make gets a fatal signal while a shell is executing, it may
delete the target file that the recipe was supposed to update. This is
done if the target file’s last-modification time has changed since
make first checked it.
The purpose of deleting the target is to make sure that it is remade from
make is next run. Why is this? Suppose you type
Ctrl-c while a compiler is running, and it has begun to write an
object file foo.o. The Ctrl-c kills the compiler, resulting
in an incomplete file whose last-modification time is newer than the source
file foo.c. But
make also receives the Ctrl-c signal
and deletes this incomplete file. If
make did not do this, the next
make would think that foo.o did not require
updating—resulting in a strange error message from the linker when it
tries to link an object file half of which is missing.
You can prevent the deletion of a target file in this way by making the
.PRECIOUS depend on it. Before remaking a target,
make checks to see whether it appears on the prerequisites of
.PRECIOUS, and thereby decides whether the target should be deleted
if a signal happens. Some reasons why you might do this are that the
target is updated in some atomic fashion, or exists only to record a
modification-time (its contents do not matter), or must exist at all
times to prevent other sorts of trouble.
make does its best to clean up there are certain situations
in which cleanup is impossible. For example,
make may be killed by
an uncatchable signal. Or, one of the programs make invokes may be killed
or crash, leaving behind an up-to-date but corrupt target file:
will not realize that this failure requires the target to be cleaned. Or
make itself may encounter a bug and crash.
For these reasons it’s best to write defensive recipes, which won’t leave behind corrupted targets even if they fail. Most commonly these recipes create temporary files rather than updating the target directly, then rename the temporary file to the final target name. Some compilers already behave this way, so that you don’t need to write a defensive recipe.