GNU `make'

Node:Wildcard Pitfall, Next:Wildcard Function, Previous:Wildcard Examples, Up:Wildcards

#### 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. 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.