GNU使产量“命令在第一个目标之前开始".错误 [英] GNU make yields "commands commence before first target" error

问题描述

在我的makefile文件中，我想检查是否存在库并给出提示性错误消息.我创建了一个条件，当找不到文件时，该条件应退出制作过程:

In my makefile, I would like to check for the existence of a library and give an informative error message. I created a conditional that should exit the make process when the file is not found:

 9: ifeq ($(${JSONLIBPATH}),)
10:    JSONLIBPATH = ${ALTJSONLIBDIR}/${LIBJSON}
11: endif
12: ifeq ($(${JSONLIBPATH}),)
13:    $(error JSON library is not found. Please install libjson before building)
14: endif 

我的makefile卡在第13行:

My makefile gets stuck on line 13:

Makefile:13: *** commands commence before first target.  Stop.

第13行之后，我的makefile有其目标.

After line 13, my makefile has its targets.

我尝试将此条件块放入目标(例如称为isJSONLibraryInstalled的目标)中，但这无法正确执行.

I tried putting this conditional block into a target (e.g. a target called isJSONLibraryInstalled) but this does not execute correctly.

在处理目标之前，我将如何检查文件的存在并处理错误情况?抱歉，这是一个愚蠢的问题.

How would I check for a file's existence and handle the error case, before processing targets? Apologies if this is a dumb question.

推荐答案

首先，您正在查看以当前路径命名的变量的内容，这可能不是您想要的.一个简单的环境变量引用是$(name)或${name}，而不是$(${name}).因此，始终会评估第13行.

First of all, you are looking at the contents of a variable that is named after the current path, which is probably not what you want. A simple environment variable reference is $(name) or ${name}, not $(${name}). Due to this, line 13 is always evaluated.

第二，我认为这使$(error ...)表达式的缩进令人窒息.当表达式解析为空字符串时，该行的开头仍然有一个制表符，它表示命令，而该命令又不能存在于规则之外.

Second, I think it is choking on the indentation of the $(error ...) expression. While the expression resolves to an empty string, there is still a tab character at the start of the line, which indicates a command, which in turn cannot exist outside a rule.

我认为使用空格而不是制表符来缩进是可行的.

I think using spaces rather than tabs to indent would work.

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