如何从Go语言调用此C函数（使用Cgo工具）

Question

Here is this C function declaration

CURL_EXTERN CURLcode curl_easy_setopt(CURL *curl, CURLoption option, ...);

how do I call this function from Go?

type Easy struct {
    curl unsafe.Pointer
    code C.CURLcode
}

func (e *Easy)SetOption(option C.CURLoption, ...) {
    e.code = C.curl_easy_setopt(e.curl, option, ????))
}

Answer 1

You can't call it directly. CGO does not play well with vararg functions on the C side. Ideally, you could create a C wrapper which accepts a list of options you want to pass. The C function should then expand that list into the variable arguments required by curl_easy_set_opt(). But I am not sure if that is possible or how to go about doing it.

The signature for your Go function is also incorrect:

type Option C.CURLoption

func (e *Easy) SetOption(options ...Option) {
    // 'options' is now accessible as a slice: []Option
    // Turn this slice into a list of C.CURLoption pointers and pass it to
    // your C wrapper.

    if len(options) == 0 {
        return // No use continuing.
    }

    // Here is one way to convert the option slice to a list
    // that C can work with.
    size := int(unsafe.Sizeof(options[0]))
    list := C.malloc(C.size_t(size * len(options)))
    defer C.free(unsafe.Pointer(list)) // Free this after use!

    for i := range options {
        ptr := unsafe.Pointer( uintptr(list) + uintptr(size * i) )
        *(*C.CURLoption)(ptr) = C.CURLoption(options[i])
    }

    C.my_setopt_wrapper(e.curl, list, C.int(len(options)))
}

Note that the type of the option parameter has been changed to a go version of it. When someone uses your package, they have no access to the C.xxx types. So you should not use those in your public api. They are only meant for internal use in your package.