curl / Mailing Lists / curl-library / Single Mail

Buy commercial curl support. We help you work out your issues, debug your libcurl applications, use the API, port to new platforms, add new features and more. With a team lead by the curl founder Daniel himself.

Re: transactions stall after March 6 commit to improve wakeup and wait code in multi

Contemporary messages sorted: [ by date ] [ by thread ] [ by subject ] [ by author ] [ by messages with attachments ]

From: Bryan Henderson via curl-library <curl-library_at_lists.haxx.se>
Date: 16 Jun 2026 14:48:35 +0000

> >> Any chance you can for example modify an existing example source code to
> >> reproduce this behavior? Then we would have a good reproducer to work with!
>
> I'll see what I can do.

This might be easier than I thought to reproduce.

I use an easy handle with no options set whatsoever, not even a URL.
Apparently, this is legal and just means performing the transaction is a
no-op. At least that's what Curl did before that March commit.

But following the commit, if I add that handle to a multi manager,
'curl_multi_fdset' says there is a file descriptor which I have to wait for to
become ready before that transaction can proceed, and that file descriptor
never becomes ready.

I don't know where to find a published example of using curl_multi_fdset to
use as a reproducer, but I made a very simple one that must look a lot like
any such example. Code is below.

When I run this against old libcurl, it prints, "There are no file descriptors
to wait for" and exits. With new libcurl, it prints, "libcurl supplied a file
descriptor [...] waiting now ..." and then hangs.

-------------------------------------------------------------------------------
#include <assert.h>
#include <stdio.h>
#include <sys/select.h>

#include <curl/curl.h>
#include <curl/easy.h>
#include <curl/multi.h>

int
main(int , const char ** ) {

    CURLMcode rc;

    printf("Hello world.\n");

    CURL * const curlSessionP = curl_easy_init();

    assert(curlSessionP);

    CURLM * const curlMultiP = curl_multi_init();

    assert(curlMultiP);

    rc = curl_multi_add_handle(curlMultiP, curlSessionP);

    assert(rc == CURLM_OK);

    fd_set readFdSet, writeFdSet, exceptFdSet;
    int maxFd;

    rc = curl_multi_fdset(curlMultiP,
                          &readFdSet, &writeFdSet, &exceptFdSet,
                          &maxFd);

    assert(rc == CURLM_OK);

    if (maxFd == -1)
        // With libcurl 8.19, this is the case.
        fprintf(stderr, "There are no file descriptors to wait for\n");
    else {
        fprintf(stderr, "libcurl supplied a file descriptor to "
                "wait for (maxFd=%u). Waiting now ...\n", maxFd);

        pselect(maxFd + 1, &readFdSet, &writeFdSet, &exceptFdSet,
                NULL /* timeout */, NULL /* signal mask */);

        // Problem: This 'pselect never completes

        fprintf(stderr, "A file descriptor is ready.\n");
    }

    return 0;
}
-------------------------------------------------------------------------------

--
Bryan Henderson                                   Omak, Washington
-- 
Unsubscribe: https://lists.haxx.se/mailman/listinfo/curl-library
Etiquette:   https://curl.se/mail/etiquette.html

Received on 2026-06-16

Contemporary messages sorted: [ by date ] [ by thread ] [ by subject ] [ by author ] [ by messages with attachments ]