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Appending More Text to Variables

   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

   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.  Note: 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 (Note: 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 (*note 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; Note: 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.

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