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curl_easy_pause() documentation question

From: XSLT2.0 via curl-library <>
Date: Mon, 28 Dec 2020 11:22:40 +0100


curl_easy_pause() documentation says:
> While it may feel tempting, take care and notice that you cannot
> call this function from another thread. To unpause...

I have tested the opposite: we can although the document says "you
cannot"... with http(s) 1.1, with threads reading in a round robin mode
-very stupid, but just for testing-, protected by semaphores,
pausing in one thread and unpausing in another, it works fine (amd64 and

Code sample of the threaded reader:

    while(!fcurl_eof(fcurl)) {
        sz = fcurl_read(buf, 1, BUFSIZE, fcurl);
        if (sz > 0) {
            wsz = write(STDOUT_FILENO, buf, sz);
            if (wsz != sz) {
                fprintf(stderr, "write() %ld bytes failed\n", (long)sz);
        if (0 == --chunks) {
            curl_easy_pause(fcurl->curl, CURLPAUSE_ALL);
            sem_post(&sem[(i + 1) % NTHREADS]);
            chunks = CHUNKS;
            curl_easy_pause(fcurl->curl, CURLPAUSE_CONT);

The only visible effect is that on "verbose" mode, on my Raspberry Pi 4
with curl 7.64, it is spitting a ton of messages:

* Expire in 0 ms for 6 (transfer 0x....)

(I can't find this message in the current source of libcurl, and it is
not displayed either on my Ubuntu's curl 7.68)

Questions: what could fail here if we do what the documentation says we
cannot do, but seems to work with proper synchronisation primitives:
semaphores or the like?

Apart from Async DNS, I can't see any "mutexes" in the library code,
other protocols have that kind of thing that would make the above code
enter in a deadlock?

Subsidiary question, I have made a version of fcurl totally replacing
mallocs/free by pauses/resume (no multi-threading here!)

The code is shorter and simpler, and let libcurl do its job with memory

Since it is not at all a bug fix (your version was working fine after my
initial PR) but a different algorithm, would you like a PR?



Received on 2020-12-28