How to change current directory in GNU Make

9
votes

I want to separate the directory with sources from the directory with targets. And it seems that changing the current working directory from Makefile should be the simplest solution.

Explicit path to targets is not sufficient because of the following drawbacks:

  1. Redundant code in Makefile since every reference to target should be prefixed with variable.
  2. More complex command line to build particular intermediate target (worse for debugging).

See also Pauls's rule #3:

Life is simplest if the targets are built in the current working directory.

Regarding VPATH — I also agree that requiring developers "to change to the target directory before running make is a pain".

4
makefilegnu-makeguile

4 Answers

8
votes

Building targets in a separate directory is a commonplace make practice that GNU make conveniently supports without changing directory or invoking auxiliary tools. Here is a routine illustration:

Makefile

srcs := main.c foo.c
blddir := bld
objs := $(addprefix $(blddir)/,$(srcs:.c=.o))
exe := $(blddir)/prog

.PHONY: all clean

all: $(exe)

$(blddir):
    mkdir -p $@

$(blddir)/%.o: %.c | $(blddir)
    $(CC) $(CFLAGS) $(CPPFLAGS) -c -o $@ $<

$(exe) : $(objs)
    $(CC) -o $@ $^ $(LDFLAGS) $(LDLIBS)

clean:
    rm -fr $(blddir)

which runs like:

$ make
mkdir -p bld
cc   -c -o bld/main.o main.c
cc   -c -o bld/foo.o foo.c
cc -o bld/prog bld/main.o bld/foo.o

Cribs:-

There can be powerful reasons to make make change its working directory but merely putting build products in a separate directory isn't one.

6
votes

In the GNU Make program I am using built for mingw64 (windows),

GNU Make 4.2.1 Built for x86_64-w64-mingw32

I am able to use this target with this command,

debug:
    cd $(PREFIX) && $(GDB) kiwigb.exe

The results are the directory change is temporary, but all works.

3
votes

Known methods overview

The excellent research of various methods how to separate the source and target directories was made by Paul D. Smith in "Multi-Architecture Builds" paper. The following methods are described (with their drawbacks):

  • Source copy
  • Explicit path (reference to every target is prefixed with the pathname)
  • VPATH (invoke build from the target directory)
  • Advanced VPATH (auto recursive invocation)

Yet another method

However I found the simpler solution — with smaller boilerplate and without recursive invocation of make. In case of GNU Make with Guile support we can just use Guile chdir function to change the current working directory from Makefile. Also we can create directory via mkdir before that.

data ?= ./data/

# Create $(data) directory if it is not exist (just for example)
$(guile (if (not (access? "$(data)" F_OK)) (mkdir "$(data)") ))

# Set the new correct value of CURDIR (before changing directory)
CURDIR := $(abspath $(data))

# Change the current directory to $(data)
$(guile (chdir "$(data)"))

# Another way of updating CURDIR
#  — via sub-shell call after changing directory
# CURDIR := $(shell pwd)


# Don't try to recreate Makefile file
# that is disappeared now from the current directory
Makefile : ;

$(info     CURDIR = $(CURDIR)     )
$(info        PWD = $(shell pwd)  )

Final boilerplate to change the current directory

The assumptions: data variable is available in the context and the parent of $(data) directory is accessible, the path can be relative.

srcdir := $(realpath $(dir $(lastword $(MAKEFILE_LIST))))
ifeq (,$(filter guile,$(.FEATURES)))
  $(warning Guile is required to change the current directory.)
  $(error Your Make version $(MAKE_VERSION) is not built with support for Guile)
endif
$(MAKEFILE_LIST): ;
$(guile (if (not (file-exists? "$(data)")) (mkdir "$(data)") ))
ORIGCURDIR  := $(CURDIR)
CURDIR      := $(realpath $(data))
$(guile (chdir "$(data)"))
ifneq ($(CURDIR),$(realpath .))
  $(error Cannot change the current directory)
endif
$(warning CURDIR is changed to "$(data)")

Remember that relative path in include directive is calculated from the current directory by default, hence it depends on the location — is it used before this boilerplate or after.

NB: $(data) should not be used in the rules; $(srcdir) can be used to specify a file relative to this Makefile file location.

Found issues

This method was tested in GNU Make 4.0 and 4.2.1

One minor issue was observed. abspath function works incorrectly after changing the current directory — it continues resolving relative paths according to the old CURDIR; realpath works correctly.

Also this method may have other yet unknown drawbacks.

2
votes

The correct way is to open a man page:

man make

and search for -C by inputting /-C and pressing Enter once. You will find something like this:

   -C dir, --directory=dir
        Change  to  directory dir before reading the makefiles or doing
        anything else.  If multiple -C options are specified,  each  is
        interpreted relative to the previous one: -C / -C etc is equiv‐
        alent to -C /etc.  This is typically used with recursive  invo‐
        cations of make.

So you can use it:

make -C <desired directory> ...