## Using make

### Overview

make is a tool that is used to automate the building of complex applications. Specifically, it uses the information in a makefile to “automatically” compile, link, etc… the parts of a complex application.

A makefile contains a list of targets (i.e., source files, object files, and executables), dependents (i.e., the files that a target depends on), and commands/recipes (i.e., what needs to be done if the target does not exist, if the target has an earlier time stamp than a dependent, or if the target is actually a “pseudotarget” without an associated file)

### Syntax

The make command has the following syntax:

 make [-option…]

where option denotes an option or switch (see below).

### Options

The list of valid options/switches for the make command includes the following:

OptionPurpose
ABuild all evaluated targets
BBuild if times stamps are equal
CSuppress output messages
fmakefileUse the given makefile
NDisplay commands but do not execute them

### The makefile

makefiles are normally named makefile. They typically contain 2 (or more) lines for each target. The first line contains the target, a colon, and the dependents (delimited by spaces). The second line contains a tab character and the command/recipe that should be used to create the target.

#### Targets

A target is a valid file name or directory name. A target cannot contain more than 256 characters but it can contain path information.

Multiple targets can be specified in the same dependency line; separate them using spaces or tabs.

#### Dependents

A dependent is a valid file name. It can contain path information.

Multiple dependents can be specified in the same dependency line; seperate them using spaces or tabs.

Of course, a dependent can also be a target elsewhere in the makefile. The make command will update dependents that are targets first. However, some care needs to be taken when creating makefiles.

Note: To split a “long” line, use the backslash (i.e., the \ character) after a dependent.

#### Example

The following makefile has 3 description blocks.

test: test.o module1.o
g++ test.o module1.o -o test

test.o: test.c module1.h
g++ -c test.c

module1.o: module1.h module1.c
g++ -c module1.c


The first description block says that the target test depends on two files, test.o and module1.o. If test has an earlier time stamp than either of these files, test should be rebuilt using the g++ compiler.

The second description block says that test.o depends on test.c and module1.h (because test.c#includes module1.h), and that it can be created with the g++ compiler.

The third description block says that module1.o depends on module1.h (which contains the declarations) and module1.c (which contains the implemntations), and that it can be created with the g++ compiler.

Assuming that this makefile is named makefile, the application test can be built with the following command:

make

#### Processing

make starts with the first target (test in the example above). But, before it executes the command/recipe for this target it checks to see if any of the dependents (test.o and module1.o in the example above) are, themselves, targets (which both are in the example above). It then treats the processing of each such dependent as a prerequisite to the processing of the current target. Of course, the same process is applied to each prerequisite.

#### Predefined Rules

The make command uses a number of predefined/implicit rules that can greatly simplify makefiles. For example, .o files will automatically be created from .c or .cpp files using the GCC.

While you are still learning to use the make command you should not make use of these predefined rules.

#### Using Variables in Place of Literals

Since one often needs to compile/link using different options at different times, people often use variables in place of literals. For example:

CC = cc -pedantic -Wall -ansi

$(CC) -c example.c -o example.o is equivalent to: cc -pedantic -Wall -ansi -c example.c -o example.o You could also accomplish the same thing as follows: CFLAGS = -pedantic -Wall -ansi cc$(CFLAGS) -c example.c -o example.o

### Error and Warning Messages

To get a description of an error or warning message, see the comprehensive list of errors generated by make .