How to Pass a Go Slice Pointer to a C Function?

Passing Pointer to Slice to C Function in Go

When calling C functions from Go using cgo, it is often necessary to pass pointers to Go slices as arguments. However, this can be challenging as Go slices are not directly compatible with C arrays.

To address this issue, it is necessary to allocate the array in C memory and pass a pointer to that array to the C function. This ensures that the C function can modify the array and return it back to Go.

Here's an example that demonstrates how to allocate an array in C and pass a pointer to it from a Go slice:

// C function signature: int f(int *count, char ***strs)
import "C"

func go_f(strs []string) int {
    count := len(strs)
    c_count := C.int(count)

    // Allocate an array in C memory to store the strings
    cArray := C.malloc(C.size_t(c_count) * C.size_t(unsafe.Sizeof(uintptr(0))))

    // Convert the C array to a Go array so we can access its elements
    a := (*[1<<30 - 1]*C.char)(cArray)

    // Copy the Go strings into the C array
    for index, value := range strs {
        a[index] = C.CString(value)
    }

    // Call the C function with a pointer to the C array
    err := C.f(&c_count, (***C.char)(unsafe.Pointer(&cArray)))

    // Free the C array
    C.free(cArray)

    return int(err)
}

By allocating the array in C memory, the C function can modify its contents and the changes will be propagated back to the Go slice when the C function returns.

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