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A conditional causes part of a makefile to be obeyed or ignored
+depending on the values of variables. Conditionals can compare the
+value of one variable to another, or the value of a variable to
+a constant string. Conditionals control what make actually
+“sees” in the makefile, so they cannot be used to control shell
+commands at the time of execution.
+
| 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. + |
| [ < ] | +[ > ] | ++ | [ << ] | +[ Up ] | +[ >> ] | ++ | + | + | + | [Top] | +[Contents] | +[Index] | +[ ? ] | +
The following example of a conditional tells make to use one set
+of libraries if the CC variable is ‘gcc’, and a different
+set of libraries otherwise. It works by controlling which of two
+command lines will be used as the command for a rule. The result is
+that ‘CC=gcc’ as an argument to make changes not only which
+compiler is used but also which libraries are linked.
+
libs_for_gcc = -lgnu +normal_libs = + +foo: $(objects) +ifeq ($(CC),gcc) + $(CC) -o foo $(objects) $(libs_for_gcc) +else + $(CC) -o foo $(objects) $(normal_libs) +endif + |
This conditional uses three directives: one ifeq, one else
+and one endif.
+
The ifeq directive begins the conditional, and specifies the
+condition. It contains two arguments, separated by a comma and surrounded
+by parentheses. Variable substitution is performed on both arguments and
+then they are compared. The lines of the makefile following the
+ifeq are obeyed if the two arguments match; otherwise they are
+ignored.
+
The else directive causes the following lines to be obeyed if the
+previous conditional failed. In the example above, this means that the
+second alternative linking command is used whenever the first alternative
+is not used. It is optional to have an else in a conditional.
+
The endif directive ends the conditional. Every conditional must
+end with an endif. Unconditional makefile text follows.
+
As this example illustrates, conditionals work at the textual level: +the lines of the conditional are treated as part of the makefile, or +ignored, according to the condition. This is why the larger syntactic +units of the makefile, such as rules, may cross the beginning or the +end of the conditional. +
+When the variable CC has the value ‘gcc’, the above example has
+this effect:
+
foo: $(objects) + $(CC) -o foo $(objects) $(libs_for_gcc) + |
When the variable CC has any other value, the effect is this:
+
foo: $(objects) + $(CC) -o foo $(objects) $(normal_libs) + |
Equivalent results can be obtained in another way by conditionalizing a +variable assignment and then using the variable unconditionally: +
+libs_for_gcc = -lgnu +normal_libs = + +ifeq ($(CC),gcc) + libs=$(libs_for_gcc) +else + libs=$(normal_libs) +endif + +foo: $(objects) + $(CC) -o foo $(objects) $(libs) + |
| [ < ] | +[ > ] | ++ | [ << ] | +[ Up ] | +[ >> ] | ++ | + | + | + | [Top] | +[Contents] | +[Index] | +[ ? ] | +
The syntax of a simple conditional with no else is as follows:
+
conditional-directive +text-if-true +endif + |
The text-if-true may be any lines of text, to be considered as part +of the makefile if the condition is true. If the condition is false, no +text is used instead. +
+The syntax of a complex conditional is as follows: +
+conditional-directive +text-if-true +else +text-if-false +endif + |
or: +
+conditional-directive +text-if-one-is-true +else conditional-directive +text-if-true +else +text-if-false +endif + |
There can be as many “else conditional-directive”
+clauses as necessary. Once a given condition is true,
+text-if-true is used and no other clause is used; if no
+condition is true then text-if-false is used. The
+text-if-true and text-if-false can be any number of lines
+of text.
+
The syntax of the conditional-directive is the same whether the
+conditional is simple or complex; after an else or not. There
+are four different directives that test different conditions. Here is
+a table of them:
+
ifeq (arg1, arg2)ifeq 'arg1' 'arg2'ifeq "arg1" "arg2"ifeq "arg1" 'arg2'ifeq 'arg1' "arg2"Expand all variable references in arg1 and arg2 and +compare them. If they are identical, the text-if-true is +effective; otherwise, the text-if-false, if any, is effective. +
+Often you want to test if a variable has a non-empty value. When the
+value results from complex expansions of variables and functions,
+expansions you would consider empty may actually contain whitespace
+characters and thus are not seen as empty. However, you can use the
+strip function (see section Functions for String Substitution and Analysis) to avoid interpreting
+whitespace as a non-empty value. For example:
+
ifeq ($(strip $(foo)),) +text-if-empty +endif + |
will evaluate text-if-empty even if the expansion of
+$(foo) contains whitespace characters.
