
所以你可能想libavcodec在调用 gcc 时添加链接器选项来链接：
-lavcodec
Please also note that libraries need to be specified on the command line afterthe files needing them:
另请注意，需要在需要它们的文件之后在命令行上指定库：
gcc -I$HOME/ffmpeg/include program.c -lavcodec
Notlike this:
不是这样的：
gcc -lavcodec -I$HOME/ffmpeg/include program.c
Update:
更新：
Referring to Wyzard's comment the complete command might look like this:
参考Wyzard的评论，完整的命令可能如下所示：
gcc -I$HOME/ffmpeg/include program.c -L$HOME/ffmpeg/lib -lavcodec
For libraries not stored in the linkers standard location the option -Lspecifies an additional search path to lookup libraries specified using the -loption, that is libavcodec.x.y.zin this case.
对于未存储在链接器标准位置中的库，该选项-L指定了一个额外的搜索路径来查找使用该-l选项指定的库，即libavcodec.x.y.z在本例中。
For a detailed reference on GCC's linker option, please read here.
有关 GCC 链接器选项的详细参考，请阅读此处。
回答by Technophile
Are you mixing C and C++? One issue that can occur is that the declarations in the .h file for a .c file need to be surrounded by:
你在混合 C 和 C++ 吗？可能发生的一个问题是 .c 文件的 .h 文件中的声明需要被包围：
#if defined(__cplusplus)
extern "C" { // Make sure we have C-declarations in C++ programs
#endif
and:
和：
#if defined(__cplusplus)
}
#endif
