How to include path prefix in GNU Make pattern rule

Question

Consider the following:

%.foo: %.bar
    echo $< > $@ 

Assuming we have one file 1.bar, the command executed is simply echo 1.bar > 1.foo. However, when % contains a path, rather than just a file name, it start becoming finicky. My problem is that I want to prepend another path to %.bar, the pattern becomes completely mangled. I.e., when %.bar is nice/path/1.bar, this becomes impossible:

%.foo: /project/something/%.bar
    echo $< > $@

This will run, but it executes echo nice/path//project/something/1.bar > 1.foo in stead of echo /project/something/nice/path1.bar > 1.foo

The reason for this is in how make does its pattern rules. From the docs:

When the target pattern does not contain a slash (and it usually does not), directory names in the file names are removed from the file name before it is compared with the target prefix and suffix. [...] The directories are ignored only for the purpose of finding an implicit rule to use, not in the application of that rule. Thus, ‘e%t’ matches the file name src/eat, with ‘src/a’ as the stem. When prerequisites are turned into file names, the directories from the stem are added at the front, while the rest of the stem is substituted for the ‘%’. The stem ‘src/a’ with a prerequisite pattern ‘c%r’ gives the file name src/car

Is there any way I can turn this off for a specific rule?

Answer 1

You may like to read up on How Patterns Match:

When the target pattern does not contain a slash (and it usually does not), directory names in the file names are removed from the file name before it is compared with the target prefix and suffix. After the comparison of the file name to the target pattern, the directory names, along with the slash that ends them, are added on to the prerequisite file names generated from the pattern rule’s prerequisite patterns and the file name. The directories are ignored only for the purpose of finding an implicit rule to use, not in the application of that rule. Thus, ‘e%t’ matches the file name src/eat, with ‘src/a’ as the stem. When prerequisites are turned into file names, the directories from the stem are added at the front, while the rest of the stem is substituted for the ‘%’. The stem ‘src/a’ with a prerequisite pattern ‘c%r’ gives the file name src/car.

The above explains why nice/path/ is prepended to prerequisite /project/something/1.bar.

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One fix would be to use full file names in rules, e.g.:

${CURDIR}/nice/path/%.foo: /project/something/%.bar