Access C array of type const char * from Go

Question

I have a C file with an array of type const char *, lets call it myStringArray[], something like:

const char *myStringArray[] = {
  "NAME_OF_FIRST_THING",
  "NAME_OF_SECOND_THING",
  "NAME_OF_THIRD_THING"}

I need Go to index into that C array, using cgo, and convert an array entry into a Go string. The following code compiles but does not work correctly; you can see from the output that follows, it is indexing along the strings rather than up the array:

myGoString := C.GoString((*C.char) (unsafe.Pointer(uintptr(unsafe.Pointer(C.myStringArray)) + uintptr(index) * unsafe.Sizeof(C.myStringArray))))

...result being this:

NAME_OF_FIRST_THING

FIRST_THING

ING

FYI, the purpose of this is to decode on a server a log file that was created on an embedded platform running C, hence the need to re-use the same logging index files.

I've been stabbing at this randomly for a few hours now without success. Can anyone correct my construction, or provide an alternative?

Answer 1

While you could do the pointer arithmetic yourself, it's safer and more convenient to convert the C array into a Go slice first.

arraySize := 3
cStrings := (*[1 << 30]*C.char)(unsafe.Pointer(&C.myStringArray))[:arraySize:arraySize]

for _, cString := range cStrings {
    fmt.Println(C.GoString(cString))
}

// prints:
// NAME_OF_FIRST_THING
// NAME_OF_SECOND_THING
// NAME_OF_THIRD_THING

Relevant cgo wiki entry: https://github.com/golang/go/wiki/cgo#turning-c-arrays-into-go-slices