Makefile: allow using BUILD_DIR for outputs#536
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The Makefile now supports an optional `BUILD_DIR` variable for out-of-tree builds. When not set, it builds in-tree exactly as before. Key mechanisms: `SRCDIR` — automatically derived from the Makefile's own location using `$(abspath $(lastword $(MAKEFILE_LIST)))`, so it always points to the source tree regardless of where `make` is invoked from. `BUILD_DIR` — user-supplied, optional. When set, all build artifacts (`.o`, `.a`, `.so`, binaries) go there instead of the source tree. `_OBJDIR` — internal variable that equals `BUILD_DIR` when set, or `SRCDIR` when not (backward compat). `VPATH := $(SRCDIR)` — lets Make find `.c` source files in the source tree when building from a different directory. `-I$(SRCDIR)` — replaces the old `-I.` so header includes always resolve against the source tree. `.SECONDEXPANSION:` with `| $$(dir $$@)` order-only prerequisites — ensures build subdirectories (`linux/`, `libhfcommon/`, etc.) are created in `BUILD_DIR` before compilation. All output paths (`OBJS`, `BIN`, `HFUZZ_CC_BIN`, `LHFUZZ_ARCH`, `LCOMMON_ARCH`, `LNETDRIVER_ARCH`, etc.) are prefixed with `$(_OBJDIR)/`.
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Thank you Paul! |
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Hi!
We recently got an issue (rust-fuzz/honggfuzz-rs#116) with
honggfuzz-rswhen buildinghonggfuzzfrom a read-only directory. This happens for example when using Nix as a build system.Rust's cargo usually writes all its artifacts in a
targetdirectory but until now myhonggfuzz-rsproject didn't respect that and builthonggfuzz's objects files directly in the source directory.This violates cargo's mechanism and breaks Nix builds.
One easy solution is to copy the whole
honggfuzzproject in thetargetdirectory each time and build from here. I think this is not great ashonggfuzzis a hundred or so MBs and cargo will duplicate it as many times as there will be build configurations.We could avoid copying the
examplesdir or using symlinks/hardlinks but I think the best solution would be have theMakefilebe able to use a separate output directory, similarly to cmake.To illustrate the idea I send you this PR but don't hesitate to reject it, I didn't even write it as I only know the basics of Makefiles. This code was written by Claude Opus 4.6 and I don't fully understand it. It seems to work fine though.
Summary of Changes (written by Claude)
The Makefile now supports an optional
BUILD_DIRvariable for out-of-tree builds. When not set, it builds in-tree exactly as before.Key mechanisms:
SRCDIR— automatically derived from the Makefile's own location using$(abspath $(lastword $(MAKEFILE_LIST))), so it always points to the source tree regardless of wheremakeis invoked from.BUILD_DIR— user-supplied, optional. When set, all build artifacts (.o,.a,.so, binaries) go there instead of the source tree._OBJDIR— internal variable that equalsBUILD_DIRwhen set, orSRCDIRwhen not (backward compat).VPATH := $(SRCDIR)— lets Make find.csource files in the source tree when building from a different directory.-I$(SRCDIR)— replaces the old-I.so header includes always resolve against the source tree..SECONDEXPANSION:with| $$(dir $$@)order-only prerequisites — ensures build subdirectories (linux/,libhfcommon/, etc.) are created inBUILD_DIRbefore compilation.All output paths (
OBJS,BIN,HFUZZ_CC_BIN,LHFUZZ_ARCH,LCOMMON_ARCH,LNETDRIVER_ARCH, etc.) are prefixed with$(_OBJDIR)/.