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102 GNU make

modiﬁcation times remain the same. You can use the ‘-W’ ﬂag in conjunction

with the ‘-n’ ﬂag to see what would happen if you were to modify speciﬁc ﬁles.

With the ‘-n’ ﬂag, make prints the recipe that it would normally execute but usually

does not execute it.

With the ‘-t’ ﬂag, make ignores the recipes in the rules and uses (in eﬀect) the command

touch for each target that needs to be remade. The touch command is also printed, unless

‘-s’ or .SILENT is used. For speed, make does not actually invoke the program touch. It

does the work directly.

With the ‘-q’ ﬂag, make prints nothing and executes no recipes, but the exit status code

it returns is zero if and only if the targets to be considered are already up to date. If the

exit status is one, then some updating needs to be done. If make encounters an error, the

exit status is two, so you can distinguish an error from a target that is not up to date.

It is an error to use more than one of these three ﬂags in the same invocation of make.

The ‘-n’, ‘-t’, and ‘-q’ options do not aﬀect recipe lines that begin with ‘+’ characters

or contain the strings ‘$(MAKE)’ or ‘${MAKE}’. Note that only the line containing the ‘+’

character or the strings ‘$(MAKE)’ or ‘${MAKE}’ is run regardless of these options. Other

lines in the same rule are not run unless they too begin with ‘+’ or contain ‘$(MAKE)’ or

‘${MAKE}’ (See

Section 5.7.1 [How the MAKE Variable Works], page 51.)

The ‘-t’ ﬂag prevents phony targets (see Section 4.5 [Phony Targets], page 29) from

being updated, unless there are recipe lines beginning with ‘+’ or containing ‘$(MAKE)’ or

‘${MAKE}’.

The ‘-W’ ﬂag provides two features:

• If you also use the ‘-n’ or ‘-q’ ﬂag, you can see what make would do if you were to

modify some ﬁles.

• Without the ‘-n’ or ‘-q’ ﬂag, when make is actually executing recipes, the ‘-W’ ﬂag

can direct make to act as if some ﬁles had been modiﬁed, without actually running the

recipes for those ﬁles.

Note that the options ‘-p’ and ‘-v’ allow you to obtain other information about make or

about the makeﬁles in use (see Section 9.7 [Summary of Options], page 104).

9.4 Avoiding Recompilation of Some Files

Sometimes you may have changed a source ﬁle but you do not want to recompile all the

ﬁles that depend on it. For example, suppose you add a macro or a declaration to a header

ﬁle that many other ﬁles depend on. Being conservative, make assumes that any change

in the header ﬁle requires recompilation of all dependent ﬁles, but you know that they do

not need to be recompiled and you would rather not waste the time waiting for them to

compile.

If you anticipate the problem before changing the header ﬁle, you can use the ‘-t’ ﬂag.

This ﬂag tells make not to run the recipes in the rules, but rather to mark the target up to

date by changing its last-modiﬁcation date. You would follow this procedure:

1. Use the command ‘make’ to recompile the source ﬁles that really need recompilation,

ensuring that the object ﬁles are up-to-date before you begin.

2. Make the changes in the header ﬁles.

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