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UNSUPPORTED_PROTOCOL from curl_easy_send on handle removed from curl_multi #9244

redbaron opened this issue Aug 3, 2022 · 3 comments


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redbaron commented Aug 3, 2022

I did this

If CURLOPT_CONNECT_ONLY socket was connected using mutli interface, then curl_easy_send returns CURLE_UNSUPPORTED_PROTOCOL. Here is minimal reproducer.


#include <curl/curl.h>
#include <cassert>

int main(int, char**) {
    CURL* e = curl_easy_init();
    curl_easy_setopt(e, CURLOPT_URL, "");
    curl_easy_setopt(e, CURLOPT_CONNECT_ONLY, 1);
    curl_easy_setopt(e, CURLOPT_VERBOSE, 1);

    CURLM* m = curl_multi_init();
    curl_multi_add_handle(m, e);

    int count;
    do {
        curl_multi_perform(m, &count);
    } while (count > 0);

    CURLMsg* msg = curl_multi_info_read(m, &count);
    assert(count == 0);
    assert(msg->easy_handle == e);
    assert(msg->data.result == CURLE_OK);

    curl_multi_remove_handle(m, e);

    const char buf[] = "XYZ";
    size_t sent;
    CURLcode res = curl_easy_send(e, buf, sizeof(buf), &sent);
    assert(res == CURLE_OK); // CURLE_UNSUPPORTED_PROTOCOL is returned

    return 0;

I expected the following

curl_easy_send is usable after curl_multi_remove_handle returns;

curl/libcurl version


operating system

Linux WSL2

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redbaron commented Aug 3, 2022

Hm, commenting curl_multi_remove_handle call makes it work. Reading curl_multi_add_handle doc I was under impression that I shouldn't call any curl_easy_* IO functions until handle is removed from multi, even though docs talks about curl_easy_perform only.

I am not sure if not calling curl_multi_remove_handle is an intended use of CURLOPT_CONNECT_ONLY sockets or just a workaround sidestepping legitimate bug.

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bagder commented Aug 4, 2022

I think this is mostly a case where we should update/clarify the documentation.

bagder added a commit that referenced this issue Aug 6, 2022
Reported-by: Maxim Ivanov
Fixes #9244
Closes #
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bagder commented Aug 6, 2022

#9262 is my attempt to clarify how to use this feature with the multi interface

@bagder bagder closed this as completed in ba670b3 Aug 7, 2022
vszakats added a commit to vszakats/curl that referenced this issue Oct 11, 2022
PR curl#9244 aimed to fix a Cygwin/MSYS issue. It used the `CURL_WIN32`
macro, but that one is never defined while compiling curl itself. This
patch changes this to `WIN32`, assuming this was the original intent.

Regression from 1c52e8a.

Ref: curl#8220
Ref: curl#9255

Closes #xxxx
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2 participants