Make/Conditional-Syntax
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7.2 Syntax of Conditionals
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-one text-if-one-is-true else conditional-directive-two text-if-two-is-true else text-if-one-and-two-are-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
andarg2
and compare them. If they are identical, thetext-if-true
is effective; otherwise, thetext-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 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.ifneq (arg1, arg2)
ifneq 'arg1' 'arg2'
ifneq "arg1" "arg2"
ifneq "arg1" 'arg2'
ifneq 'arg1' "arg2"
Expand all variable references in
arg1
andarg2
and compare them. If they are different, thetext-if-true
is effective; otherwise, thetext-if-false
, if any, is effective.ifdef variable-name
The
ifdef
form takes the name of a variable as its argument, not a reference to a variable. If the value of that variable has a non-empty value, thetext-if-true
is effective; otherwise, thetext-if-false
, if any, is effective. Variables that have never been defined have an empty value. The textvariable-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
$(foo)
is expanded, yieldingbar
, which is considered to be the name of a variable. The variablebar
is not expanded, but its value is examined to determine if it is non-empty.Note that
ifdef
only tests whether a variable has a value. It does not expand the variable to see if that value is nonempty. Consequently, tests usingifdef
return true for all definitions except those likefoo =
. To test for an empty value, useifeq ($(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-name
If the variable
variable-name
has an empty value, thetext-if-true
is effective; otherwise, thetext-if-false
, if any, is effective. The rules for expansion and testing ofvariable-name
are identical to theifdef
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 part of a recipe 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 recipes 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
write an include
directive within a conditional, provided you do
not attempt to terminate the conditional inside the included file.
Next: Testing Flags, Previous: Conditional Example, Up: Conditionals [Contents][Index]