make: Appending
6.6 Appending More Text to Variables
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Often it is useful to add more text to the value of a variable already
defined. You do this with a line containing '+=', like this:
objects += another.o
This takes the value of the variable 'objects', and adds the text
'another.o' to it (preceded by a single space). Thus:
objects = main.o foo.o bar.o utils.o
objects += another.o
sets 'objects' to 'main.o foo.o bar.o utils.o another.o'.
Using '+=' is similar to:
objects = main.o foo.o bar.o utils.o
objects := $(objects) another.o
but differs in ways that become important when you use more complex
values.
When the variable in question has not been defined before, '+=' acts
just like normal '=': it defines a recursively-expanded variable.
However, when there _is_ a previous definition, exactly what '+=' does
depends on what flavor of variable you defined originally. The
Two Flavors of Variables Flavors, for an explanation of the two flavors
of variables.
When you add to a variable's value with '+=', 'make' acts essentially
as if you had included the extra text in the initial definition of the
variable. If you defined it first with ':=' or '::=', making it a
simply-expanded variable, '+=' adds to that simply-expanded definition,
and expands the new text before appending it to the old value just as
':=' does (see Setting Variables Setting, for a full explanation
of ':=' or '::='). In fact,
variable := value
variable += more
is exactly equivalent to:
variable := value
variable := $(variable) more
On the other hand, when you use '+=' with a variable that you defined
first to be recursively-expanded using plain '=', 'make' does something
a bit different. Recall that when you define a recursively-expanded
variable, 'make' does not expand the value you set for variable and
function references immediately. Instead it stores the text verbatim,
and saves these variable and function references to be expanded later,
when you refer to the new variable (The Two Flavors of Variables
Flavors.). When you use '+=' on a recursively-expanded variable, it is
this unexpanded text to which 'make' appends the new text you specify.
variable = value
variable += more
is roughly equivalent to:
temp = value
variable = $(temp) more
except that of course it never defines a variable called 'temp'. The
importance of this comes when the variable's old value contains variable
references. Take this common example:
CFLAGS = $(includes) -O
...
CFLAGS += -pg # enable profiling
The first line defines the 'CFLAGS' variable with a reference to another
variable, 'includes'. ('CFLAGS' is used by the rules for C compilation;
Catalogue of Built-In Rules Catalogue of Rules.) Using '=' for
the definition makes 'CFLAGS' a recursively-expanded variable, meaning
'$(includes) -O' is _not_ expanded when 'make' processes the definition
of 'CFLAGS'. Thus, 'includes' need not be defined yet for its value to
take effect. It only has to be defined before any reference to
'CFLAGS'. If we tried to append to the value of 'CFLAGS' without using
'+=', we might do it like this:
CFLAGS := $(CFLAGS) -pg # enable profiling
This is pretty close, but not quite what we want. Using ':=' redefines
'CFLAGS' as a simply-expanded variable; this means 'make' expands the
text '$(CFLAGS) -pg' before setting the variable. If 'includes' is not
yet defined, we get ' -O -pg', and a later definition of 'includes' will
have no effect. Conversely, by using '+=' we set 'CFLAGS' to the
_unexpanded_ value '$(includes) -O -pg'. Thus we preserve the reference
to 'includes', so if that variable gets defined at any later point, a
reference like '$(CFLAGS)' still uses its value.