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

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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.
CH04.fm Page 133 Monday, October 7, 2002 8:54 PM Getting the Latest Version and Installation 133 4.6.4 The ifndef Directive The ifndef directive is similar to the ifdef directive. However selection of a block of commands is reversed as compared to ifdef directive. Its format is as follows: ifndef variable block if variable is empty else block if variable is not empty endif 4.6.5 The for Control Structure You have already used the for directive in the example of using make with multiple directories. This directive is used to perform repeated operation on multiple items. The following rule in the makefile is used to go into multiple directories and remove object files in each of these directories: SUBDIRS = ftp-dir tftp-dir common-dir clean: for dir in $(SUBDIRS) ; do \ ( cd $$dir ; rm *.o) ; \ done Each time a new value is assigned to variable dir from the values listed by the SUBDIRS variable until all of the values are used. 4.7 Getting the Latest Version and Installation Almost all of the Linux distributions come with make. The Red Hat 7.1 distribution that we have used for this book already has the latest version of make. However if you want to get the latest version of GNU make, it is available at the following FTP site. ftp://ftp.gnu.org/pub/gnu/make/ This FTP site has multiple versions of the make utility. Download the latest version, which will be a zipped tar file. The latest version at the time of writing this book is 3.79.1 and the filename is make-3.79.1.tar.gz. 4.7.1 Compilation After downloading this file, just un-tar it into a directory using the following command: tar zxvf make-3.79.1.tar.gz The source files will be extracted into directory make-3.79.1. Change to this directory and run the configure script. It will create the makefiles appropriate for you. You should have some previous version of make in order to build the latest version. Use the make command; the default rule contains information about how to build make. Use the following com-
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