Apr 17, 20173 min read

make -p and make-ing helloworld without a Makefile

Overview

This page discusses make's internal database. This database is useful because it allows users to leverage "implicit" rules to create easy to maintain makefiles or even to build code without a makefile, using make!

Example

Create a file called helloworld.c and enter:

Then type:

How Does make helloworld Work?

The --trace and -B make command line options give hints:

make --help lists:

--trace

Prints tracing information.

-B, --always-make

Unconditionally make all targets.

Add --debug=v

man make gives what the v flag means to the --debug argument: more verbose basic debugging.

--debug[=FLAGS] Print debugging information in addition to normal processing. If the FLAGS are omitted, then the behavior is the same as if -d was specified. FLAGS may be a for all debugging output (same as using -d), b for basic debugging, v for more verbose basic debugging, i for showing implicit rules, j for details on invocation of com‐ mands, and m for debugging while remaking makefiles. Use n to disable all previous debugging flags.

Make's Internal Database

Make has an internal database of rules and variables that users can leverage to write Makefiles or build a program sans Makefile. make helloworld leverages this database.

Printing Make's Internal Database

To print make's internal database type: make -p or make --print-data-base

Beginning of the Database

Here's the beginning of a listing with explanations

Some Internal Make Database Statistics

1306 lines in total

453 non-white space lines

325 lines that don't start with a white space character

167 rules

158 variables

Which Rule is Being Used?

To figure out which internal database rule is being used we can print the database to a file, use that file as our Makefile and leverage the "SHELL hack."

The "SHELL hack"

10 years ago (2007) John Graham-Cumming published the SHELL hack in a Dr Dobbs article titled "Debugging Makefiles."

To use the SHELL hack to figure out what rule is being used redirect the output of make -p to a file:

Next edit Makefile.db and change

Now run make with a few more flags:

You should see something like:

Makefile.db:362: [helloworld (helloworld.c) (helloworld.c)] + cc helloworld.c -o helloworld

@ 362 in my Makefile.db is this:

This is a pattern rule. for make helloworld it expands into:

grepping the Makefile.db shows:

grep 'LINK.c' Makefile.db

shows

LINK.c = $(CC) $(CFLAGS) $(CPPFLAGS) $(LDFLAGS) $(TARGET_ARCH)

grep 'LOADLIBES' Makefile.db and grep 'LDLIBS' Makefile.db show both variables used but not defined.

$^ is an automatic variable that is the names of all the prerequisites, with spaces between them.

$@ is an automatic variable that is the file name of the target

grep 'CC =' Makefile.db

shows

CC = cc

grep 'CFLAGS' Makefile.db and grep 'CPPFLAGS' Makefile.db and grep 'LDFLAGS' Makefile.db and grep 'TARGET_ARCH' Makefile.db

shows that these are all used but not defined. With this the final expansion yields:

Reference

1. make --version

GNU Make 4.1 Built for x86_64-pc-linux-gnu Copyright (C) 1988-2014 Free Software Foundation, Inc. License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html> This is free software: you are free to change and redistribute it. There is NO WARRANTY, to the extent permitted by law.