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.
See The Function wildcard.
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.