Environment variables¶

Variables set by just-buildit¶

just-buildit sets these before calling your build command:

Variable	Example value	Purpose
`JUST_BUILDIT_NAME`	`mylib`	Normalized package name — use as the base of your output filename
`JUST_BUILDIT_PYTHON`	`/usr/bin/python3.12`	Absolute path to the Python interpreter running the build
`JUST_BUILDIT_INCLUDE_DIR`	`/usr/include/python3.12`	Python header directory — pass as `-I$JUST_BUILDIT_INCLUDE_DIR`
`JUST_BUILDIT_OUTPUT_DIR`	`/tmp/just-buildit-xyz/output`	Wheel content root — write all output here
`JUST_BUILDIT_EXT_SUFFIX`	`.cpython-312-x86_64-linux-gnu.so`	Full platform extension suffix for `.so` / `.pyd` naming

Output directory

$JUST_BUILDIT_OUTPUT_DIR is packaged verbatim into the wheel. Write every file your package needs there — extensions, pure-Python sources, data files — and preserve the directory structure you want users to import.

Platform-specific link flags¶

Variable	Linux	macOS	Windows (MinGW)
`JUST_BUILDIT_LDFLAGS`	`-shared -fPIC`	`-dynamiclib -undefined dynamic_lookup`	`-shared`
`JUST_BUILDIT_LIBS`	(empty)	(empty)	`-L/ucrt64/lib -lpython3.14`

JUST_BUILDIT_LIBS is only non-empty when a custom command is configured and the platform is Windows/MinGW — where Python's import library must be linked explicitly. On Linux and macOS it is always empty; JUST_BUILDIT_LDFLAGS is sufficient.

Link order on Linux and macOS

Always place $JUST_BUILDIT_LIBS after the output file (-o) in your linker invocation:

# CORRECT — library after the object file
$(CC) $(JUST_BUILDIT_LDFLAGS) -I$(JUST_BUILDIT_INCLUDE_DIR) \
    src/mylib.c -o $(TARGET) $(JUST_BUILDIT_LIBS)

# WRONG — library before the object file (ld drops it silently)
$(CC) $(JUST_BUILDIT_LDFLAGS) $(JUST_BUILDIT_LIBS) \
    -I$(JUST_BUILDIT_INCLUDE_DIR) src/mylib.c -o $(TARGET)

GNU ld uses --as-needed by default on Debian/Ubuntu; any library that appears before the object files that reference it is silently dropped. This is a no-op on Linux/macOS (where JUST_BUILDIT_LIBS is empty anyway), but the ordering habit matters on Windows/MinGW.

Variables read by just-buildit¶

These are not set by just-buildit — they are read from the environment if already present.

Variable	Default	Effect
`CC`	`cc`	C compiler for zero-config builds (no `command` set)
`SOURCE_DATE_EPOCH`	(current time)	Unix timestamp clamped into wheel and sdist archive entries

Selecting the C compiler

Override CC to use a specific compiler without changing any config:

CC=gcc pip install .
CC=clang pip install .

This only affects zero-config builds (no command in [tool.just-buildit]). Custom-command builds receive no CC injection — your Makefile or CMake handles compiler selection directly.

Reproducible builds

Set SOURCE_DATE_EPOCH to a fixed Unix timestamp to make wheel and sdist archives byte-for-byte identical across builds:

SOURCE_DATE_EPOCH=1700000000 pip wheel .

This follows the Reproducible Builds standard and is respected by most build backends and archive tools. Both pip wheel and python -m build propagate it automatically when they detect it in the environment.