Chapter 3. Variables and Macros

     

Chapter 3. Variables and Macros

We've been looking at makefile variables for a while now and we've seen many examples of how they're used in both the built-in and user -defined rules. But the examples we've seen have only scratched the surface. Variables and macros get much more complicated and give GNU make much of its incredible power.

Before we go any further, it is important to understand that make is sort of two languages in one. The first language describes dependency graphs consisting of targets and prerequisites. (This language was covered in Chapter 2.) The second language is a macro language for performing textual substitution. Other macro languages you may be familiar with are the C preprocessor, m4 , TEX, and macro assemblers. Like these other macro languages, make allows you to define a shorthand term for a longer sequence of characters and use the shorthand in your program. The macro processor will recognize your shorthand terms and replace them with their expanded form. Although it is easy to think of makefile variables as traditional programming language variables, there is a distinction between a macro "variable" and a "traditional" variable. A macro variable is expanded "in place" to yield a text string that may then be expanded further. This distinction will become more clear as we proceed.

A variable name can contain almost any characters including most punctuation. Even spaces are allowed, but if you value your sanity you should avoid them. The only characters actually disallowed in a variable name are :, #, and =.

Variables are case-sensitive, so cc and CC refer to different variables. To get the value of a variable, enclose the variable name in $( ) . As a special case, single-letter variable names can omit the parentheses and simply use $ letter . This is why the automatic variables can be written without the parentheses. As a general rule you should use the parenthetical form and avoid single letter variable names.

Variables can also be expanded using curly braces as in ${CC} and you will often see this form, particularly in older makefile s. There is seldom an advantage to using one over the other, so just pick one and stick with it. Some people use curly braces for variable reference and parentheses for function call, similar to the way the shell uses them. Most modern makefile s use parentheses and that's what we'll use throughout this book.

Variables representing constants a user might want to customize on the command line or in the environment are written in all uppercase, by convention. Words are separated by underscores. Variables that appear only in the makefile are all lowercase with words separated by underscores. Finally, in this book, user-defined functions in variables and macros use lowercase words separated by dashes. Other naming conventions will be explained where they occur. (The following example uses features we haven't discussed yet. I'm using them to illustrate the variable naming conventions, don't be too concerned about the righthand side for now.)

 # Some simple constants. CC    := gcc MKDIR := mkdir -p # Internal variables. sources = *.c objects = $(subst .c,.o,$(sources)) # A function or two. maybe-make-dir  = $(if $(wildcard ),,$(MKDIR) ) assert-not-null = $(if ,,$(error Illegal null value.)) 

The value of a variable consists of all the words to the right of the assignment symbol with leading space trimmed. Trailing spaces are not trimmed . This can occasionally cause trouble, for instance, if the trailing whitespace is included in the variable and subsequently used in a command script:

 LIBRARY = libio.a # LIBRARY has a trailing space. missing_file:         touch $(LIBRARY)         ls -l  grep '$(LIBRARY)' 

The variable assignment contains a trailing space that is made more apparent by the comment (but a trailing space can also be present without a trailing comment). When this makefile is run, we get:

 $ make touch libio.a  ls -l  grep 'libio.a ' make: *** [missing_file] Error 1 

Oops, the grep search string also included the trailing space and failed to find the file in ls 's output. We'll discuss whitespace issues in more detail later. For now, let's look more closely at variables.



Managing Projects with GNU make
Managing Projects with GNU Make (Nutshell Handbooks)
ISBN: 0596006101
EAN: 2147483647
Year: 2003
Pages: 131

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