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22 GNU make

The criterion for being out of date is speciﬁed in terms of the prerequisites, which

consist of ﬁle names separated by spaces. (Wildcards and archive members (see Chapter 11

[Archives], page 129) are allowed here too.) A target is out of date if it does not exist or if it

is older than any of the prerequisites (by comparison of last-modiﬁcation times). The idea is

that the contents of the target ﬁle are computed based on information in the prerequisites,

so if any of the prerequisites changes, the contents of the existing target ﬁle are no longer

necessarily valid.

How to update is speciﬁed by a recipe. This is one or more lines to be executed by

the shell (normally ‘sh’), but with some extra features (see Chapter 5 [Writing Recipes in

Rules], page 41).

4.2 Types of Prerequisites

There are actually two diﬀerent types of prerequisites understood by GNU make: normal

prerequisites such as described in the previous section, and order-only prerequisites. A

normal prerequisite makes two statements: ﬁrst, it imposes an order in which recipes will

be invoked: the recipes for all prerequisites of a target will be completed before the recipe

for the target is run. Second, it imposes a dependency relationship: if any prerequisite is

newer than the target, then the target is considered out-of-date and must be rebuilt.

Normally, this is exactly what you want: if a target’s prerequisite is updated, then the

target should also be updated.

Occasionally, however, you have a situation where you want to impose a speciﬁc ordering

on the rules to be invoked without forcing the target to be updated if one of those rules is

executed. In that case, you want to deﬁne order-only prerequisites. Order-only prerequisites

can be speciﬁed by placing a pipe symbol (|) in the prerequisites list: any prerequisites to

the left of the pipe symbol are normal; any prerequisites to the right are order-only:

targets : normal-prerequisites | order-only-prerequisites

The normal prerequisites section may of course be empty. Also, you may still declare

multiple lines of prerequisites for the same target: they are appended appropriately (normal

prerequisites are appended to the list of normal prerequisites; order-only prerequisites are

appended to the list of order-only prerequisites). Note that if you declare the same ﬁle to

be both a normal and an order-only prerequisite, the normal prerequisite takes precedence

(since they have a strict superset of the behavior of an order-only prerequisite).

Consider an example where your targets are to be placed in a separate directory, and that

directory might not exist before make is run. In this situation, you want the directory to

be created before any targets are placed into it but, because the timestamps on directories

change whenever a ﬁle is added, removed, or renamed, we certainly don’t want to rebuild

all the targets whenever the directory’s timestamp changes. One way to manage this is with

order-only prerequisites: make the directory an order-only prerequisite on all the targets:

OBJDIR := objdir

OBJS := $(addprefix $(OBJDIR)/,foo.o bar.o baz.o)

$(OBJDIR)/%.o : %.c

$(COMPILE.c) $(OUTPUT_OPTION) $<

all: $(OBJS)

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