GNU make

The GNU make utility automatically determines which pieces of a large program need to be recompiled, and issues the commands to recompile them.

This edition of the GNU Make Manual, last updated 08 July 2002, documents GNU make Version 3.80.

This manual describes make and contains the following chapters:

1. Overview of make
2. An Introduction to Makefiles		An introduction to make.
3. Writing Makefiles		Makefiles tell make what to do.
4. Writing Rules		Rules describe when a file must be remade.
5. Writing the Commands in Rules		Commands say how to remake a file.
6. How to Use Variables		You can use variables to avoid repetition.
7. Conditional Parts of Makefiles		Use or ignore parts of the makefile based on the values of variables.
8. Functions for Transforming Text		Many powerful ways to manipulate text.
9. How to Run make		How to invoke make on the command line.
10. Using Implicit Rules		Use implicit rules to treat many files alike, based on their file names.
11. Using make to Update Archive Files		How make can update library archives.
12. Features of GNU make		Features GNU make has over other makes.
13. Incompatibilities and Missing Features		What GNU make lacks from other makes.
14. Makefile Conventions		Conventions for writing makefiles for GNU programs.
A. Quick Reference		A quick reference for experienced users.
B. Errors Generated by Make		A list of common errors generated by make.
C. Complex Makefile Example		A real example of a straightforward, but nontrivial, makefile.

D. GNU Free Documentation License		License for copying this manual
Index of Concepts
Index of Functions, Variables, & Directives

 -- The Detailed Node Listing ---

Overview of make

Preparing and Running Make
1.1 How to Read This Manual		On Reading this Text
1.2 Problems and Bugs

An Introduction to Makefiles

2.1 What a Rule Looks Like		What a rule looks like.
2.2 A Simple Makefile
2.3 How make Processes a Makefile		How make Processes This Makefile
2.4 Variables Make Makefiles Simpler
2.5 Letting make Deduce the Commands
2.6 Another Style of Makefile
2.7 Rules for Cleaning the Directory

Writing Makefiles

3.1 What Makefiles Contain		What makefiles contain.
3.2 What Name to Give Your Makefile		How to name your makefile.
3.3 Including Other Makefiles		How one makefile can use another makefile.
3.4 The Variable MAKEFILES		The environment can specify extra makefiles.
3.5 The Variable MAKEFILE_LIST		Discover which makefiles have been read.
3.6 Other Special Variables		Other special variables.
3.7 How Makefiles Are Remade		How makefiles get remade.
3.8 Overriding Part of Another Makefile		How to override part of one makefile with another makefile.
3.9 How make Reads a Makefile		How makefiles are parsed.

Writing Rules

4.1 Rule Example		An example explained.
4.2 Rule Syntax		General syntax explained.
4.3 Types of Prerequisites		There are two types of prerequisites.
4.4 Using Wildcard Characters in File Names		Using wildcard characters such as `*'.
4.5 Searching Directories for Prerequisites		Searching other directories for source files.
4.6 Phony Targets		Using a target that is not a real file's name.
4.7 Rules without Commands or Prerequisites		You can use a target without commands or prerequisites to mark other targets as phony.
4.8 Empty Target Files to Record Events		When only the date matters and the files are empty.
4.9 Special Built-in Target Names		Targets with special built-in meanings.
4.10 Multiple Targets in a Rule		When to make use of several targets in a rule.
4.11 Multiple Rules for One Target		How to use several rules with the same target.
4.12 Static Pattern Rules		Static pattern rules apply to multiple targets and can vary the prerequisites according to the target name.
4.13 Double-Colon Rules		How to use a special kind of rule to allow several independent rules for one target.
4.14 Generating Prerequisites Automatically		How to automatically generate rules giving prerequisites from source files themselves.

Using Wildcard Characters in File Names

4.4.1 Wildcard Examples		Several examples
4.4.2 Pitfalls of Using Wildcards		Problems to avoid.
4.4.3 The Function wildcard		How to cause wildcard expansion where it does not normally take place.

Searching Directories for Prerequisites

4.5.1 VPATH: Search Path for All Prerequisites		Specifying a search path that applies to every prerequisite.
4.5.2 The vpath Directive		Specifying a search path for a specified class of names.
4.5.3 How Directory Searches are Performed		When and how search paths are applied.
4.5.4 Writing Shell Commands with Directory Search		How to write shell commands that work together with search paths.
4.5.5 Directory Search and Implicit Rules		How search paths affect implicit rules.
4.5.6 Directory Search for Link Libraries		Directory search for link libraries.

Static Pattern Rules

4.12.1 Syntax of Static Pattern Rules		The syntax of static pattern rules.
4.12.2 Static Pattern Rules versus Implicit Rules		When are they better than implicit rules?

Writing the Commands in Rules

5.1 Command Echoing		How to control when commands are echoed.
5.2 Command Execution		How commands are executed.
5.3 Parallel Execution		How commands can be executed in parallel.
5.4 Errors in Commands		What happens after a command execution error.
5.5 Interrupting or Killing make		What happens when a command is interrupted.
5.6 Recursive Use of make		Invoking make from makefiles.
5.7 Defining Canned Command Sequences		Defining canned sequences of commands.
5.8 Using Empty Commands		Defining useful, do-nothing commands.

