lambda to curl callback function

Question

I am trying to pass a lambda to the CURLOPT_WRITEFUNCTION.

The function expects a static function, however, I learned from this question that a lambda would be implicitly converted, and I could call the member function from the lambda.

auto callback = [](char * ptr_data, size_t size, size_t nmemb, string * writerData)
->size_t
{
    if(writerData == NULL)
        return 0;
    size_t data_size = size * nmemb;
    writerData->append(ptr_data, data_size);
    return (int)data_size;
};

CURLcode code = curl_easy_setopt(conn, CURLOPT_WRITEFUNCTION, callback);

This actually compiles, but curl segfaults: Segmentation fault: 11

I pasted the full example here.

Looking at your snippet, don't you want: `code = curl_easy_setopt(conn, CURLOPT_WRITEDATA, &buffer);` to be called before you call `CURLOPT_WRITEFUNCTION`? Otherwise the pointer to pass to the callback is not set. — Jesse Good, Apr 13 '15 at 03:00
@JesseGood, The order doesn't matter as long as I set them before calling `curl_easy_perform`. — Francisco Aguilera, Apr 13 '15 at 03:04

score 8 · Accepted Answer · edited Aug 08 '23 at 12:38

The magic is, add the leading "+" at the non-captured lambda expression, which will trigger conversion to plain C function pointer.

curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION,
  /* NOTE: Leader '+' trigger conversion from non-captured Lambda Object to plain C pointer */
  +[](void *buffer, size_t size, size_t nmemb, void *userp) -> size_t {
    // invoke the member function via userp
    return size * nmemb;
  });

My understanding is, curl_easy_setopt wants a void*, not an explicit function type, so the compiler just gives the address of lambda object; if we do a function pointer operation on lambda object, the compiler will return the function pointer from lambda object.

Drew Dormann · Answer 2 · 2015-04-13T02:41:35.883

3

A lambda closure is convertible to a function pointer only if the lambda has no captures.

Your lambda is capturing [this] so it cannot be converted.

Edit: this was an answer to the original revision of the question.

edited Apr 13 '15 at 02:41

answered Apr 13 '15 at 02:20

Drew Dormann

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While not curl specific, [these](http://stackoverflow.com/questions/15657011) other [questions](http://stackoverflow.com/questions/28746744) are similar. – Drew Dormann Apr 13 '15 at 02:23
see my edit, I was aware of this, and was testing different configurations, because curl still segfaults in all cases. – Francisco Aguilera Apr 20 '15 at 12:12

lambda to curl callback function

2 Answers2