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cmd/cgo: clarify implicit "cgo" build constraint

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When using the special import "C", the "cgo" build constraint is implied for the go file,
potentially triggering unclear "undefined" error messages.
Explicitly explain this in the documentation.

Updates #24068

Change-Id: Ib656ceccd52c749ffe7fb2d3db9ac144f17abb32
GitHub-Last-Rev: 5a13f00a9b
GitHub-Pull-Request: golang/go#24072
Reviewed-on: https://go-review.googlesource.com/96655
Reviewed-by: Ian Lance Taylor <iant@golang.org>
This commit is contained in:
Rens Rikkerink 2018-02-26 18:23:00 +00:00 committed by Ian Lance Taylor
parent 515fa58ac9
commit cbfda7f892
1 changed files with 5 additions and 1 deletions
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6
src/cmd/cgo/doc.go
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@ -109,7 +109,11 @@ it is expected to work. It is disabled by default when
cross-compiling. You can control this by setting the CGO_ENABLED
environment variable when running the go tool: set it to 1 to enable
the use of cgo, and to 0 to disable it. The go tool will set the
build constraint "cgo" if cgo is enabled.
build constraint "cgo" if cgo is enabled. The special import "C"
implies the "cgo" build constraint, as though the file also said
"// +build cgo". Therefore, if cgo is disabled, files that import
"C" will not be built by the go tool. (For more about build constraints
see https://golang.org/pkg/go/build/#hdr-Build_Constraints).
When cross-compiling, you must specify a C cross-compiler for cgo to
use. You can do this by setting the generic CC_FOR_TARGET or the