pkg-config命令了解和使用

1.首先，什么是pkg-config：

pkg-config 当你从源代码编译软件时，用来提供依赖库的信息的软件。pkg-config原本是设计用于Linux的，但现在在各个版本的BSD、windows、Mac OS X和Solaris上都有着可用的版本。它输出已安装的库的各个信息，包括：1)C或C++编译器需要的参数2)链接器需要的参数3)已安装软件包的版本信息

2.什么时候用的着？

当在Linux/Unix重 编写完程序，开始编译（寻找头文件）和链接（寻找链接库）时，就用上它了。

3.怎么用？

最常用的有：

pkg-config 库的名称(比如，OpenCV） --libs
pkg-config 库的名称(比如，opencv) --cflags
或者：
pkg-config 库的名称(比如，opencv） --libs --cflags

1：pkg-config --libs --cflags   输出：编译和链接使用

-I/usr/local/include/opencv -I/usr/local/include  -L/usr/local/lib -lopencv_shape -lopencv_stitching -lopencv_objdetect -lopencv_superres -lopencv_videostab -lopencv_calib3d -lopencv_features2d -lopencv_highgui -lopencv_videoio -lopencv_imgcodecs -lopencv_video -lopencv_photo -lopencv_ml -lopencv_imgproc -lopencv_flann -lopencv_core  
2：pkg-config --libs 输出：链接器使用  -L/usr/local/lib -lopencv_shape -lopencv_stitching -lopencv_objdetect -lopencv_superres -lopencv_videostab -lopencv_calib3d -lopencv_features2d -lopencv_highgui -lopencv_videoio -lopencv_imgcodecs -lopencv_video -lopencv_photo -lopencv_ml -lopencv_imgproc -lopencv_flann -lopencv_core  
3：pkg-config --cflags 输出： 编译器使用  -I/usr/local/include/opencv -I/usr/local/include  

4.最后：pkg-config工作原理：命令的输出来源于那里呢？

pkg-config命令通过检索存放于系统的*.pc文件来给出相应的库的输出。比如，如果在系统中安装了opencv库，则对应的在系统中存放着opencv.pc；而这个文件就记录着opencv库的版本、编译参数、链接参数等信息。pkg-config搜索过程：1：opencv  was   found in the pkg-config search path.
2: you should add the directory containing `opencv.pc' to the PKG_CONFIG_PATH environment variable

# Package Information for pkg-config

prefix=/usr/local
exec_prefix=${prefix}
libdir=${exec_prefix}/lib
includedir_old=${prefix}/include/opencv
includedir_new=${prefix}/include

Name: OpenCV
Description: Open Source Computer Vision Library
Version: 3.2.0
Libs: -L${exec_prefix}/lib -lopencv_shape -lopencv_stitching -lopencv_objdetect -lopencv_superres -lopencv_videostab -lopencv_calib3d -lopencv_features2d -lopencv_highgui -lopencv_videoio -lopencv_imgcodecs -lopencv_video -lopencv_photo -lopencv_ml -lopencv_imgproc -lopencv_flann -lopencv_core
Libs.private: -ldl -lm -lpthread -lrt
Cflags: -I${includedir_old} -I${includedir_new}

这个文件来源于那里呢？这个*.pc文件在你的库安装包里，一般是库的打包者或发行者设置好的。当安装库时，*.pc文件被放于你系统重的某一个文件夹中（根据你的系统设置，一般存放的位置有/usr/lib/pkgconfig/等）。--cflags 输出：

Cflags: -I${includedir_old} -I${includedir_new}

--libs 输出：

-L${exec_prefix}/lib -lopencv_shape -lopencv_stitching -lopencv_objdetect -lopencv_superres -lopencv_videostab -lopencv_calib3d -lopencv_features2d -lopencv_highgui -lopencv_videoio -lopencv_imgcodecs -lopencv_video -lopencv_photo -lopencv_ml -lopencv_imgproc -lopencv_flann -lopencv_core