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cmd/cgo: document generated cgo directives

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This took me a while to figure out. Save the next person some trouble.

Change-Id: Ifab2d426f67c21b08ef225c79125805a9008e578
Reviewed-on: https://go-review.googlesource.com/c/go/+/309336
Trust: Austin Clements <austin@google.com>
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>
This commit is contained in:
Austin Clements 2021-04-11 17:19:59 -04:00
parent 7b19fb1d56
commit 10f883deb7
1 changed files with 17 additions and 3 deletions
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20
src/cmd/cgo/out.go
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@ -1021,11 +1021,25 @@ func (p *Package) writeExports(fgo2, fm, fgcc, fgcch io.Writer) {
}
fmt.Fprintf(fgcc, "}\n")
// In internal linking mode, the Go linker sees both
// the C wrapper written above and the Go wrapper it
// references. Hence, export the C wrapper (e.g., for
// if we're building a shared object). The Go linker
// will resolve the C wrapper's reference to the Go
// wrapper without a separate export.
fmt.Fprintf(fgo2, "//go:cgo_export_dynamic %s\n", exp.ExpName)
// cgo_export_static refers to a symbol by its linker
// name, so set the linker name of the Go wrapper.
fmt.Fprintf(fgo2, "//go:linkname _cgoexp%s_%s _cgoexp%s_%s\n", cPrefix, exp.ExpName, cPrefix, exp.ExpName)
// In external linking mode, the Go linker sees the Go
// wrapper, but not the C wrapper. For this case,
// export the Go wrapper so the host linker can
// resolve the reference from the C wrapper to the Go
// wrapper.
fmt.Fprintf(fgo2, "//go:cgo_export_static _cgoexp%s_%s\n", cPrefix, exp.ExpName)
// Build the wrapper function compiled by cmd/compile.
// This unpacks the argument struct above and calls the Go function.
fmt.Fprintf(fgo2, "//go:cgo_export_dynamic %s\n", exp.ExpName)
fmt.Fprintf(fgo2, "//go:linkname _cgoexp%s_%s _cgoexp%s_%s\n", cPrefix, exp.ExpName, cPrefix, exp.ExpName)
fmt.Fprintf(fgo2, "//go:cgo_export_static _cgoexp%s_%s\n", cPrefix, exp.ExpName)
fmt.Fprintf(fgo2, "func _cgoexp%s_%s(a *%s) {\n", cPrefix, exp.ExpName, gotype)
fmt.Fprintf(fm, "int _cgoexp%s_%s;\n", cPrefix, exp.ExpName)