12.18.4 Automatic Rule Rewriting

Some make implementations, such as Solaris and Tru64, search for prerequisites in VPATH and then rewrite each occurrence as a plain word in the rule. For instance:

# This isn't portable to GNU make.
VPATH = ../pkg/src
f.c: if.c
        cp if.c f.c

executes cp ../pkg/src/if.c f.c if if.c is found in ../pkg/src.

However, this rule leads to real problems in practice. For example, if the source directory contains an ordinary file named test that is used in a dependency, Solaris make rewrites commands like ‘if test -r foo; …’ to ‘if ../pkg/src/test -r foo; …’, which is typically undesirable. In fact, make is completely unaware of shell syntax used in the rules, so the VPATH rewrite can potentially apply to any whitespace-separated word in a rule, including shell variables, functions, and keywords.

$ mkdir build
$ cd build
$ cat > Makefile <<'END'
VPATH = ..
all: arg func for echo
        func () { for arg in "$$@"; do echo $$arg; done; }; \
        func "hello world"
END
$ touch ../arg ../func ../for ../echo
$ make
../func () { ../for ../arg in "$@"; do ../echo $arg; done; }; \
../func "hello world"
sh: syntax error at line 1: `do' unexpected
*** Error code 2

To avoid this problem, portable makefiles should never mention a source file or dependency whose name is that of a shell keyword like for or until, a shell command like cat or gcc or test, or a shell function or variable used in the corresponding Makefile recipe.

Because of these problems GNU make and many other make implementations do not rewrite commands, so portable makefiles should search VPATH manually. It is tempting to write this:

# This isn't portable to Solaris make.
VPATH = ../pkg/src
f.c: if.c
        cp `test -f if.c || echo $(VPATH)/`if.c f.c

However, the “prerequisite rewriting” still applies here. So if if.c is in ../pkg/src, Solaris and Tru64 make execute

cp `test -f ../pkg/src/if.c || echo ../pkg/src/`if.c f.c

which reduces to

cp if.c f.c

and thus fails. Oops.

A simple workaround, and good practice anyway, is to use ‘$?’ and ‘$@’ when possible:

VPATH = ../pkg/src
f.c: if.c
        cp $? $@

but this does not generalize well to commands with multiple prerequisites. A more general workaround is to rewrite the rule so that the prerequisite if.c never appears as a plain word. For example, these three rules would be safe, assuming if.c is in ../pkg/src and the other files are in the working directory:

VPATH = ../pkg/src
f.c: if.c f1.c
        cat `test -f ./if.c || echo $(VPATH)/`if.c f1.c >$@
g.c: if.c g1.c
        cat `test -f 'if.c' || echo $(VPATH)/`if.c g1.c >$@
h.c: if.c h1.c
        cat `test -f "if.c" || echo $(VPATH)/`if.c h1.c >$@

Things get worse when your prerequisites are in a macro.

VPATH = ../pkg/src
HEADERS = f.h g.h h.h
install-HEADERS: $(HEADERS)
        for i in $(HEADERS); do \
          $(INSTALL) -m 644 \
            `test -f $$i || echo $(VPATH)/`$$i \
            $(DESTDIR)$(includedir)/$$i; \
        done

The above install-HEADERS rule is not Solaris-proof because for i in $(HEADERS); is expanded to for i in f.h g.h h.h; where f.h and g.h are plain words and are hence subject to VPATH adjustments.

If the three files are in ../pkg/src, the rule is run as:

for i in ../pkg/src/f.h ../pkg/src/g.h h.h; do \
  install -m 644 \
     `test -f $i || echo ../pkg/src/`$i \
     /usr/local/include/$i; \
done

where the two first install calls fail. For instance, consider the f.h installation:

install -m 644 \
  `test -f ../pkg/src/f.h || \
    echo ../pkg/src/ \
  `../pkg/src/f.h \
  /usr/local/include/../pkg/src/f.h;

It reduces to:

install -m 644 \
  ../pkg/src/f.h \
  /usr/local/include/../pkg/src/f.h;

Note that the manual VPATH search did not cause any problems here; however this command installs f.h in an incorrect directory.

Trying to quote $(HEADERS) in some way, as we did for foo.c a few makefiles ago, does not help:

install-HEADERS: $(HEADERS)
        headers='$(HEADERS)'; \
        for i in $$headers; do \
          $(INSTALL) -m 644 \
            `test -f $$i || echo $(VPATH)/`$$i \
            $(DESTDIR)$(includedir)/$$i; \
        done

Now, headers='$(HEADERS)' macro-expands to:

headers='f.h g.h h.h'

but g.h is still a plain word. (As an aside, the idiom headers='$(HEADERS)'; for i in $$headers; is a good idea if $(HEADERS) can be empty, because some shells diagnose a syntax error on for i in;.)

One workaround is to strip this unwanted ../pkg/src/ prefix manually:

VPATH = ../pkg/src
HEADERS = f.h g.h h.h
install-HEADERS: $(HEADERS)
        headers='$(HEADERS)'; \
        for i in $$headers; do \
          i=`expr "$$i" : '$(VPATH)/\(.*\)'`;
          $(INSTALL) -m 644 \
            `test -f $$i || echo $(VPATH)/`$$i \
            $(DESTDIR)$(includedir)/$$i; \
        done

Automake does something similar. However the above hack works only if the files listed in HEADERS are in the current directory or a subdirectory; they should not be in an enclosing directory. If we had HEADERS = ../f.h, the above fragment would fail in a VPATH build with Tru64 make. The reason is that not only does Tru64 make rewrite dependencies, but it also simplifies them. Hence ../f.h becomes ../pkg/f.h instead of ../pkg/src/../f.h. This obviously defeats any attempt to strip a leading ../pkg/src/ component.

The following example makes the behavior of Tru64 make more apparent.

$ cat Makefile
VPATH = sub
all: ../foo
        echo ../foo
$ ls
Makefile foo
$ make
echo foo
foo

Dependency ../foo was found in sub/../foo, but Tru64 make simplified it as foo. (Note that the sub/ directory does not even exist, this just means that the simplification occurred before the file was checked for.)

For the record here is how SunOS 4 make behaves on this example.

$ make
make: Fatal error: Don't know how to make target `../foo'
$ mkdir sub
$ make
echo sub/../foo
sub/../foo