Recursive Use of make

5.6.1 How the MAKE Variable Works		The special effects of using `$(MAKE)'.
5.6.2 Communicating Variables to a Sub-make		How to communicate variables to a sub-make.
5.6.3 Communicating Options to a Sub-make		How to communicate options to a sub-make.
5.6.4 The `--print-directory' Option		How the `-w' or `--print-directory' option helps debug use of recursive make commands.

How to Use Variables

6.1 Basics of Variable References		How to use the value of a variable.
6.2 The Two Flavors of Variables		Variables come in two flavors.
6.3 Advanced Features for Reference to Variables		Advanced features for referencing a variable.
6.4 How Variables Get Their Values		All the ways variables get their values.
6.5 Setting Variables		How to set a variable in the makefile.
6.6 Appending More Text to Variables		How to append more text to the old value of a variable.
6.7 The override Directive		How to set a variable in the makefile even if the user has set it with a command argument.
6.8 Defining Variables Verbatim		An alternate way to set a variable to a verbatim string.
6.9 Variables from the Environment		Variable values can come from the environment.
6.10 Target-specific Variable Values		Variable values can be defined on a per-target basis.
6.11 Pattern-specific Variable Values		Target-specific variable values can be applied to a group of targets that match a pattern.

Advanced Features for Reference to Variables

6.3.1 Substitution References		Referencing a variable with substitutions on the value.
6.3.2 Computed Variable Names		Computing the name of the variable to refer to.

Conditional Parts of Makefiles

7.1 Example of a Conditional		Example of a conditional
7.2 Syntax of Conditionals		The syntax of conditionals.
7.3 Conditionals that Test Flags		Conditionals that test flags.

Functions for Transforming Text

8.1 Function Call Syntax		How to write a function call.
8.2 Functions for String Substitution and Analysis		General-purpose text manipulation functions.
8.3 Functions for File Names		Functions for manipulating file names.
8.4 The foreach Function		Repeat some text with controlled variation.
8.5 The if Function		Conditionally expand a value.
8.6 The call Function		Expand a user-defined function.
8.7 The value Function		Return the un-expanded value of a variable.
8.8 The eval Function		Evaluate the arguments as makefile syntax.
8.9 The origin Function		Find where a variable got its value.
8.10 The shell Function		Substitute the output of a shell command.
8.11 Functions That Control Make		Functions that control how make runs.

How to Run make

9.1 Arguments to Specify the Makefile		How to specify which makefile to use.
9.2 Arguments to Specify the Goals		How to use goal arguments to specify which parts of the makefile to use.
9.3 Instead of Executing the Commands		How to use mode flags to specify what kind of thing to do with the commands in the makefile other than simply execute them.
9.4 Avoiding Recompilation of Some Files		How to avoid recompiling certain files.
9.5 Overriding Variables		How to override a variable to specify an alternate compiler and other things.
9.6 Testing the Compilation of a Program		How to proceed past some errors, to test compilation.
9.7 Summary of Options

Using Implicit Rules

10.1 Using Implicit Rules		How to use an existing implicit rule to get the commands for updating a file.
10.2 Catalogue of Implicit Rules		A list of built-in implicit rules.
10.3 Variables Used by Implicit Rules		How to change what predefined rules do.
10.4 Chains of Implicit Rules		How to use a chain of implicit rules.
10.5 Defining and Redefining Pattern Rules		How to define new implicit rules.
10.6 Defining Last-Resort Default Rules		How to defining commands for rules which cannot find any.
10.7 Old-Fashioned Suffix Rules		The old-fashioned style of implicit rule.
10.8 Implicit Rule Search Algorithm		The precise algorithm for applying implicit rules.

Defining and Redefining Pattern Rules

10.5.1 Introduction to Pattern Rules		An introduction to pattern rules.
10.5.2 Pattern Rule Examples		Examples of pattern rules.
10.5.3 Automatic Variables		How to use automatic variables in the commands of implicit rules.
10.5.4 How Patterns Match		How patterns match.
10.5.5 Match-Anything Pattern Rules		Precautions you should take prior to defining rules that can match any target file whatever.
10.5.6 Canceling Implicit Rules		How to override or cancel built-in rules.

Using make to Update Archive Files

11.1 Archive Members as Targets		Archive members as targets.
11.2 Implicit Rule for Archive Member Targets		The implicit rule for archive member targets.
11.3 Dangers When Using Archives		Dangers to watch out for when using archives.
11.4 Suffix Rules for Archive Files		You can write a special kind of suffix rule for updating archives.

Implicit Rule for Archive Member Targets

11.2.1 Updating Archive Symbol Directories

How to update archive symbol directories.

