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
conditional-directive text-if-one-is-true else conditional-directive text-if-true else text-if-false endif
There can be as many “
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
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:
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 Text Functions) 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.
ifdefform 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
$(foo) is expanded, yielding
which is considered to be the name of a variable. The variable
bar is not expanded, but its value is examined to determine if
it is non-empty.
ifdef only tests whether a variable has a value. It
does not expand the variable to see if that value is nonempty.
Consequently, tests using
ifdef return true for all definitions
except those like
foo =. To test for an empty value, use
ifeq ($(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’.
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
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 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
include directive within a conditional, provided you do
not attempt to terminate the conditional inside the included file.