NOTES ON MAKEFILES

1. GENERAL

Makefiles are files performing certain actions useful to compile a program or library, archive a document, update a website, etc. These actions are executed only depending on the last modification date of the various source files.

A makefile is composed of several rules with the form:

<target>: <dependency>
          <commands>

The commands must be preceded by a TAB. Comments are indicated with #.

1.1. Executing

The general command to execute a makefile is:

make -f <mymakefile>.mk <rules>

If no file name is given, make will look for a makefile file in the current directory.
If no rule is given, make executes the first rule in the file.

1.2. Rule Intrication

If a dependency is the target of another rule, this rule is then executed.
If the target does not exist or is older than one dependency, the command of the rule is executed.

1.3. Specific Rules

1.3.1. The rule `all`

It should appear first. Its dependencies are all the executables to be produced.

1.3.2. The rule `clean`

It suppresses the intermediary files.intermédiaires.

1.3.3. The rule `mrproper`

It suppresses everything generated by the makefile.

1.3.4. The rule `.PHONY`

With a line such as:

.PHONY: <target1> ... <targetN>

the targets <target1> to <targetN> are systematically rebuilt, even if they are more recent than their dependencies.

1.3.5. The rule `.PRECIOUS`

With a line such as:

.PRECIOUS: %.o

the makefile does not suppress the .o files, which should still be suppressed as intermediary files.

1.4. Silent Commands

To make a command silent, it needs to be preceded by @. Only error messages are printed in this case.

2. VARIABLES

2.1. Variable Definitions

One can, in the first part of a makefile, define several variables, such as: <var> = <value>, <value> can eventually be empty. Most of the time, those are compilation options or file lists, such as:

FC=h5fc
CFLAGS=-O3 - g
LDFLAGS=
MODS=<mod1>.f90 \
     ...
     <modN.f90

One can also import variables from another file: include <path>/<make_common.mk>.

2.2. Internal Variables

$@: → the target;
$<: → the first dependency;
$^: → the list of dependencies;
$?: → the list of dependencies more recent than the target;
S*: → the file without extension.

2.3. Inference Rules

Example of building several .o files from their .f90:

%.o: %.f90
       <commands>

2.4. Generating a List of Files

A declaration such as:

OBJ=$(SRC:.f90=.o)

attributes to the variable $(OBJ) the list of $(SRC) files, but replacing the extension .f90 by .o.

2.5. Using Metacharacters

One can use UNIX metacharacters with the command wildcard, such as:

SRC=$(wildcard *.f90)

2.6. Variables `shell`

One can use shell variables with the shell command, such as:

HOST=($shell HOSTNAME)

3. ADVANCED FUNCTIONALITIES

3.1. Decision Scheme

One can implement a classic decision scheme such as:

ifeq ($(VAR),value1)
       <command1>
else ifeq ($(VAR),value2)
       <command2>
else
      <command3>
endif

3.2. Search Directories

One can specify in which directories searching for targets and dependencies, by declaring the variable VPATH = <dir1>:...:<dirN>.

One can also specify more precisely the search with the directive vpath, such as:

vpath %.f90 ../Programs:../Tests
vpath %.o ../Programs
vpath lib%.a ../Programs

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