Datasheet

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Exploring the Software-Development Tools in Linux
Suppose that you have a C++ source file named form.C that contains the
following preprocessor directive:
#include “form.h” // Include header file
The object file form.o clearly depends on the source file form.C and the
header file form.h. In addition to these dependencies, you must specify
how make converts the form.C file to the object file form.o. Suppose that
you want make to invoke g++ (because the source file is in C++) with these
options:
✦ -c (compile only)
✦ -g (generate debugging information)
✦ -O2 (optimize some)
In the makefile, you can express these options with the following rule:
# This a comment in the makefile
# The following lines indicate how form.o depends
# on form.C and form.h and how to create form.o.
form.o: form.C form.h
g++ -c -g -O2 form.C
In this example, the first noncomment line shows form.o as the target and
form.C and form.h as the dependent files.
The line following the dependency indicates how to build the target from its
dependents. This line must start with a tab. Otherwise, the make command
exits with an error message, and you’re left scratching your head because
when you look at the makefile in a text editor, you can’t tell the differ-
ence between a tab and a space. Now that you know the secret, the fix is to
replace the spaces at the beginning of the offending line with a single tab.
The benefit of using make is that it prevents unnecessary compilations. After
all, you can run g++ (or gcc) from a shell script to compile and link all the
files that make up your application, but the shell script compiles everything,
even if the compilations are unnecessary. GNU make, on the other hand,
builds a target only if one or more of its dependents have changed since the
last time the target was built. make verifies this change by examining the
time of the last modification of the target and the dependents.
make treats the target as the name of a goal to be achieved; the target
doesn’t have to be a file. You can have a rule such as this one:
clean:
rm -f *.o
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