I found it hard to understand in detail the exact legal syntax of the Makefile conditionals, and the code that goes inside of them, and this was the clearest explanation I found on the web.
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.
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) $(libs)
The syntax of a simple conditional with no
else is as follows:
conditional-directive
text-if-true
endif
The
text-if-true may be any lines of text, to be considered as part of the makefile if the condition is true. If the condition is false, no text is used instead.
The syntax of a complex conditional is as follows:
conditional-directive
text-if-true
else
text-if-false
endif
If the condition is true,
text-if-true is used; otherwise,
text-if-false is used instead. The
text-if-false can be any number of lines of text.
The syntax of the
conditional-directive is the same whether the conditional is simple or complex. There are four different directives that test different conditions. Here is a table of them:
ifeq (arg1, arg2)ifeq 'arg1' 'arg2'ifeq "arg1" "arg2"ifeq "arg1" 'arg2'ifeq 'arg1' "arg2"- Expand all variable references in arg1 and arg2 and compare them. If they are identical, the text-if-true is effective; otherwise, the text-if-false, if any, is effective.Often you want to test if a variable has a non-empty value. When the value results from complex expansions of variables and functions, expansions you would consider empty may actually contain whitespace characters and thus are not seen as empty. However, you can use the
strip function (see section Functions for String Substitution and Analysis) to avoid interpreting whitespace as a non-empty value. For example:
ifeq ($(strip $(foo)),)
text-if-empty
endif
will evaluate text-if-empty even if the expansion of $(foo) contains whitespace characters.
ifneq (arg1, arg2)ifneq 'arg1' 'arg2'ifneq "arg1" "arg2"ifneq "arg1" 'arg2'ifneq 'arg1' "arg2"- Expand all variable references in arg1 and arg2 and compare them. If they are different, the text-if-true is effective; otherwise, the text-if-false, if any, is effective.
ifdef variable-name- If the variable variable-name has a non-empty value, the text-if-true is effective; otherwise, the text-if-false, if any, is effective. Variables that have never been defined have an empty value.Note that
ifdef only tests whether a variable has a value. It does not expand the variable to see if that value is nonempty. Consequently, tests using ifdef return true for all definitions except those like foo =. To test for an empty value, use ifeq ($(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-name- If the variable variable-name has an empty value, the text-if-true is effective; otherwise, the text-if-false, if any, is effective.
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.
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.
GNU Make - Conditional Parts of Makefiles
'via Blog this'
Originally Posted by hazel 
