libCurl.继续进行时取消请求 [英] libCurl. Cancel request while proceed

问题描述

已更新:问题不在libcurl中.如果从回调非零值返回，则取消请求的正确方法.我使用了curl_progress_callback函数，一切正常.

UPDATED: Problem was not in libcurl. The right way to cancel request if to return from callback non-zero value. I used curl_progress_callback function, and everything works fine.

推荐答案

您需要了解的是CURL是C库.通常，您不能将指针传递给C ++对象或函数，因为C对C ++的类和调用约定一无所知.

What you need to understand is that CURL is a C library. In general, you cannot pass pointers to C++ objects or functions because C does not know anything about C++'s classes and calling convention.

例如，此行不正确:

curl_easy_setopt(curl, CURLOPT_ERRORBUFFER, errorBuffer);

CURL setopt CURLOPT_ERRORBUFFER期望第三个参数是一个指向C样式字符串(C样式char数组)的指针，该字符串具有用于CURL_ERROR_SIZE字符的空间.而是将指针传递给std::string对象.由于用C编写的CURL不知道std::string是什么，因此它会简单地用错误数据覆盖具有sizeof (std::string)字节的字节表示形式，因为它认为指针是C风格的char数组.

CURL setopt CURLOPT_ERRORBUFFER expects that the third parameter is a pointer to a C-style string (C-style char array) having space for CURL_ERROR_SIZE chars. You are instead passing a pointer to a std::string object. As CURL, being written in C, does not know what a std::string is, it simply overwrites the byte representation having sizeof (std::string) bytes with the error data because it thinks that the pointer is to a C-style char array.

改为使用此:

char errorBuffer[CURL_ERROR_SIZE + 1]; errorBuffer[CURL_ERROR_SIZE] = '\0';
curl_easy_setopt(curl, CURLOPT_ERRORBUFFER, errorBuffer);

errorBuffer仅在curl_easy_perform()返回错误代码时才用有效的字符串填充.

errorBuffer is only filled with a valid string if curl_easy_perform() returns an error code.

此外，Uploader::WriteResponce是C ++函数.在这一行:

Also, Uploader::WriteResponce is a C++ function. With this line:

curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, WriteResponce);

CURL希望第三个参数是C语言的指针.在这里，您正在传递一个指向C ++的函数.您需要将对WriteResponce的调用包装在调用C ++函数的extern "C"函数中:

CURL expects the third parameter to be a pointer-to-C function. Here, you are passing a pointer-to-C++ function. You need to wrap the call to WriteResponce in an extern "C" function that calls the C++ function:

extern "C" size_t call_uploader_writeresponce(char *ptr, size_t size, size_t nmemb, void *user_data) {
    return Uploader::WriteResponce(ptr, size, nmemb, static_cast<std::string *>(user_data));
}


// ...
curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, &call_uploader_writeresponce);

写入功能"需要返回size_t，而不是int； Uploader::WriteResponce需要返回size_t.

The "write function" needs to return size_t, not int; Uploader::WriteResponce needs to return size_t.

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