makefile - GNU make yields "commands commence before first target" error

Question

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 

My makefile gets stuck on line 13:

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

After line 13, my makefile has its targets.

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

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

Answer 1

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.

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.

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