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GNU Make Reference Manual

It has at lot to do with files, C, text processing, jobs-parallel-recursive functions.

Basics

• mytarget: prerequisites; recepies or with new line
• % is matching anything
• MS-DOS and Windows has makefiles too

Targets

• first target is default
• can be file
• can be non-file, .PHONY
• uses timestamp of update of a file to decide if target is up to date by looking at prerequisites
• use .PHONY: clean make run tests to make sure name of target does not collide with file if file exists

Rules

• if some rule fails, then make fails
• -gcc ... use - at rule start to not fail if command fails
• each rule is a separate shell, so no env variables shared or cd
• to run rules in same shell use .ONESHELL
• explicit rule, defined by explicit targets
• implicit rule like %.c: ; gcc ... is matching pattern
• abs: ; explicit empty rule
• prerequisites can be order-only (p.36), uses foo: bar | acs; gcc .. style
• $? files that changed
• $@ target file

Useful to update timestamp of a file:

print: a.c b.h
    ...
    touch print

• .PRECIOUS will keep file even if it is temporary. Will not delete a file.
• .SILENT do not print
• a.o b.o &: .. is grouped targets. weird. do not use.

How it runs?

• 1 read all makefiles files one by one
• 2 build DAG of targets
• 3 run targets

• 3.1 there is second pass if specified (secondary expansion)

• has parallel version
• has recursive version
• MAKELEVEL has level at which current make is running

How to run

• make -O line to make output linewise, useful for parallel mode to make sure output is synchronized
• default goal is first target
• any target can be a goal unless it starts with - or contains =
• make --warn-undefined-variables
• make clean build processes multiple target goals in order
• MAKECMDGOALS list of targets set automatically
• make -n shows targets needs to be run, but not running them. prevents PHONY targets
• make -t touch, makes up to date without changing
• make -q whether needs to run anything, only status code 1 signals this, 0 otherwise
• make CFLAGS='-g' to override variable
• make -k keep-going to continue run on error. useful to get all the errors
• make -L consider timestamps of symbolic links in addition to timestamps of files referenced by those links
• make -O sync output, can be target (default), line, recourse, none
• make -p print database of rules and variables
• make -r no built-in rules
• make -s silent, not printing recepies as they are executed

.. more complicated things

Variables

• all variables are stings
• $(myvar) or ${myvar} which is same
• a = ... is recursive variable, uses data at runtime
• a := ... is simply expanded variable (or ::=), uses data only at time of defining
• a ?= ... if not defined yet
• a != ls -la execute shell and get output with new lines replaced with spaces
• a += ... line concatenate
• if not defined then whitespace
• $(a:b=c) substitution of suffix
• undefine a

Conditionals

• if, else, endif, ifeq
• also text comparisons and operators

Text Functions

• iterables are strings with space between them
• handles lists with whitespace separation
• $(function arg1 arg2) or ${function arg1 arg2}
• subst, patsbust, strip, findstring, filter, sort, word, and more on p.88
• $(dir names...), suffix, basename, join
• $(wildcard pattern) matches all existing files matching pattern
• conditionals like $(if ...)
• iteration $(foreach...)
• $(file ...) can read or write to file
• $(eval ...) is complex
• $(origin variable) shows where var is coming from
• $(flavor variable) shows if it is undefined recursive or simple
• $(error msg) generates fatal error
• $(warning msg)
• $(info msg)
• $(shell cat foo) returns result of execution of shell. converts new lines to spaces.

.. and even more arcane things

Implicit Rules

• basically are patterns
• chained
• .mod is one of suffixes! so collides with go.mod!

Archives

• only for ar
• myarchive(myfile): ... targets inside files

.. and even more strange things

Extending

• GNU Guile, which is a Scheme language
• do not use load, which is something like C linked libs?
• there is protocol for communicating with Make job server for jobslots

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