How do I convert a Go array of strings to a C array of strings?

Question

I am using cgo in a project, and I want to export a function for use. Here's an example of what I want to achieve:

package csplit

import (
    "C"
    "strings"
)

//export Split
/* The Split function takes two C strings, the second of which represents
   a substring to split on, and returns an array of strings. Example:
       Split("1,2", ",") // gives ["1", "2"]
*/
func Split(original *C.char, split *C.char) []*C.char {
        goResult := strings.Split(C.GoString(original), C.GoString(split))
        cResult := make([]*C.char, len(goResult))

        for idx, substring := range goResult {
                cResult[idx] = C.CString(substring)
        }

        return cResult
}

The problem is that the return type is Go allocated data, and not moved into the C heap. This panics with: runtime error: cgo result has Go pointer

Answer 1

You're returning a Go slice which is allocated in Go, and is a different structure than a C array. You need to allocate an array in C:

//export Split
func Split(original *C.char, split *C.char) **C.char {
    goResult := strings.Split(C.GoString(original), C.GoString(split))
    cArray := C.malloc(C.size_t(len(goResult)) * C.size_t(unsafe.Sizeof(uintptr(0))))

    // convert the C array to a Go Array so we can index it
    a := (*[1<<30 - 1]*C.char)(cArray)

    for idx, substring := range goResult {
        a[idx] = C.CString(substring)
    }

    return (**C.char)(cArray)
}