https://github.com/hydrogen602/makefile-smart-dependencies

A demo for how to intelligently and automatically determine dependencies in makefiles
https://github.com/hydrogen602/makefile-smart-dependencies

Last synced: about 2 months ago
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A demo for how to intelligently and automatically determine dependencies in makefiles

• Host: GitHub
• URL: https://github.com/hydrogen602/makefile-smart-dependencies
• Owner: hydrogen602
• License: mit
• Created: 2022-09-16T02:34:07.000Z (over 2 years ago)
• Default Branch: main
• Last Pushed: 2022-09-16T02:56:02.000Z (over 2 years ago)
• Last Synced: 2023-02-26T08:08:11.552Z (almost 2 years ago)
• Language: Python
• Size: 7.81 KB
• Stars: 0
• Watchers: 1
• Forks: 0
• Open Issues: 0
• Metadata Files:
  • Readme: README.md
  • License: LICENSE

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README

        
# Determining Dependencies in Make rules automatically and intelligently

When using general makefile rules, often all headers need to be added as a dependency to the rule, and thus a single header file change will trigger a full rebuild - even if the header is only used in a few files. The usual way around this is to have CMAKE generate the makefile or to handwrite a rule for every file specifically, but both those solutions are painful. 

```Make

%.o: %.c $(ALL_HEADERS)

	@echo "dependencies:" $^ 

	$(CC) -c -o $@ $<

```

However, GNU makefiles have a feature that allow passing variables like `$@` and `%.c` into a shell script which can then determine the dependencies. This is called second expansion and allows make to compute dependencies for each files automatically.

```Make

.SECONDEXPANSION:

%.o: %.c $$(shell ./dependency.py %.c)

	@echo "dependencies:" $^ 

	$(CC) -c -o $@ $<

```

`dependency.py` is just a simple script that uses regex to recursively search for includes using quotes (`"`) in each `%.c` file.

Makefile output

```

dependencies: main.c main.h extra.h base.h other.h

gcc -c -o main.o main.c

dependencies: other.c other.h

gcc -c -o other.o other.c

gcc main.o other.o -o main

```

Notice that the dependencies are only those used by the specific C source file.

## Notes

Secondary expansion is a GNU make specific feature. While the example is for C files, this can easily be extending for C++ or anything else where the dependencies can be computed from the source file.