6.6 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 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 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 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 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 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.