File: make.info,  Node: Wildcard Pitfall,  Next: Wildcard Function,  Prev: Wildcard Examples,  Up: Wildcards

4.4.2 Pitfalls of Using Wildcards
---------------------------------

Now here is an example of a naive way of using wildcard expansion, that
does not do what you would intend.  Suppose you would like to say that
the executable file 'foo' is made from all the object files in the
directory, and you write this:

     objects = *.o

     foo : $(objects)
             cc -o foo $(CFLAGS) $(objects)

The value of 'objects' is the actual string '*.o'.  Wildcard expansion
happens in the rule for 'foo', so that each _existing_ '.o' file becomes
a prerequisite of 'foo' and will be recompiled if necessary.

   But what if you delete all the '.o' files?  When a wildcard matches
no files, it is left as it is, so then 'foo' will depend on the
oddly-named file '*.o'.  Since no such file is likely to exist, 'make'
will give you an error saying it cannot figure out how to make '*.o'.
This is not what you want!

   Actually it is possible to obtain the desired result with wildcard
expansion, but you need more sophisticated techniques, including the
'wildcard' function and string substitution.  *Note The Function
'wildcard': Wildcard Function.

   Microsoft operating systems (MS-DOS and MS-Windows) use backslashes
to separate directories in pathnames, like so:

       c:\foo\bar\baz.c

   This is equivalent to the Unix-style 'c:/foo/bar/baz.c' (the 'c:'
part is the so-called drive letter).  When 'make' runs on these systems,
it supports backslashes as well as the Unix-style forward slashes in
pathnames.  However, this support does _not_ include the wildcard
expansion, where backslash is a quote character.  Therefore, you _must_
use Unix-style slashes in these cases.