为什么 CMake 生成的 Makefile 找不到外部库？

Question

我第一次尝试使用 CMake，但收效甚微。我的 CMakeLists.txt 是：

# Sets the version of CMake that is required.
cmake_minimum_required(VERSION 2.8.10)

# Name of the project.
project(QTCODERLIB)

# Adds common directories to the build.
include_directories(/usr/local/include)
link_directories(/usr/local/lib)

# Check for header files that we depend on.
include(CheckIncludeFiles)
check_include_files(libavcodec/avcodec.h HAVE_AVCODEC_H)

# Adds all of the source files.
file(GLOB SOURCE RELATIVE ${CMAKE_CURRENT_SOURCE_DIR} *.cxx)

# Builds a library named `qtcoder` from source.
add_library(qtcoder SHARED ${SOURCE})

# Links the library against third-party dependencies.
target_link_libraries(qtcoder avcodec)

我的 test.cxx 文件是：

#include "config.h"
#include <libavcodec/avcodec.h>

void f(void) {
  avcodec_register_all();
}

cmake 。 && make VERBOSE=1 结果如下：

/usr/local/Cellar/cmake/2.8.10.1/bin/cmake -H/Users/phowes/Personal/QTCoder/QTCoderLib -B/Users/phowes/Personal/QTCoder/QTCoderLib --check-build-system CMakeFiles/Makefile.cmake 0
/usr/local/Cellar/cmake/2.8.10.1/bin/cmake -E cmake_progress_start /Users/phowes/Personal/QTCoder/QTCoderLib/CMakeFiles /Users/phowes/Personal/QTCoder/QTCoderLib/CMakeFiles/progress.marks
make -f CMakeFiles/Makefile2 all
make -f CMakeFiles/qtcoder.dir/build.make CMakeFiles/qtcoder.dir/depend
cd /Users/phowes/Personal/QTCoder/QTCoderLib && /usr/local/Cellar/cmake/2.8.10.1/bin/cmake -E cmake_depends "Unix Makefiles" /Users/phowes/Personal/QTCoder/QTCoderLib /Users/phowes/Personal/QTCoder/QTCoderLib /Users/phowes/Personal/QTCoder/QTCoderLib /Users/phowes/Personal/QTCoder/QTCoderLib /Users/phowes/Personal/QTCoder/QTCoderLib/CMakeFiles/qtcoder.dir/DependInfo.cmake --color=
make -f CMakeFiles/qtcoder.dir/build.make CMakeFiles/qtcoder.dir/build
/usr/local/Cellar/cmake/2.8.10.1/bin/cmake -E cmake_progress_report /Users/phowes/Personal/QTCoder/QTCoderLib/CMakeFiles 1
[100%] Building CXX object CMakeFiles/qtcoder.dir/test.cxx.o
/usr/bin/c++   -Dqtcoder_EXPORTS -fPIC -I/usr/local/include    -o CMakeFiles/qtcoder.dir/test.cxx.o -c /Users/phowes/Personal/QTCoder/QTCoderLib/test.cxx
Linking CXX shared library libqtcoder.dylib
/usr/local/Cellar/cmake/2.8.10.1/bin/cmake -E cmake_link_script CMakeFiles/qtcoder.dir/link.txt --verbose=1
/usr/bin/c++   -dynamiclib -Wl,-headerpad_max_install_names   -o libqtcoder.dylib -install_name /Users/phowes/Personal/QTCoder/QTCoderLib/libqtcoder.dylib CMakeFiles/qtcoder.dir/test.cxx.o  -L/usr/local/lib -lavcodec 
Undefined symbols for architecture x86_64:
  "avcodec_register_all()", referenced from:
      f() in test.cxx.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
make[2]: *** [libqtcoder.dylib] Error 1
make[1]: *** [CMakeFiles/qtcoder.dir/all] Error 2
make: *** [all] Error 2

目标文件和库都编译为 64 位：

$ lipo -info CMakeFiles/qtcoder.dir/test.cxx.o
Non-fat file: CMakeFiles/qtcoder.dir/test.cxx.o is architecture: x86_64

$ lipo -info /usr/local/lib/libavcodec.dylib
Non-fat file: /usr/local/lib/libavcodec.dylib is architecture: x86_64

函数存在于库中：

$ nm /usr/local/lib/libavcodec.dylib | grep avcodec_register_all
0000000000033c01 T _avcodec_register_all

我在这里做错了什么？

Answer 1

avcodec 库是一个 C 库，查看您正在为 CXX 编译的 cmake 输出。这个问题解决了如何精确地处理这种情况：g++ Linking Error on Mac while compiling FFMPEG

Answer 2

通常你会使用find_package 来完成这些繁重的任务。默认情况下，CMake 附带了很多模块，但有时您必须为使用较少的项目下载一个。

这是how to find libraries 的一些文档。

这是他们人为设计的示例，说明您希望看到的内容。

cmake_minimum_required(VERSION 2.8)
project(helloworld)
add_executable(helloworld hello.c)
find_package (BZip2)
if (BZIP2_FOUND)
  include_directories(${BZIP_INCLUDE_DIRS})
  target_link_libraries (helloworld ${BZIP2_LIBRARIES})
endif (BZIP2_FOUND)

从简单的网络搜索中，我不清楚 CMake 是否提供了 AVCodec 模块。我在网上找到了一些：

https://github.com/arjanhouben/SDL_ffmpeg/blob/master/Findavcodec.cmake http://whispercast.org/trac/browser/trunk/cmake/FindLibAvCodec.cmake

我看到大多数使用这个库的人都对 FFMPEG 感兴趣，这可能也是你感兴趣的。

https://github.com/zinnschlag/openmw/blob/master/cmake/FindFFMPEG.cmake

如果您确实需要使用您下载的一个，这里是how to use custom modoules 的一些说明。