I modified my example based on the code example you gave at the bottom for the pointer arithmetic and that compiled without any go vet errors. I attempted to use the slice whose backing array is the C array https://play.golang.org/p/rJoDMu5YAQ but I get a compile error: main.go:14: cannot convert cArr (type *Tag) to type *[1048576]Tag
Another question, if I update my code to use the slice whose backing array is the C array, I see the slice type supports 1048576 entries. If I only need at most 100 entries, is there a memory penalty imposed by declaring the type to support 1048576 entries? If so, how great is that penalty? On Wednesday, August 31, 2016 at 3:17:30 PM UTC-5, Ian Lance Taylor wrote: > On Wed, Aug 31, 2016 at 12:47 PM, Luke Mauldin <lukem...@gmail.com > <javascript:>> wrote: > > > > I have questions about pointer arithmetic illustrated by this Play > example: > > https://play.golang.org/p/-cZteTY_M2 > > > > Questions: > > 1) Is this the best way to do pointer arithmetic in Go to process a C > array > > and convert it to a Go slice? > > 2) Go vet gives an error on line 18 but doesn't give much information on > how > > to fix the usage of unsafe.Pointer. Any recommendations? > > It's usually simplest to write an expression like > s := (*[1 << 20]Tag)(Carr)[:arrLength:arrLength] > That will give you a slice whose backing array is the C array, without > requiring any copying. Of course you then have to make sure that C > array lives on the heap as least as long as the slice does. If that > is an issue, then write > s2 := make([]Tag, arrLength) > copy(s2, s) > To be clear, this assume there are fewer than 1 << 20 entries in the > array, so adjust as needed. > > > The vet errors for your code are false positives, assuming that cArr > is allocated in C memory. That said, it's easy to avoid them by > writing code like > > q := unsafe.Pointer(cArr) > for i := 0; i < arrLength; i++ { > p := (*Tag)(q) > ret = append(ret, int(*p)) > q = unsafe.Pointer(uintptr(q) + unsafe.Sizeof(q)) > } > > The point is: always keep pointers as pointers, except in expressions > that convert to uintptr and back in a single expression. > > Ian > -- You received this message because you are subscribed to the Google Groups "golang-nuts" group. To unsubscribe from this group and stop receiving emails from it, send an email to golang-nuts+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/d/optout.
