The Linux Development Platform Configuring, Using, and ... - Classes

CH04.fm Page 132 Monday, October 7, 2002 8:54 PM
132 Chapter 4 • Using GNU make
4.6.1 The ifeq Directive
The ifeq directive is used to compare two values and make a decision based upon the
result. The general format of this directive is as follows:
ifeq (value1, value2)
block if value 1 is equal to value2
else
block if value 1 is not equal to value2
endif
The else part is optional and may not be present. This structure is useful if you want to
make a decision on the basis of some criteria. For example, based upon the value of type of build
(temporary or final) you want to use different levels of optimization. The following code in a
makefile does that.
ifeq ($(BUILD), final)
$(CC) -c -O2 ftp.c
else
$(CC) -c -O1 ftp.c
endif
Please note that there is no TAB character before ifeq, else and endif words.
4.6.2 The ifneq Directive
The ifneq directive is similar to ifeq directive. The only difference is that sense of
equality is reversed. The general syntax is as follows:
ifneq (value1, value2)
block if value 1 is not equal to value2
else
block if value 1 is equal to value2
endif
4.6.3 The ifdef Directive
The ifdef directive checks if the value of the variable is empty or not. If the variable is
not empty, the ifdef block is executed, otherwise the else part is executed. The general structure
of the directive is as follows:
ifdef variable
block if variable is non-empty
else
block if variable is empty
endif
This directive is useful to verify if a variable is defined or not. The else part is optional.