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路径环境变量中的陷阱 | Traps in path related environment variables

最近需要在没有root的机器上配一个自己的编译环境，于是需要改一些环境变量，比如

CFLAGS="-I#/include"
LDFLAGS="-L#/lib"
PATH="#/bin"
LD_LIBRARY_PATH="#/lib"
LIBRARY_PATH="#/lib"
MANPATH="#/man"
C_INCLUDE_PATH="#/include"
CPLUS_INCLUDE_PATH="#/include"
PKG_CONFIG_PATH="#/lib/pkgconfig"

当然可能还需要其他的。

如果就按这么写明显有问题，比如PATH原先可能不是空的，所以应该写成PATH="#/bin:$PATH"，其他变量也类似。但是这样会有个更严重的问题，困扰了我一天。

问题出在C_INCLUDE_PATH,一般这个变量都是空的，所以按上面的方法添加自定义路径后变成了"#/include:"，而这个末尾的":"会被理解为末尾有个当前目录，即'.'，开头的':'以及中间的'::'会有同样情况。

另一方面，在使用automake编译的程序中，经常会出现-I.的编译参数，而且通常出现在最开始以保证当前目录是第一个被搜寻的路径。但是如果'.'出现了C_INCLUDE_PATH中，gcc会认为它是系统标准路径，于是命令行中的-I.会被忽略掉。这样搜索路径的顺序就改变了，就会出问题。
我是在编译binutils时发现的问题，很多目录里都有config.h，于是搜索路径的顺序很重要。

解决方法当然也很简单，就是麻烦一些，即判断一下原来的路径是否为空，避免添加多余的分隔符':'。


Recently I need to set up a development environment without root privilege, therefore I need to tweak several environment variables, for example:

CFLAGS="-I#/include"
LDFLAGS="-L#/lib"
PATH="#/bin"
LD_LIBRARY_PATH="#/lib"
LIBRARY_PATH="#/lib"
MANPATH="#/man"
C_INCLUDE_PATH="#/include"
CPLUS_INCLUDE_PATH="#/include"
PKG_CONFIG_PATH="#/lib/pkgconfig"

and maybe more.

Well there is a problem here, for example PATH may not be empty before we set it, so it would be better to write as PATH="#/bin:$PATH". This is also true for other variables. However this may lead to a more serious problem, which made me spending a whole day before resolving it.

It was all because of C_INCLUDE_PATH, usually it is empty, so if I prepend something in the way mentioned above, it'll become "#/include:". The trailing ':' will actually be interpreted as a 'current directory' in the end, that is '.'. And it also happen for a staring ':' and a '::' in the middle.

On the other hand, for programs utilizing automake, we often see a parameter '-I.', which is often appears in the beginning in order to make '.' the first searched directory for header files. However if '.' appears in C_INCLUDE_PATH, gcc will then consider it a 'system standard path', and therefore ignore the parameter '-I.'. In this way the order of searching directories is changed, then issues may appear.
I found this when I was compiling binutils, where the file 'config.h' appears in many directories, such that the order of searching directories is quite important.

To resolve it, simple but trouble, just to check whether the original variable is empty before prepend anything, in order to avoid a trouble-making ':'.
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