Makefile Conventions

14.1 General Conventions for Makefiles
14.2 Utilities in Makefiles
14.3 Variables for Specifying Commands
14.4 Variables for Installation Directories
14.5 Standard Targets for Users
14.6 Install Command Categories		Three categories of commands in the `install'

Copying This Manual

1. Overview of make

The make utility automatically determines which pieces of a large program need to be recompiled, and issues commands to recompile them. This manual describes GNU make, which was implemented by Richard Stallman and Roland McGrath. Development since Version 3.76 has been handled by Paul Smith.

GNU make conforms to section 6.2 of IEEE Standard 1003.2-1992 (POSIX.2).

Our examples show C programs, since they are most common, but you can use make with any programming language whose compiler can be run with a shell command. Indeed, make is not limited to programs. You can use it to describe any task where some files must be updated automatically from others whenever the others change.

Preparing and Running Make
1.1 How to Read This Manual		On Reading this Text
1.2 Problems and Bugs

Preparing and Running Make

To prepare to use make, you must write a file called the makefile that describes the relationships among files in your program and provides commands for updating each file. In a program, typically, the executable file is updated from object files, which are in turn made by compiling source files.

Once a suitable makefile exists, each time you change some source files, this simple shell command:

make

suffices to perform all necessary recompilations. The make program uses the makefile data base and the last-modification times of the files to decide which of the files need to be updated. For each of those files, it issues the commands recorded in the data base.

You can provide command line arguments to make to control which files should be recompiled, or how. See section How to Run make.

1.1 How to Read This Manual

If you are new to make, or are looking for a general introduction, read the first few sections of each chapter, skipping the later sections. In each chapter, the first few sections contain introductory or general information and the later sections contain specialized or technical information. The exception is the second chapter, An Introduction to Makefiles, all of which is introductory.

If you are familiar with other make programs, see Features of GNU make, which lists the enhancements GNU make has, and Incompatibilities and Missing Features, which explains the few things GNU make lacks that others have.

For a quick summary, see 9.7 Summary of Options, A. Quick Reference, and 4.9 Special Built-in Target Names.

1.2 Problems and Bugs

If you have problems with GNU make or think you've found a bug, please report it to the developers; we cannot promise to do anything but we might well want to fix it.

Before reporting a bug, make sure you've actually found a real bug. Carefully reread the documentation and see if it really says you can do what you're trying to do. If it's not clear whether you should be able to do something or not, report that too; it's a bug in the documentation!

Before reporting a bug or trying to fix it yourself, try to isolate it to the smallest possible makefile that reproduces the problem. Then send us the makefile and the exact results make gave you, including any error or warning messages. Please don't paraphrase these messages: it's best to cut and paste them into your report. When generating this small makefile, be sure to not use any non-free or unusual tools in your commands: you can almost always emulate what such a tool would do with simple shell commands. Finally, be sure to explain what you expected to occur; this will help us decide whether the problem was really in the documentation.

Once you have a precise problem you can report it in one of two ways. Either send electronic mail to:

bug-make@gnu.org

or use our Web-based project management tool, at:

http://savannah.gnu.org/projects/make/

In addition to the information above, please be careful to include the version number of make you are using. You can get this information with the command `make --version'. Be sure also to include the type of machine and operating system you are using. One way to obtain this information is by looking at the final lines of output from the command `make --help'.

2. An Introduction to Makefiles

You need a file called a makefile to tell make what to do. Most often, the makefile tells make how to compile and link a program.

In this chapter, we will discuss a simple makefile that describes how to compile and link a text editor which consists of eight C source files and three header files. The makefile can also tell make how to run miscellaneous commands when explicitly asked (for example, to remove certain files as a clean-up operation). To see a more complex example of a makefile, see C. Complex Makefile Example.

When make recompiles the editor, each changed C source file must be recompiled. If a header file has changed, each C source file that includes the header file must be recompiled to be safe. Each compilation produces an object file corresponding to the source file. Finally, if any source file has been recompiled, all the object files, whether newly made or saved from previous compilations, must be linked together to produce the new executable editor.

2.1 What a Rule Looks Like		What a rule looks like.
2.2 A Simple Makefile
2.3 How make Processes a Makefile		How make Processes This Makefile
2.4 Variables Make Makefiles Simpler
2.5 Letting make Deduce the Commands
2.6 Another Style of Makefile
2.7 Rules for Cleaning the Directory

2.1 What a Rule Looks Like

A simple makefile consists of "rules" with the following shape:

target ... : prerequisites ... command ... ...

A target is usually the name of a file that is generated by a program; examples of targets are executable or object files. A target can also be the name of an action to carry out, such as `clean' (see section 4.6 Phony Targets).

A prerequisite is a file that is used as input to create the target. A target often depends on several files.

A command is an action that make carries out. A rule may have more than one command, each on its own line. Please note: you need to put a tab character at the beginning of every command line! This is an obscurity that catches the unwary.

Usually a command is in a rule with prerequisites and serves to create a target file if any of the prerequisites change. However, the rule that specifies commands for the target need not have prerequisites. For example, the rule containing the delete command associated with the target `clean' does not have prerequisites.

