Go 'cgo: C compiler "gcc" not found' or 'build constraints exclude all Go files' (cgo disabled): when do I need CGO_ENABLED=0 and how do I get a static build?

Cause: Packages that import "C", plus the race detector, need cgo, which needs a C compiler. Per the cgo docs, cgo is enabled by default for native builds, but is off by default when cross-compiling or when CC is unset and no default C compiler (gcc or clang) is found on PATH. Setting CGO_ENABLED explicitly overrides this. With cgo disabled, files that import "C" are skipped by the build constraint, which can end in 'build constraints exclude all Go files' or undefined symbols. In minimal builder images (alpine, slim, scratch) the compiler simply isn't installed. Fix (pick by need): 1. Pure-Go app, want a static binary or cross-compile: CGO_ENABLED=0 go build -o app ./cmd/app. Many standard-library features (net, os/user) fall back to pure-Go implementations. Check that none of your dependencies requires cgo (some SQLite drivers do; pure-Go alternatives exist). 2. You really need cgo: install a compiler in the build stage (for Debian images apt-get install gcc, for Alpine apk add build-base) and keep CGO_ENABLED=1. 3. Cross-compiling with cgo: set CC (and CXX) to a cross-compiler for the target, as the cgo docs describe. 4. For the race detector, keep cgo on and a C compiler present, since -race requires it. 5. Set the variable per command or in the Dockerfile with ENV CGO_ENABLED=0, and confirm with go env CGO_ENABLED. Verify: go build succeeds; for a CGO_ENABLED=0 build, running file or ldd on the binary reports it as statically linked, and it starts in a FROM scratch or distroless static container. Sources: https://pkg.go.dev/cmd/cgo https://go.dev/doc/articles/race_detector