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When to Use (and Not Use) --old-file=file in Your Makefile


  • Ignores File Changes
    Any changes made to the file itself are ignored by Make during this build process.
  • Forces No Rebuild
    Make won't attempt to remake the specified file even if its modification time suggests it's out of date.

Essentially, it tells Make to trust that the file is up-to-date and avoid rebuilding it or anything that depends on it.

Here are some key points to remember about --old-file:

  • Use Case
    This option is helpful when you know for certain that the specified file hasn't changed and rebuilding it would be unnecessary. It can save time during builds, especially for large files.

Imagine you have a Makefile with a rule to compile your program based on a configuration file (config.txt). You know that config.txt rarely changes, and rebuilding the program every time is time-consuming.

Makefile

program: source.cpp config.txt
  g++ source.cpp -o program $(LDFLAGS) $(CFLAGS)

clean:
  rm -f program

Scenario

  • You don't want to rebuild if config.txt hasn't changed (since it rarely does).
  • You modify source.cpp and need to recompile the program.

Running Make

make --old-file=config.txt

This command tells Make to treat config.txt as very old, even if its modification time is newer than source.cpp. As a result, Make will only recompile the program based on the changes in source.cpp and avoid unnecessary processing of config.txt.

Caution
This example demonstrates a potential misuse of --old-file. It's generally not recommended unless you fully understand the implications.

Let's say your Makefile generates a temporary file during the build process (temp.txt). You know this file is always overwritten during the build and doesn't need to be considered for rebuilding other targets.

Makefile (Not Recommended)

build: source.cpp
  g++ source.cpp -o program $(LDFLAGS) $(CFLAGS) > temp.txt

clean:
  rm -f program temp.txt

make --old-file=temp.txt

Leverage Timestamp Independence

  • For example, if a script generates a configuration file (config.txt) during the build process, the Makefile rule for compiling your program could depend on the script itself instead of config.txt. This ensures the program rebuilds whenever the script or its logic changes, updating the configuration accordingly.
  • If your target doesn't inherently rely on the modification time of a specific file, consider adjusting your Makefile rules. This eliminates the need to manipulate Make's behavior with options like --old-file.

Use Phony Targets (For Manual Intervention)

  • This approach allows you to explicitly control when the configuration file is regenerated without relying on file timestamps.
  • You could create a phony target named "rebuild-config" that triggers the script to regenerate config.txt. Then, your program's build rule depends on both the source code and the "rebuild-config" target.
  • Phony targets are special targets in Make that don't correspond to actual files. They are useful for defining manual build steps or tasks.

Leverage Build System Features (For Complex Scenarios)

  • If you're dealing with more complex build scenarios, consider using features offered by advanced build systems like CMake or Apache Buildr. These tools can handle dependencies and configuration management more effectively, potentially eliminating the need for options like --old-file.
  • In specific situations (use with caution!), you can use the touch command within your Makefile to manipulate timestamps. However, this approach can be fragile and lead to unexpected behavior if not used carefully. It's generally better to rely on the logic of your build process rather than timestamp manipulation.