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gnu make dependency checking with directories and symbolic links

Friday, January 11th, 2008

Note: The googlefood category is just for things that I needed to know but couldn’t find on google.

When using gnu make:

  1. file dependency checks that involve symbolic links check only the modification time of the file the symlink points to. The creation time of the symbolic link itself is irrelevant.
  2. File dependency checks that involve directories as pre-requisites will trigger if either:
    • the directory itself is newer
    • any file within the top level of the directory is newer
  3. Directories within directories or files within non-toplevel subdirectories will not trigger.

Example in a Makefile


all: slink dir

# This rule is triggered if 'filename' is newer than file 'slink'
# where 'file-symlinked-to-filename' is a symlink to 'filename'
# The creation and modification times of 'file-symlinked-to-filename' itself are ignored
slink: file-symlinked-to-filename
    @echo Rebuild slink
    touch slink

# This rule is triggered if 'directory' is newer than file 'filedir'
# 'directory' is newer if mtime(directory) > mtime(filedir)
# 'directory' is newer if mtime(directory/any-file) > mtime(filedir)
# 'directory' is NOT newer if mtime(directory/dir2) > mtime(filedir)
# 'directory' is NOT newer if mtime(directory/*/any-file) > mtime(filedir)

filedir: directory
    @echo Rebuild filedir
    touch filedir

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