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. See section The Two Flavors of Variables, 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 `:=',
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 section Setting Variables, for a full explanation of `:=').
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 (see section The Two Flavors of Variables). 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 = $(includes) -O ... CFLAGS += -pg # enable profiling
The first line defines the CFLAGS variable with a reference to another
variable, includes. (CFLAGS is used by the rules for C
compilation; see section Catalogue of Implicit Rules.)
Using `=' for the definition makes CFLAGS a recursively-expanded
variable, meaning `$(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.