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 *Base; /* Base address. */
uint64_t 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 call C `writev` style API 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].Base = unsafe.Pointer(&bs[0])
iovec[i].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 code 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.
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 a
*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
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