+
ifneq (arg1, arg2)ifneq 'arg1' 'arg2'ifneq "arg1" "arg2"ifneq "arg1" 'arg2'ifneq 'arg1' "arg2"Expand all variable references in arg1 and arg2 and +compare them. If they are different, the text-if-true is +effective; otherwise, the text-if-false, if any, is effective. +
+ifdef variable-nameThe ifdef form takes the name of a variable as its
+argument, not a reference to a variable. The value of that variable
+has a non-empty value, the text-if-true is effective; otherwise,
+the text-if-false, if any, is effective. Variables that have
+never been defined have an empty value. The text variable-name
+is expanded, so it could be a variable or function that expands
+to the name of a variable. For example:
+
bar = true +foo = bar +ifdef $(foo) +frobozz = yes +endif + |
The variable reference $(foo) is expanded, yielding bar,
+which is considered to be the name of a variable. The variable
+bar is not expanded, but its value is examined to determine if
+it is non-empty.
+
Note that ifdef only tests whether a variable has a value. It
+does not expand the variable to see if that value is nonempty.
+Consequently, tests using ifdef return true for all definitions
+except those like foo =. To test for an empty value, use
+ifeq ($(foo),). For example,
+
bar = +foo = $(bar) +ifdef foo +frobozz = yes +else +frobozz = no +endif + |
sets ‘frobozz’ to ‘yes’, while: +
+foo = +ifdef foo +frobozz = yes +else +frobozz = no +endif + |
sets ‘frobozz’ to ‘no’. +
+ifndef variable-nameIf the variable variable-name has an empty value, the
+text-if-true is effective; otherwise, the text-if-false,
+if any, is effective. The rules for expansion and testing of
+variable-name are identical to the ifdef directive.
+
Extra spaces are allowed and ignored at the beginning of the conditional +directive line, but a tab is not allowed. (If the line begins with a tab, +it will be considered a command for a rule.) Aside from this, extra spaces +or tabs may be inserted with no effect anywhere except within the directive +name or within an argument. A comment starting with ‘#’ may appear at +the end of the line. +
+The other two directives that play a part in a conditional are else
+and endif. Each of these directives is written as one word, with no
+arguments. Extra spaces are allowed and ignored at the beginning of the
+line, and spaces or tabs at the end. A comment starting with ‘#’ may
+appear at the end of the line.
+
Conditionals affect which lines of the makefile make uses. If
+the condition is true, make reads the lines of the
+text-if-true as part of the makefile; if the condition is false,
+make ignores those lines completely. It follows that syntactic
+units of the makefile, such as rules, may safely be split across the
+beginning or the end of the conditional.
+
make evaluates conditionals when it reads a makefile.
+Consequently, you cannot use automatic variables in the tests of
+conditionals because they are not defined until commands are run
+(see section Automatic Variables).
+
To prevent intolerable confusion, it is not permitted to start a
+conditional in one makefile and end it in another. However, you may
+write an include directive within a conditional, provided you do
+not attempt to terminate the conditional inside the included file.
+
| [ < ] | +[ > ] | ++ | [ << ] | +[ Up ] | +[ >> ] | ++ | + | + | + | [Top] | +[Contents] | +[Index] | +[ ? ] | +
You can write a conditional that tests make command flags such as
+‘-t’ by using the variable MAKEFLAGS together with the
+findstring function
+(see section Functions for String Substitution and Analysis).
+This is useful when touch is not enough to make a file appear up
+to date.
+
The findstring function determines whether one string appears as a
+substring of another. If you want to test for the ‘-t’ flag,
+use ‘t’ as the first string and the value of MAKEFLAGS as
+the other.
+
For example, here is how to arrange to use ‘ranlib -t’ to finish +marking an archive file up to date: +
+archive.a: … +ifneq (,$(findstring t,$(MAKEFLAGS))) + +touch archive.a + +ranlib -t archive.a +else + ranlib archive.a +endif + |
The ‘+’ prefix marks those command lines as “recursive” so
+that they will be executed despite use of the ‘-t’ flag.
+See section Recursive Use of make.
+
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+
+ This document was generated by Manoj Srivastava on August, 17 2009 using texi2html 1.78.
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