Some tricks which i got from linux makefile

[Aerosol]

I'm not superuser of make, but sometime i use it. My knowledge about the files usually end in the next things: I know that if I'll create Makefile file, write some simple rules which are in general form:

target:
        command

And than execute make target, it executes command. Last time ago i started to learn some low-level stuffs as assembly programming, operating system theory and practice (as you can read and understand from previous blog posts about assembly programming). Some days ago i started to read Linux kernel Makefile and here i want to share some make syntax features which i learned from there. So this post is not for people who used make much time and knows much about it.

origin

origin - is a builtin function which returns information about variable. It's general form is:

$(origin var)

You can pass any $(variable) to it and it will return info about it. Return value of origin can be:

• undefined - variable didn't defined
  
• default - default var, like $(CC) and etc...
  
• environemnt - from environment
  
• environment override - return environemnt which was overridden with make -e
  
• file - from makefile
  
• command line - from make command line arguments
  
• automatic - see bellow
  

So we can use it like this:

all:
    ifeq ("$(origin A)", "command line")
        ALL = $(A)
    endif

@

Usually makefile prints every action after executing it, but sometimes we no need in it. We can use @ prefix for it, for example:

[CODE @Echo "Done." line, but just prints Done.

filter

filter function removes all space separated words from text, which doesn't match no one template. General view of it:

'$(filter template..., text)' 

For example, we have targets list and we need to check is there build target there or not:

targets := all clean  

findTargets: $(targets)   
    ifeq $($(filter build, $(targets),)   

Check empty value

We can check empty value with following:

ifeq ($(VAR_CAN_BE_EMPTY)),)

Current directory

Previously when i was need to get current directory path, i used something like this:

curr = $(shell pwd)

Now i know about builtin variable:

$(CURDIR)

Errors, Warning and Info

There are 3 builtin functions for printing errors, warning and info:

ifeq $($(USE_VAR1),)
    $(USER_VAR1 must be not empty)

ifeq $($(USE_VAR2), 1)
    $(USE_VAR2 1)

ifneq $($(USE_VAR3),)
    $(info $(USE_VAR1) is not empty)

VPATH

VPATH variable defines list of directories where template rules will search dependencies:

VPATH = src

%.o: %.c
    gcc -c $< 

Automatic variables

There are some special macros:

• $| - Names of all the order-only prerequisites, with spaces between them
  
• $@ - Filename of the target of the rule
  
• $? - Names of all the prerequisites that are newer than the target
  
• $< - Name of the first prerequisite
  

Source