A rule, then, explains how and when to remake certain files which are the targets of the particular rule. make carries out the commands on the prerequisites to create or update the target. A rule can also explain how and when to carry out an action. See section Writing Rules.

A makefile may contain other text besides rules, but a simple makefile need only contain rules. Rules may look somewhat more complicated than shown in this template, but all fit the pattern more or less.

2.2 A Simple Makefile

Here is a straightforward makefile that describes the way an executable file called edit depends on eight object files which, in turn, depend on eight C source and three header files.

In this example, all the C files include `defs.h', but only those defining editing commands include `command.h', and only low level files that change the editor buffer include `buffer.h'.

edit : main.o kbd.o command.o display.o \ insert.o search.o files.o utils.o cc -o edit main.o kbd.o command.o display.o \ insert.o search.o files.o utils.o main.o : main.c defs.h cc -c main.c kbd.o : kbd.c defs.h command.h cc -c kbd.c command.o : command.c defs.h command.h cc -c command.c display.o : display.c defs.h buffer.h cc -c display.c insert.o : insert.c defs.h buffer.h cc -c insert.c search.o : search.c defs.h buffer.h cc -c search.c files.o : files.c defs.h buffer.h command.h cc -c files.c utils.o : utils.c defs.h cc -c utils.c clean : rm edit main.o kbd.o command.o display.o \ insert.o search.o files.o utils.o

We split each long line into two lines using backslash-newline; this is like using one long line, but is easier to read.

To use this makefile to create the executable file called `edit', type:

make

To use this makefile to delete the executable file and all the object files from the directory, type:

make clean

In the example makefile, the targets include the executable file `edit', and the object files `main.o' and `kbd.o'. The prerequisites are files such as `main.c' and `defs.h'. In fact, each `.o' file is both a target and a prerequisite. Commands include `cc -c main.c' and `cc -c kbd.c'.

When a target is a file, it needs to be recompiled or relinked if any of its prerequisites change. In addition, any prerequisites that are themselves automatically generated should be updated first. In this example, `edit' depends on each of the eight object files; the object file `main.o' depends on the source file `main.c' and on the header file `defs.h'.

A shell command follows each line that contains a target and prerequisites. These shell commands say how to update the target file. A tab character must come at the beginning of every command line to distinguish commands lines from other lines in the makefile. (Bear in mind that make does not know anything about how the commands work. It is up to you to supply commands that will update the target file properly. All make does is execute the commands in the rule you have specified when the target file needs to be updated.)

The target `clean' is not a file, but merely the name of an action. Since you normally do not want to carry out the actions in this rule, `clean' is not a prerequisite of any other rule. Consequently, make never does anything with it unless you tell it specifically. Note that this rule not only is not a prerequisite, it also does not have any prerequisites, so the only purpose of the rule is to run the specified commands. Targets that do not refer to files but are just actions are called phony targets. See section 4.6 Phony Targets, for information about this kind of target. See section Errors in Commands, to see how to cause make to ignore errors from rm or any other command.

2.3 How make Processes a Makefile

By default, make starts with the first target (not targets whose names start with `.'). This is called the default goal. (Goals are the targets that make strives ultimately to update. See section Arguments to Specify the Goals.)

In the simple example of the previous section, the default goal is to update the executable program `edit'; therefore, we put that rule first.

Thus, when you give the command:

make

make reads the makefile in the current directory and begins by processing the first rule. In the example, this rule is for relinking `edit'; but before make can fully process this rule, it must process the rules for the files that `edit' depends on, which in this case are the object files. Each of these files is processed according to its own rule. These rules say to update each `.o' file by compiling its source file. The recompilation must be done if the source file, or any of the header files named as prerequisites, is more recent than the object file, or if the object file does not exist.

The other rules are processed because their targets appear as prerequisites of the goal. If some other rule is not depended on by the goal (or anything it depends on, etc.), that rule is not processed, unless you tell make to do so (with a command such as make clean).

Before recompiling an object file, make considers updating its prerequisites, the source file and header files. This makefile does not specify anything to be done for them--the `.c' and `.h' files are not the targets of any rules--so make does nothing for these files. But make would update automatically generated C programs, such as those made by Bison or Yacc, by their own rules at this time.

After recompiling whichever object files need it, make decides whether to relink `edit'. This must be done if the file `edit' does not exist, or if any of the object files are newer than it. If an object file was just recompiled, it is now newer than `edit', so `edit' is relinked.

Thus, if we change the file `insert.c' and run make, make will compile that file to update `insert.o', and then link `edit'. If we change the file `command.h' and run make, make will recompile the object files `kbd.o', `command.o' and `files.o' and then link the file `edit'.

2.4 Variables Make Makefiles Simpler

In our example, we had to list all the object files twice in the rule for `edit' (repeated here):

edit : main.o kbd.o command.o display.o \ insert.o search.o files.o utils.o cc -o edit main.o kbd.o command.o display.o \ insert.o search.o files.o utils.o

Such duplication is error-prone; if a new object file is added to the system, we might add it to one list and forget the other. We can eliminate the risk and simplify the makefile by using a variable. Variables allow a text string to be defined once and substituted in multiple places later (see section How to Use Variables).

