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:
#include <stdio.h>
int main(int argc, char *argv[])
{
printf("Hello, World\n");
}
Then type:
pfefferz@pfefferz-VirtualBox:~/helloworld$ make helloworld
cc helloworld.c -o helloworld
pfefferz@pfefferz-VirtualBox:~/helloworld$ ./helloworld
Hello, World
How Does make helloworld Work?
The –trace and -B make command line options give hints:
pfefferz@pfefferz-VirtualBox:~/helloworld$ make -B --trace helloworld
: update target 'helloworld' due to: helloworld.c
cc helloworld.c -o helloworld
make –help lists:
--trace
Prints tracing information.
-B, –always-make
Unconditionally make all targets.
Add –debug=v
pfefferz@pfefferz-VirtualBox:~/helloworld$ make -B --trace --debug=v helloworld
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
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.
Reading makefiles...
Updating goal targets....
Considering target file 'helloworld'.
Considering target file 'helloworld.c'.
Finished prerequisites of target file 'helloworld.c'.
No need to remake target 'helloworld.c'.
Finished prerequisites of target file 'helloworld'.
Prerequisite 'helloworld.c' is older than target 'helloworld'.
Making 'helloworld' due to always-make flag.
Must remake target 'helloworld'.
: update target 'helloworld' due to: helloworld.c
cc helloworld.c -o helloworld
Successfully remade target file 'helloworld'.
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
# 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.
# Make data base, printed on Sat Apr 8 11:26:50 2017
# Variables
# automatic
<D = $(patsubst %/,%,$(dir $<))
Some Internal Make Database Statistics
1306 lines in total
pfefferz@pfefferz-VirtualBox:~/helloworld$ make -p | wc -l
make: *** No targets specified and no makefile found. Stop.
1306
453 non-white space lines
pfefferz@pfefferz-VirtualBox:~/helloworld$ make -p | grep -v '^$' | grep -v '^#' | wc -l
make: *** No targets specified and no makefile found. Stop.
453
325 lines that don’t start with a white space character
pfefferz@pfefferz-VirtualBox:~/helloworld$ make -p | grep -v '^$' | grep -v '^#' | grep -v '^\s' | wc -l
make: *** No targets specified and no makefile found. Stop.
325
167 rules
pfefferz@pfefferz-VirtualBox:~/helloworld$ make -p | grep -v '^$' | grep -v '^#' | grep -v '^\s' | grep -v '=' | wc -l
make: *** No targets specified and no makefile found. Stop.
167
158 variables
pfefferz@pfefferz-VirtualBox:~/helloworld$ make -p | grep -v '^$' | grep -v '^#' | grep -v '^\s' | grep '=' | wc -l
make: *** No targets specified and no makefile found. Stop.
158
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:
make -p > Makefile.db
Next edit Makefile.db and change
SHELL := /bin/sh
to
OLD_SHELL := /bin/sh
SHELL = $(warning [$@ ($^) ($?)])$(OLD_SHELL) -x
Now run make with a few more flags:
make -rRB -f Makefile.db helloworld > Makefile.db.out
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:
%: %.c
# recipe to execute (built-in):
$(LINK.c) $^ $(LOADLIBES) $(LDLIBS) -o $@
This is a pattern rule. for make helloworld it expands into:
helloworld: helloworld.c
# recipe to execute (built-in):
$(LINK.c) $^ $(LOADLIBES) $(LDLIBS) -o $@
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:
helloworld: helloworld.c
# recipe to execute (built-in):
cc helloworld.c -o helloworld
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.
3. Everything You Need to Know About HTML’s ‘pre’ Element
4. GNU make: a program for directed recompilation: GNU make version 3.79.1
Richard Stallman - Roland McGrath - Paul D.Smith - GNU Press - 2004
ISBN 1-882114-81-7
[Amazon]