What causes Make to delete intermediate files?

Question

I wrote a Makefile and found out that when executing make command, an unexpected rm was executed, after all command in Mmakefile were done. But I didn't write the rm command in the Makefile.

run-%: d/%.out
    $<

d/%.out: d/%.c
    gcc -o $(subst .c,.out,$<) $<

Output of running make run-a:

gcc -o d/a.out d/a.c
d/a.out
rm d/a.out

Notice the trailing rm d/a.out, which I didn't write.

Under what circumstance will the automatic rm command be added?

Answer 1

When you did make run-a, the file d/a.out was created by an intermediate target (a dependency of run-a) which, when the main target (run-a) has been built and that intermediate target's result is no longer needed, is removed.

You may prevent the removal of d/a.out by declaring it as "precious":

.PRECIOUS: d/%.out

See also "Chains of Implicit Rules" in the GNU Make manual:

Intermediate files are remade using their rules just like all other files. But intermediate files are treated differently in two ways.

The first difference is [...]

The second difference is that if make does create b in order to update something else, it deletes b later on after it is no longer needed. Therefore, an intermediate file which did not exist before make also does not exist after make. make reports the deletion to you by printing a rm -f command showing which file it is deleting.