Re: Bug or Feature? make follows symbolic link for current directory

[Kaz Kylheku]

On 2023-07-07 09:52, Christian Hujer wrote:
> Hello folks,
> 
> Is this a bug or a feature?
> 
> mkdir foo
> ln -s foo bar
> cd bar
> echo -e 'all:\n\tpwd' >Makefile
> make
> 
> I would have expected the output to be …/bar but it actually is …/foo.

When you chdir() to a directory through a symlink, that information is
lost. The process hangs on to the inode (or other low-level pointer-like
thing) of the directory object.

The pwd command most likely uses the getcwd() function calculate
the absolute path to the current directory. The getcwd() function
somehow figures out the absolute path of that directory, without
caring how the process ended up in that directory.

Make itself uses getcwd() in order to print the "Entering directory"
and "Leaving directory" messages when you use "make -w".

Now there is a related convention in this area. The environment variable
PWD, which is maintained by the shell, tracks the abstract path followed
by cd commands.

GNU Make doesn't make use of the PWD environment variable.

We can simulate that ourselves, look:

  $ make
  make: Entering directory '/home/kaz/junk/make-symlink/bar'
  /home/kaz/junk/make-symlink/bar
  make: Leaving directory '/home/kaz/junk/make-symlink/bar'

The code:

  .PHONY: prolog all epilog

  epilog: all

  all: prolog

  all:
        @printf "%s\n" "$$PWD"

  prolog:
        @printf "make: Entering directory '%s'\n" "$$PWD"

  epilog:
        @printf "make: Leaving directory '%s'\n" "$$PWD"

We implemented our own Entering/Leaving messages which look like
those of GNU make, and replaced the pwd command with one
that prints the contents of PWD instead.

There are caveats with PWD, such as if the directory is changed
by some program that doesn't update PWD, it will have incorrect
contents.