It is standard practice for every makefile to have a variable named objects, OBJECTS, objs, OBJS, obj, or OBJ which is a list of all object file names. We would define such a variable objects with a line like this in the makefile:

objects = main.o kbd.o command.o display.o \ insert.o search.o files.o utils.o

Then, each place we want to put a list of the object file names, we can substitute the variable's value by writing `$(objects)' (see section How to Use Variables).

Here is how the complete simple makefile looks when you use a variable for the object files:

objects = main.o kbd.o command.o display.o \ insert.o search.o files.o utils.o edit : $(objects) cc -o edit $(objects) main.o : main.c defs.h cc -c main.c kbd.o : kbd.c defs.h command.h cc -c kbd.c command.o : command.c defs.h command.h cc -c command.c display.o : display.c defs.h buffer.h cc -c display.c insert.o : insert.c defs.h buffer.h cc -c insert.c search.o : search.c defs.h buffer.h cc -c search.c files.o : files.c defs.h buffer.h command.h cc -c files.c utils.o : utils.c defs.h cc -c utils.c clean : rm edit $(objects)

2.5 Letting make Deduce the Commands

It is not necessary to spell out the commands for compiling the individual C source files, because make can figure them out: it has an implicit rule for updating a `.o' file from a correspondingly named `.c' file using a `cc -c' command. For example, it will use the command `cc -c main.c -o main.o' to compile `main.c' into `main.o'. We can therefore omit the commands from the rules for the object files. See section Using Implicit Rules.

When a `.c' file is used automatically in this way, it is also automatically added to the list of prerequisites. We can therefore omit the `.c' files from the prerequisites, provided we omit the commands.

Here is the entire example, with both of these changes, and a variable objects as suggested above:

objects = main.o kbd.o command.o display.o \ insert.o search.o files.o utils.o edit : $(objects) cc -o edit $(objects) main.o : defs.h kbd.o : defs.h command.h command.o : defs.h command.h display.o : defs.h buffer.h insert.o : defs.h buffer.h search.o : defs.h buffer.h files.o : defs.h buffer.h command.h utils.o : defs.h .PHONY : clean clean : rm edit $(objects)

This is how we would write the makefile in actual practice. (The complications associated with `clean' are described elsewhere. See 4.6 Phony Targets, and Errors in Commands.)

Because implicit rules are so convenient, they are important. You will see them used frequently.

2.6 Another Style of Makefile

When the objects of a makefile are created only by implicit rules, an alternative style of makefile is possible. In this style of makefile, you group entries by their prerequisites instead of by their targets. Here is what one looks like:

objects = main.o kbd.o command.o display.o \ insert.o search.o files.o utils.o edit : $(objects) cc -o edit $(objects) $(objects) : defs.h kbd.o command.o files.o : command.h display.o insert.o search.o files.o : buffer.h

Here `defs.h' is given as a prerequisite of all the object files; `command.h' and `buffer.h' are prerequisites of the specific object files listed for them.

Whether this is better is a matter of taste: it is more compact, but some people dislike it because they find it clearer to put all the information about each target in one place.

2.7 Rules for Cleaning the Directory

Compiling a program is not the only thing you might want to write rules for. Makefiles commonly tell how to do a few other things besides compiling a program: for example, how to delete all the object files and executables so that the directory is `clean'.

Here is how we could write a make rule for cleaning our example editor:

clean: rm edit $(objects)

In practice, we might want to write the rule in a somewhat more complicated manner to handle unanticipated situations. We would do this:

.PHONY : clean clean : -rm edit $(objects)

This prevents make from getting confused by an actual file called `clean' and causes it to continue in spite of errors from rm. (See 4.6 Phony Targets, and Errors in Commands.)

A rule such as this should not be placed at the beginning of the makefile, because we do not want it to run by default! Thus, in the example makefile, we want the rule for edit, which recompiles the editor, to remain the default goal.

Since clean is not a prerequisite of edit, this rule will not run at all if we give the command `make' with no arguments. In order to make the rule run, we have to type `make clean'. See section How to Run make.

3. Writing Makefiles

The information that tells make how to recompile a system comes from reading a data base called the makefile.

3.1 What Makefiles Contain		What makefiles contain.
3.2 What Name to Give Your Makefile		How to name your makefile.
3.3 Including Other Makefiles		How one makefile can use another makefile.
3.4 The Variable MAKEFILES		The environment can specify extra makefiles.
3.5 The Variable MAKEFILE_LIST		Discover which makefiles have been read.
3.6 Other Special Variables		Other special variables.
3.7 How Makefiles Are Remade		How makefiles get remade.
3.8 Overriding Part of Another Makefile		How to override part of one makefile with another makefile.
3.9 How make Reads a Makefile		How makefiles are parsed.

3.1 What Makefiles Contain

Makefiles contain five kinds of things: explicit rules, implicit rules, variable definitions, directives, and comments. Rules, variables, and directives are described at length in later chapters.

