FoBiS.py

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Flag Heritage ​

By default, FoBiS.py uses file timestamps to decide whether to recompile an object. This works well when source files change, but if you only change compiler flags (e.g. switching from -O0 to -O3) the timestamps are unchanged and nothing is recompiled — even though all objects should be rebuilt with the new flags.

Flag heritage solves this by saving the current flags to a hidden file and forcing a full recompile whenever they differ from the saved value.

Enabling flag heritage ​

Command line ​

bash
FoBiS.py build --cflags_heritage -cflags "-c -O1"
# or the short form
FoBiS.py build -ch -cflags "-c -O1"

fobos file ​

ini
[debug]
compiler        = gnu
cflags          = -c -O0 -g
cflags_heritage = True

[release]
compiler        = gnu
cflags          = -c -O3
cflags_heritage = True

Heritage file location ​

FoBiS.py saves the flags in a hidden file:

  • <build_dir>/<target>.cflags.heritage — when a target is set
  • <build_dir>/.cflags.heritage — when no target is set

Example walkthrough ​

Given a source tree:

src/
├── main.f90
└── lib/
    └── utils.f90

First build — flags -c -O1:

bash
FoBiS.py build -ch -cflags "-c -O1"

FoBiS.py compiles everything and writes -c -O1 to .cflags.heritage.

Second build — flags unchanged:

bash
FoBiS.py build -ch -cflags "-c -O1"

FoBiS.py reads .cflags.heritage, sees no change, and skips recompilation (timestamps are also up to date).

Third build — flags changed to -c -O3:

bash
FoBiS.py build -ch -cflags "-c -O3"

FoBiS.py reads .cflags.heritage, detects the difference, forces a full recompile, and updates the heritage file to contain -c -O3.

Fourth build — heritage disabled:

bash
FoBiS.py build -cflags "-c -O1"

Without -ch, FoBiS.py checks only timestamps. Since all objects are up to date with respect to sources, nothing is recompiled and the heritage file is not updated.

Notes ​

  • Leading and trailing whitespace in the flags string is ignored during comparison.
  • The heritage file is always updated after a successful build (when heritage is enabled).
  • Combine with -force_compile to unconditionally rebuild regardless of flags or timestamps.