Roland Ver. 4.5 Información técnica descargar pdf (Pagina 77)

Chapter 6: How to Use Variables 67

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 deﬁni-

tion, exactly what ‘+=’ does depends on what ﬂavor of variable you deﬁned originally. See

Section 6.2 [The Two Flavors of Variables], page 60, 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 Section 6.5

[Setting Variables], page 65, 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 (see Section 6.2 [The Two Flavors of Variables], page 60). 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

1 2 ... 72 73 74 75 76 77 78 79 80 81 82 ... 211 212

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Roland Ver. 4.5 Información técnica Pagina 77

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