• An explicit rule says when and how to remake one or more files, called the rule's targets. It lists the other files that the targets depend on, call the prerequisites of the target, and may also give commands to use to create or update the targets. See section Writing Rules.

• An implicit rule says when and how to remake a class of files based on their names. It describes how a target may depend on a file with a name similar to the target and gives commands to create or update such a target. See section Using Implicit Rules.

• A variable definition is a line that specifies a text string value for a variable that can be substituted into the text later. The simple makefile example shows a variable definition for objects as a list of all object files (see section Variables Make Makefiles Simpler).

• A directive is a command for make to do something special while reading the makefile. These include:
  • Reading another makefile (see section Including Other Makefiles).

  • Deciding (based on the values of variables) whether to use or ignore a part of the makefile (see section Conditional Parts of Makefiles).

  • Defining a variable from a verbatim string containing multiple lines (see section Defining Variables Verbatim).

• `#' in a line of a makefile starts a comment. It and the rest of the line are ignored, except that a trailing backslash not escaped by another backslash will continue the comment across multiple lines. A line containing just a comment (with perhaps spaces before it) is effectively blank, and is ignored. If you want a literal #, escape it with a backslash (e.g., \#). Comments may appear on any line in the makefile, although they are treated specially in certain situations.

  Within a command script (if the line begins with a TAB character) the entire line is passed to the shell, just as with any other line that begins with a TAB. The shell decides how to interpret the text: whether or not this is a comment is up to the shell.

  Within a define directive, comments are not ignored during the definition of the variable, but rather kept intact in the value of the variable. When the variable is expanded they will either be treated as make comments or as command script text, depending on the context in which the variable is evaluated.

3.2 What Name to Give Your Makefile

By default, when make looks for the makefile, it tries the following names, in order: `GNUmakefile', `makefile' and `Makefile'.

Normally you should call your makefile either `makefile' or `Makefile'. (We recommend `Makefile' because it appears prominently near the beginning of a directory listing, right near other important files such as `README'.) The first name checked, `GNUmakefile', is not recommended for most makefiles. You should use this name if you have a makefile that is specific to GNU make, and will not be understood by other versions of make. Other make programs look for `makefile' and `Makefile', but not `GNUmakefile'.

If make finds none of these names, it does not use any makefile. Then you must specify a goal with a command argument, and make will attempt to figure out how to remake it using only its built-in implicit rules. See section Using Implicit Rules.

If you want to use a nonstandard name for your makefile, you can specify the makefile name with the `-f' or `--file' option. The arguments `-f name' or `--file=name' tell make to read the file name as the makefile. If you use more than one `-f' or `--file' option, you can specify several makefiles. All the makefiles are effectively concatenated in the order specified. The default makefile names `GNUmakefile', `makefile' and `Makefile' are not checked automatically if you specify `-f' or `--file'.

3.3 Including Other Makefiles

The include directive tells make to suspend reading the current makefile and read one or more other makefiles before continuing. The directive is a line in the makefile that looks like this:

include filenames...

filenames can contain shell file name patterns.

Extra spaces are allowed and ignored at the beginning of the line, but a tab is not allowed. (If the line begins with a tab, it will be considered a command line.) Whitespace is required between include and the file names, and between file names; extra whitespace is ignored there and at the end of the directive. A comment starting with `#' is allowed at the end of the line. If the file names contain any variable or function references, they are expanded. See section How to Use Variables.

For example, if you have three `.mk' files, `a.mk', `b.mk', and `c.mk', and $(bar) expands to bish bash, then the following expression

include foo *.mk $(bar)

is equivalent to

include foo a.mk b.mk c.mk bish bash

When make processes an include directive, it suspends reading of the containing makefile and reads from each listed file in turn. When that is finished, make resumes reading the makefile in which the directive appears.

One occasion for using include directives is when several programs, handled by individual makefiles in various directories, need to use a common set of variable definitions (see section Setting Variables) or pattern rules (see section Defining and Redefining Pattern Rules).

Another such occasion is when you want to generate prerequisites from source files automatically; the prerequisites can be put in a file that is included by the main makefile. This practice is generally cleaner than that of somehow appending the prerequisites to the end of the main makefile as has been traditionally done with other versions of make. See section 4.14 Generating Prerequisites Automatically.

If the specified name does not start with a slash, and the file is not found in the current directory, several other directories are searched. First, any directories you have specified with the `-I' or `--include-dir' option are searched (see section Summary of Options). Then the following directories (if they exist) are searched, in this order: `prefix/include' (normally `/usr/local/include' (1)) `/usr/gnu/include', `/usr/local/include', `/usr/include'.

If an included makefile cannot be found in any of these directories, a warning message is generated, but it is not an immediately fatal error; processing of the makefile containing the include continues. Once it has finished reading makefiles, make will try to remake any that are out of date or don't exist. See section How Makefiles Are Remade. Only after it has tried to find a way to remake a makefile and failed, will make diagnose the missing makefile as a fatal error.

If you want make to simply ignore a makefile which does not exist and cannot be remade, with no error message, use the -include directive instead of include, like this:

-include filenames...

