What version of Go are you using (go version)?

$ go version
go version go1.16 linux/amd64

Does this issue reproduce with the latest release?

All version.

What operating system and processor architecture are you using (go env)?

$ go env
$ go env
GO111MODULE="on"
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/x/.cache/go-build"
GOENV="/home/x/.config/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/home/x/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/x/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/home/x/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/home/x/go/pkg/tool/linux_amd64"
GOVCS=""
GOVERSION="go1.16.2"
GCCGO="gccgo"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/dev/null"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"

Question

Say here is a C api like ssize_t writev(const struct iovec *iov, int iovcnt) which the definition of iovec is like below:

struct iovec {
     uint8_t   *iov_base;  /* Base address. */
     uint64_t iov_len;    /* Length. */
 };

For C api which like ssize_t write(const void *buf, size_t nbyte), the solution would be quite straight forward:

bs := make([]byte, 1024*1024*512)
// without extra memory allocation and copying of the whole input byte slice :-D
rc := C.write(unsafe.Pointer(&bs[0]), C.int(len(bs)))

But how to do writev without extra memory allocation or copying of the whole byte slice vector?

bsv := make([][]byte, 1024)
for i := range bsv{
	bsv[i] = make([]byte, 5*1024*(rand.Intn(i)+1))
}
// assuming that allocation of a iovec array is acceptable
// but allocation and copying of all bsv[x] byte slice member is unacceptable
//
iovec := make([]syscall.Iovec, len(bsv))
for i := range bsv {
		bs := bsv[i]
		if len(bs) > 0 {
        iovec[i].iov_base = unsafe.Pointer(&bs[0])
        iovec[i].iov_len = uint64(len(bs))
		}
}
//
// rc := C.writev( /* how? :-( */)
rc := C.writev(unsafe.Pointer(&iovec[0]), C.int(len(iovec))) // Does this code is right and safe?

Does the codes above is right?

I have read cgo's docs carefully, and here is a constraint from https://golang.org/cmd/cgo/#hdr-Passing_pointers:

Go code may pass a Go pointer to C provided the Go memory to which it points does not contain any Go pointers.

And https://github.com/golang/proposal/blob/master/design/12416-cgo-pointers.md#proposal:

1. Go code may pass a Go pointer to C provided that the Go memory to which it points does not contain any Go pointers.

• The C code must not store any Go pointers in Go memory, even temporarily.
• When passing a pointer to a field in a struct, the Go memory in question is the memory occupied by the field, not the entire struct.
• When passing a pointer to an element in an array or slice, the Go memory in question is the entire array or the entire backing array of the slice.
• Passing a Go pointer to C code means that that Go pointer is visible to C code; passing one Go pointer does not cause any other Go pointers to become visible.
• The maximum number of Go pointers that can become visible to C code in a single function call is the number of arguments to the function.

If the Go memory pointed by unsafe.Pointer(&iovec[0]) contains pointer which points to these byte slice members of bsv, so it would be Violation of the cgo constraint above. And that means you could not call C writev style API without allocation and copying of the whole vector.

Please correct me if I get something wrong. Thanks a lot :-D