A Simple Example

Suppose you are faced with creating a Makefile that needs to deal with two files named foo bar and bar baz, with foo bar built from bar ba'.  I've used italics to make clear that these are file names that include spaces.

A naive way to write this in a Makefile would be:

foo bar: bar baz

    @echo Making $@ from $<

That doesn't work at all.  GNU Make can't differentiate between the cases where the spaces there are part of, or not part of, the file names.  In fact the Makefile above is exactly
the same as:

foo: bar baz

    @echo Making $@ from $<

bar: bar baz

    @echo Making $@ from $<

Placing quotations marks around the file names doesn't work.  If you try to do:

"foo bar": "bar baz"

    @echo Making $@ from $<

then GNU Make thinks you are talking about four files called "foo, bar", "bar and baz". 
GNU Make has totally ignored the double-quotes and split the list of file names on spaces.

One way: escape the spaces with \\

GNU Make does have an escaping operator, \\ which can be used to escape sensitive
characters (such as a literal # that musn't start a comment, or a literal % in a pattern
that shouldn't be the wildcard character). 

In a file name in a rule \\ can be used to escape a space.  The example Makefile can be
rewritten as follows:

foo\\ bar: bar\\ baz

    @echo Making $@ from $<

and it will work correctly.  The \\ is removed during the parsing of the Makefile, but the
actual target and prerequisite names correctly contain spaces.   This will be reflected in
the automatic variables (such as $@). 

When foo bar needs updating the simple Makefile will output:

Making foo bar from bar baz

The same escaping mechanism can also be used inside GNU Make's $(wildcard) function.   To
check for the existence of foo bar you can do $(wildcard foo\\ bar) and GNU Make will
treat foo bar as a single file name to look for in the file system.

Unfortunately, GNU Make's other functions that deal with space-separated list do not
respect the escaping of the space.  So, for example the output of $(sort foo\\ bar) is the
list bar foo\\ and not foo\\ bar as you might expect.

The following table shows the GNU Make functions that accept a list as an argument and
whether they respect escaping of spaces.

Function	Respect \\
addprefix	No
addsuffix	No
basename	No
dir	No
filter	No
filter-out	No
firstword	No
foreach	No
join	No
notdir	No
patsubst	No
sort	No
suffix	No
word	No
wordlist	No
words	No
wildcard	Yes

So, $(wildcard) is the lone hold out.

This leads to a problem if you have to deal with the automatic variables that contain
lists of targets.  Consider this slightly more complicated example Makefile:

foo\\ bar: bar\\ baz a\\ b

    @echo Making $@ from $<

Now foo bar has two prerequisites bar baz and a b.  What's the value of $^ (the list of
all prerequisites) in this case?  It's bar baz a b: the escaping is gone, and even if it
weren't gone the table above shows that it would be pretty much useless anyway.  $^ is,
from GNU Make's perspective, a list with four elements.

A quick look at the definitions of the automatic variables tells use which are safe to use
in the presence of spaces in file names:

Automatic Variable	Safe?
$@	Yes
$<	Yes
$%	Yes
$*	Yes
$?	No
$^	No
$+	No

And it gets a little worse... even though the first four automatic variables there are

safe to use, the modified versions with D and F suffixes (that extract the directory and
file name portions of the corresponding automatic variable) are not.   That's because they
are defined in terms of the dir and notdir functions.

Consider this example Makefile:

/tmp/foo\\ bar/baz: bar\\ baz a\\ b

    @echo Making $@ from $<

The value of $@ is /tmp/foo bar/baz as expected, but the value of $(@D) is /tmp bar (/tmp/foo bar would be expected) and the value of $(@F) is foo file (instead of file).

Another way: turn spaces into ?

In "Managing Projects with GNU Make, 3rd Edition" by Mecklenburg there's a different
suggestion: turn spaces into question marks.  Here's the original simple Makefile
transformed that way:

foo?bar: bar?baz

    @echo Making $@ from $<

Since GNU Make does globbing of target and prerequisite names (and respects any spaces found) this will work.  But the results are rather inconsistent.

If foo bar exists when this Makefile runs then the pattern foo?bar will get turned into
foo bar and that value will be used for $@.   If that file were missing when the Makefile
is parsed the pattern (and hence $@) stay as foo?bar.

(Of course, there's a another problem: that ? could match something other than a space: if
there's a file called foombar on the system the Makefile may end up working on the wrong
file).

To get around this problem Mecklenburg defines two functions to add and remove spaces
automatically.  Here's the updated Makefile using two functions (sq and qs) to add and
remove question marks.

sp :=

sp +=

qs = $(subst ?,$(sp),$1)

sq = $(subst $(sp),?,$1)

$(call sq,foo bar): $(call sq,bar baz)

    @echo Making $(call qs,$@) from $(call qs,$<)

Since we can't be sure whether the automatic variables will or will not have question
marks in them (since that depends on the state of the file system when the Makefile is
parsed), it's still impossible to use the list-based automatic variables or any GNU Make
functions that handle lists.

(Aside, the first two lines above end up putting a single space in the macro sp, it's
needed for the definition of sq since the first argument to the $(subst) needs to be a
space.)

Frankly, I think Mecklenburg's approach causes more trouble than it's worth: it doesn't
give consistent results, the values of $@ and other automatic variable have to be wrapped
in a call to a function, the ? could end up matching a non-space and all targets and
prerequisites need to be wrapped up with a $(call sq).

My advice

So given that GNU Make clearly has a really hard problem with spaces in file names what
can be done.  Here's my advice:

• Don't.  If you can rename your files to not include spaces, that's going to be the
  simplest path.  I realize that for many people this is impossible, those spaces in
  file names may have been put there by a third-party.

 

• Use 8.3 filenames.  If you are on Windows it may be possible to use short, 8.3
  file names throughout.   This will allow you to still have spaces on disk, but avoid
  them in the Makefile.

 

• Use the \\ escaping.  If you still need the spaces, then escape than with \\. 
  This does give consistent results; you'll have to avoid the automatic variables
  mentioned above.

If you do number 3 and yet you need to manipulate lists of file names that contain spaces,
then the best thing to do is transform them by substituting space with some other
character and then change them back again.

For example, the s+ and +s functions here change escaped spaces to + signs and back
again.  You can then safely manipulate lists of file names using all the GNU Make
functions.  Just be sure to remove the +'s before using these names in a rule.

s+ = $(subst \\ ,+,$1)

+s = $(subst +,\\ ,$1)

Here's a fuller example where a list of source files that contain escaped spaces is
transformed into a list of object files and then the object files are used to define the
prerequisites of the all rule.

SRCS := a\\ b.c c\\ d.c e\\ f.c

SRCS := $(call s+,$(SRCS))

OBJS := $(SRCS:.c=.o)

all: $(call +s,$(OBJS))