This acts like include in every way except that there is no error (not even a warning) if any of the filenames do not exist. For compatibility with some other make implementations, sinclude is another name for -include.

3.4 The Variable MAKEFILES

If the environment variable MAKEFILES is defined, make considers its value as a list of names (separated by whitespace) of additional makefiles to be read before the others. This works much like the include directive: various directories are searched for those files (see section Including Other Makefiles). In addition, the default goal is never taken from one of these makefiles and it is not an error if the files listed in MAKEFILES are not found.

The main use of MAKEFILES is in communication between recursive invocations of make (see section Recursive Use of make). It usually is not desirable to set the environment variable before a top-level invocation of make, because it is usually better not to mess with a makefile from outside. However, if you are running make without a specific makefile, a makefile in MAKEFILES can do useful things to help the built-in implicit rules work better, such as defining search paths (see section 4.5 Searching Directories for Prerequisites).

Some users are tempted to set MAKEFILES in the environment automatically on login, and program makefiles to expect this to be done. This is a very bad idea, because such makefiles will fail to work if run by anyone else. It is much better to write explicit include directives in the makefiles. See section Including Other Makefiles.

3.5 The Variable MAKEFILE_LIST

As make reads various makefiles, including any obtained from the MAKEFILES variable, the command line, the default files, or from include directives, their names will be automatically appended to the MAKEFILE_LIST variable. They are added right before make begins to parse them.

This means that if the first thing a makefile does is examine the last word in this variable, it will be the name of the current makefile. Once the current makefile has used include, however, the last word will be the just-included makefile.

If a makefile named Makefile has this content:

name1 := $(word $(words $(MAKEFILE_LIST)),$(MAKEFILE_LIST)) include inc.mk name2 := $(word $(words $(MAKEFILE_LIST)),$(MAKEFILE_LIST)) all: @echo name1 = $(name1) @echo name2 = $(name2)

then you would expect to see this output:

name1 = Makefile name2 = inc.mk

See section 8.2 Functions for String Substitution and Analysis, for more information on the word and words functions used above. See section The Two Flavors of Variables, for more information on simply-expanded (:=) variable definitions.

3.6 Other Special Variables

GNU make also supports a special variable. Note that any value you assign to this variable will be ignored; it will always return its special value.

The first special variable is .VARIABLES. When expanded, the value consists of a list of the names of all global variables defined in all makefiles read up until that point. This includes variables which have empty values, as well as built-in variables (see section Variables Used by Implicit Rules), but does not include any variables which are only defined in a target-specific context.

3.7 How Makefiles Are Remade

Sometimes makefiles can be remade from other files, such as RCS or SCCS files. If a makefile can be remade from other files, you probably want make to get an up-to-date version of the makefile to read in.

To this end, after reading in all makefiles, make will consider each as a goal target and attempt to update it. If a makefile has a rule which says how to update it (found either in that very makefile or in another one) or if an implicit rule applies to it (see section Using Implicit Rules), it will be updated if necessary. After all makefiles have been checked, if any have actually been changed, make starts with a clean slate and reads all the makefiles over again. (It will also attempt to update each of them over again, but normally this will not change them again, since they are already up to date.)

If you know that one or more of your makefiles cannot be remade and you want to keep make from performing an implicit rule search on them, perhaps for efficiency reasons, you can use any normal method of preventing implicit rule lookup to do so. For example, you can write an explicit rule with the makefile as the target, and an empty command string (see section Using Empty Commands).

If the makefiles specify a double-colon rule to remake a file with commands but no prerequisites, that file will always be remade (see section 4.13 Double-Colon Rules). In the case of makefiles, a makefile that has a double-colon rule with commands but no prerequisites will be remade every time make is run, and then again after make starts over and reads the makefiles in again. This would cause an infinite loop: make would constantly remake the makefile, and never do anything else. So, to avoid this, make will not attempt to remake makefiles which are specified as targets of a double-colon rule with commands but no prerequisites.

If you do not specify any makefiles to be read with `-f' or `--file' options, make will try the default makefile names; see section What Name to Give Your Makefile. Unlike makefiles explicitly requested with `-f' or `--file' options, make is not certain that these makefiles should exist. However, if a default makefile does not exist but can be created by running make rules, you probably want the rules to be run so that the makefile can be used.

Therefore, if none of the default makefiles exists, make will try to make each of them in the same order in which they are searched for (see section What Name to Give Your Makefile) until it succeeds in making one, or it runs out of names to try. Note that it is not an error if make cannot find or make any makefile; a makefile is not always necessary.

When you use the `-t' or `--touch' option (see section Instead of Executing the Commands), you would not want to use an out-of-date makefile to decide which targets to touch. So the `-t' option has no effect on updating makefiles; they are really updated even if `-t' is specified. Likewise, `-q' (or `--question') and `-n' (or `--just-print') do not prevent updating of makefiles, because an out-of-date makefile would result in the wrong output for other targets. Thus, `make -f mfile -n foo' will update `mfile', read it in, and then print the commands to update `foo' and its prerequisites without running them. The commands printed for `foo' will be those specified in the updated contents of `mfile'.

