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

CH04.fm Page 125 Monday, October 7, 2002 8:54 PM
Working with Multiple Makefiles and Directories 125
N O T E
After defining variables, we have a rule for each target. This rule basically has two commands
on the same line. The first command, cd, is used to change the directory where source
files for that target are located. In the second command, make uses makefile in that directory to
build that target. Please note that we can also use the $(MAKE) variable for this purpose.
In the clean and install rules, we use the for loop to go into each directory and execute
some commands. The for loop is explained later in this chapter.
4.4.2 Makefile in common-dir Directory
The files in the common-dir directory are used to build a simple library that is used by
other targets at link time. The makefile in this directory is listed below:
# Variable definition
SRCS = common.c
OBJS = common.o
HDRS = common.h
LIBCOMMON = libcommon.a
INSTALLDIR = /root
CC = gcc
CFLAGS = -g -O2 -c
# Default Target
Please note that the following lines:
ftp:
@cd $(FTPDIR) ; make
are not equal to the following three lines:
ftp:
@cd $(FTPDIR)
make
In the first case, make changes the directory to $(FTPDIR)
and then executes the make command in that directory, which is
the right thing to do. However in the second case, the cd command
is executed and after that the next make command is again
executed in the current directory. The effect of the cd command is
lost when make goes to the next line to execute the next command.
This is because of a new instance of sub-shell. that executes
commands in each line as discussed earlier in this chapter.
$(LIBCOMMON): $(SRCS) $(HDRS)
$(CC) $(CFLAGS) common.c
ar -cr $(LIBCOMMON) $(OBJS)
ranlib $(LIBCOMMON)