error in makefile - makefile

Question

I am using gnu Make 3.82 and have an annoying problem.
I have a rule setting dependencies between directories.
OBJDIR=../obj
$(objdir)/%.o: %.C
$(COMPILE) -MM -MT$(objdir)/$(notdir $#) $< -o $(DEPDIR)/$(notdir $(basename $<).d )
$(COMPILE) -o $(objdir)/$(notdir $# ) -c $<
In order to do this, the obj directory must exist.
I want to mkdir the directory as a prerequisite
$(objdir)/%.o: %.C $(objdir)
$(COMPILE) -MM -MT$(objdir)/$(notdir $#) $< -o $(DEPDIR)/$(notdir $(basename $<).d )
$(COMPILE) -o $(objdir)/$(notdir $# ) -c $<
$(objdir):
mkdir $(objdir)
This doesn't work, because it fails when the directory is there and then the make stops
I tried shell
if [ ! -d $(objdir) ] ; then \
mkdir $(objdir) \
fi
but obviously I've got something wrong. What's the best way of doing this?

Answer 1

One simple way is to use:
mkdir -p ../obj
It doesn't fail when the directory exists.
I usually create a macro, MKPATH, for this:
MKPATH = mkdir -p
and then reference the macro in the rule:
$(objdir):
$(MKPATH) $(objdir)
That way, I can change the behaviour without changing the makefile if it becomes necessary.
Your shell fragment:
if [ ! -d $(objdir) ] ; then
mkdir $(objdir)
fi
does not work as written because make executes each line separately.
You could write (note the added semi-colon):
if [ ! -d $(objdir) ] ; then \
$(MKPATH) $(objdir) ; \
fi
Or:
if [ ! -d $(objdir) ] ; then $(MKPATH) $(objdir); fi
Or:
[ -d $(objdir) ] || $(MKPATH) $(objdir)
Note that the command line must be successful overall, so do not try:
[ ! -d $(objdir) ] && $(MKPATH) $(objdir)
If the directory exists, the first alternative fails, but the shell exits with a non-zero status, thus failing...and causing the build to fail.

Answer 2

mkdir
"mkdir -p"
Change:
$(objdir): mkdir $(objdir)
to =>
$(objdir):
mkdir -p $(objdir)
If that particular mkdir does not have -p then:
$(objdir):
test -d $(objdir) || mkdir $(objdir)
Makefiles
Keep the target: and the comands (mkdir, etc) on seperate lines.
Also, in make, to ignore failed commands, prefix command with minus:
$(objdir):
-mkdir $(objdir)
Commands (if-then-else; for loops, etc) with multiple lines require adding `\;' to represent newlines to the shell:
$(objdir):
if [ ! -d $(objdir) ] ; then \
mkdir $(objdir) ; \
fi
This particular usage of if-then-else can also written as:
$(objdir):
if [ ! -d $(objdir) ] ; then mkdir $(objdir) ; fi
The following Makefile that demonstrates each point above
all: setup dirs report
# Create an intefering dir1
# Remove dir2. It is work to be done later.
setup:
#mkdir -p dir1
#if test -d dir2 ; then rmdir dir2 ; fi
# Continue (with dir2), even though dir1 re-creation fails
dirs:
-mkdir dir1
mkdir -v dir2
# Show we're still running
report:
#echo DIRS:
#for d in dir?; do \
test -d $$d || break ; \
echo -n "$$d " ; \
done
#echo
Output from running running make:
mkdir dir1
mkdir: cannot create directory `dir1': File exists
make: [dirs] Error 1 (ignored)
mkdir -v dir2
mkdir: created directory `dir2'
DIRS:
dir1 dir2