However, on occasion you might actually wish to prevent updating of even the makefiles. You can do this by specifying the makefiles as goals in the command line as well as specifying them as makefiles. When the makefile name is specified explicitly as a goal, the options `-t' and so on do apply to them.

Thus, `make -f mfile -n mfile foo' would read the makefile `mfile', print the commands needed to update it without actually running them, and then print the commands needed to update `foo' without running them. The commands for `foo' will be those specified by the existing contents of `mfile'.

3.8 Overriding Part of Another Makefile

Sometimes it is useful to have a makefile that is mostly just like another makefile. You can often use the `include' directive to include one in the other, and add more targets or variable definitions. However, if the two makefiles give different commands for the same target, make will not let you just do this. But there is another way.

In the containing makefile (the one that wants to include the other), you can use a match-anything pattern rule to say that to remake any target that cannot be made from the information in the containing makefile, make should look in another makefile. See section 10.5 Defining and Redefining Pattern Rules, for more information on pattern rules.

For example, if you have a makefile called `Makefile' that says how to make the target `foo' (and other targets), you can write a makefile called `GNUmakefile' that contains:

foo: frobnicate > foo %: force @$(MAKE) -f Makefile $@ force: ;

If you say `make foo', make will find `GNUmakefile', read it, and see that to make `foo', it needs to run the command `frobnicate > foo'. If you say `make bar', make will find no way to make `bar' in `GNUmakefile', so it will use the commands from the pattern rule: `make -f Makefile bar'. If `Makefile' provides a rule for updating `bar', make will apply the rule. And likewise for any other target that `GNUmakefile' does not say how to make.

The way this works is that the pattern rule has a pattern of just `%', so it matches any target whatever. The rule specifies a prerequisite `force', to guarantee that the commands will be run even if the target file already exists. We give `force' target empty commands to prevent make from searching for an implicit rule to build it--otherwise it would apply the same match-anything rule to `force' itself and create a prerequisite loop!

3.9 How make Reads a Makefile

GNU make does its work in two distinct phases. During the first phase it reads all the makefiles, included makefiles, etc. and internalizes all the variables and their values, implicit and explicit rules, and constructs a dependency graph of all the targets and their prerequisites. During the second phase, make uses these internal structures to determine what targets will need to be rebuilt and to invoke the rules necessary to do so.

It's important to understand this two-phase approach because it has a direct impact on how variable and function expansion happens; this is often a source of some confusion when writing makefiles. Here we will present a summary of the phases in which expansion happens for different constructs within the makefile. We say that expansion is immediate if it happens during the first phase: in this case make will expand any variables or functions in that section of a construct as the makefile is parsed. We say that expansion is deferred if expansion is not performed immediately. Expansion of deferred construct is not performed until either the construct appears later in an immediate context, or until the second phase.

You may not be familiar with some of these constructs yet. You can reference this section as you become familiar with them, in later chapters.

Variable Assignment

Variable definitions are parsed as follows:

immediate = deferred immediate ?= deferred immediate := immediate immediate += deferred or immediate define immediate deferred endef

For the append operator, `+=', the right-hand side is considered immediate if the variable was previously set as a simple variable (`:='), and deferred otherwise.

Conditional Statements

All instances of conditional syntax are parsed immediately, in their entirety; this includes the ifdef, ifeq, ifndef, and ifneq forms.

Rule Definition

A rule is always expanded the same way, regardless of the form:

immediate : immediate ; deferred deferred

That is, the target and prerequisite sections are expanded immediately, and the commands used to construct the target are always deferred. This general rule is true for explicit rules, pattern rules, suffix rules, static pattern rules, and simple prerequisite definitions.

4. Writing Rules

A rule appears in the makefile and says when and how to remake certain files, called the rule's targets (most often only one per rule). It lists the other files that are the prerequisites of the target, and commands to use to create or update the target.

The order of rules is not significant, except for determining the default goal: the target for make to consider, if you do not otherwise specify one. The default goal is the target of the first rule in the first makefile. If the first rule has multiple targets, only the first target is taken as the default. There are two exceptions: a target starting with a period is not a default unless it contains one or more slashes, `/', as well; and, a target that defines a pattern rule has no effect on the default goal. (See section Defining and Redefining Pattern Rules.)

Therefore, we usually write the makefile so that the first rule is the one for compiling the entire program or all the programs described by the makefile (often with a target called `all'). See section Arguments to Specify the Goals.

4.1 Rule Example		An example explained.
4.2 Rule Syntax		General syntax explained.
4.3 Types of Prerequisites		There are two types of prerequisites.
4.4 Using Wildcard Characters in File Names		Using wildcard characters such as `*'.
4.5 Searching Directories for Prerequisites		Searching other directories for source files.
4.6 Phony Targets		Using a target that is not a real file's name.
4.7 Rules without Commands or Prerequisites		You can use a target without commands or prerequisites to mark other targets as phony.
4.8 Empty Target Files to Record Events		When only the date matters and the