curl_formadd在第一次調用時返回`CURL_FORMADD_OPTION_TWICE`

Question

我試圖用libcurl構造一個HTTP表單，但是我無法讓它正常工作。每次我撥打curl_formadd時，都會返回CURL_FORMADD_OPTION_TWICE。關於這個錯誤的唯一信息表明libcurl認爲我試圖添加兩個具有相同名稱的表單元素，即使它第一次調用curl_formadd，而我只添加了一個元素！

Declare Function curl_global_init Lib "libcurl" (flags As Integer) As Integer 
    Declare Function curl_formadd Lib "libcurl" (FirstItem As Ptr, LastItem As Ptr, Option1 As Integer, Value1 As Ptr, Option2 As Integer, Value2 As Ptr, EndMarker As Integer) As Integer 

    Const CURLFORM_COPYCONTENTS = 2 
    Const CURLFORM_COPYNAME = 1 
    Const CURLFORM_END = 17 

    Dim formname, formvalue As MemoryBlock 
    formname = "NAME" 
    formvalue = "CONTENTS" 

    If curl_global_init(3) = 0 Then 
    Dim first, last As Ptr 
    Dim err As Integer 
    err = curl_formadd(first, last, CURLFORM_COPYNAME, formname, CURLFORM_COPYCONTENTS, formvalue, CURLFORM_END) 
    Break 
    ' err is 2 (CURL_FORMADD_OPTION_TWICE) 
    End If 

什麼是這個錯誤試圖告訴我？

Answer 1

閱讀curl_formadd（）的「人」頁面，它說：

指針和與firstItem應lastitem無論是在第一次調用這個函數是指向NULL。

但是，您爲這些傳遞NULL。

您似乎也錯誤地傳遞了字符串。嘗試將Value1和Value2參數定義爲「CString」，然後傳遞正常的字符串，而不是內存塊。

最後，你給了CURLFORM_COPYCONTENTS錯誤的代碼。它不是2但是4.請參閱curl.h中的CURLformoption枚舉：「CFINIT（NOTHING）」獲得值0，並且每個項目都會得到一個更高的值，因此CFINIT（COPYCONTENTS）會獲得4.

下面是代碼適用於我：

Declare Function curl_global_init Lib "libcurl" (flags As Integer) As Integer 
Declare Function curl_formadd Lib "libcurl" (ByRef FirstItem As Ptr, ByRef LastItem As Ptr, Option1 As Integer, Value1 As CString, Option2 As Integer, Value2 As CString, EndMarker As Integer) As Integer 

Const CURLFORM_COPYCONTENTS = 4 
Const CURLFORM_COPYNAME = 1 
Const CURLFORM_END = 17 
const CURL_GLOBAL_ALL = 3 

Dim formname, formvalue As String 
formname = "NAME" 
formvalue = "CONTENTS" 

If curl_global_init(CURL_GLOBAL_ALL) = 0 Then 
    Dim first, last As Ptr 
    Dim err As Integer 
    err = curl_formadd(first, last, CURLFORM_COPYNAME, formname, CURLFORM_COPYCONTENTS, formvalue, CURLFORM_END) 
    Break 
    ' err is 0 
End If