Re: valgrind drd shows potential data race in curl_easy_init

From: Aman Grewal via curl-library <curl-library_at_lists.haxx.se>
Date: Wed, 19 Nov 2025 19:51:51 -0500

I'm calling it. This is a minimal example where I'm able to reproduce it.

```
#include <pthread.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

#include <curl/curl.h>

static pthread_t thread1;
static pthread_t thread2;

struct storage {
    char *response;
    size_t size;
};

size_t cb_for_curl_easy_perform(char *data, size_t size, size_t nmemb, void
*storage) {
    size_t realsize = size * nmemb;
    struct storage *mem = (struct storage *)storage;

    char *ptr = realloc(mem->response, mem->size + realsize + 1);

    mem->response = ptr;
    memcpy(&(mem->response[mem->size]), data, realsize);
    mem->size += realsize;
    mem->response[mem->size] = 0;

    return realsize;
}

void *thread_func(void *arg __attribute__((unused))) {
    for (int i = 0; i < 2; ++i) {
        CURL *handle = curl_easy_init();
        CURLcode res = curl_easy_setopt(handle, CURLOPT_URL, "
http://localhost:8888/test.c");
        res = curl_easy_setopt(handle, CURLOPT_WRITEFUNCTION,
cb_for_curl_easy_perform);
        struct storage resp = { .response = NULL, .size = 0 };
        res = curl_easy_setopt(handle, CURLOPT_WRITEDATA, &resp);
        res = curl_easy_perform(handle);
        curl_easy_cleanup(handle);
    }
}

int main(int argc, char** argv) {
    printf("HELLO, WORLD!\n");
    fflush(stdout);
    curl_global_init(CURL_GLOBAL_NOTHING);
    pthread_create(&thread1, NULL, thread_func, NULL);
    pthread_create(&thread2, NULL, thread_func, NULL);
    pthread_join(thread1, NULL);
    pthread_join(thread2, NULL);
    printf("GOODBYE, WORLD!\n");
    fflush(stdout);
}
```


On Wed, Nov 19, 2025 at 6:43 PM Henrik Holst <henrik.holst_at_millistream.com>
wrote:

> looks like you don't call curl_global_init() before starting your threads
>
> /HH
>
> Den tors 20 nov. 2025 kl 00:36 skrev Aman Grewal via curl-library <
> curl-library_at_lists.haxx.se>:
>
>> Hi all,
>>
>> I'm running my library under valgrind to try to find any bugs. I'm seeing
>> this warning stemming from `curl_easy_init` when running drd.
>>
>> Command: `valgrind --tool=drd --read-var-info=yes --check-stack-var=yes
>> --free-is-write=yes -s --gen-suppressions=all /test`
>>
>> Truncated Output:
>> ==8== Thread 3:
>> ==8== Conflicting load by thread 3 at 0x04949724 size 4
>> ==8== at 0x48B8FA2: curl_easy_init (in
>> /usr/lib/x86_64-linux-gnu/libcurl-gnutls.so.4.8.0)
>> ==8== by 0x40014C1: thread_func (test.c:33)
>> ==8== by 0x485B948: vgDrd_thread_wrapper (drd_pthread_intercepts.c:512)
>> ==8== by 0x49E6AA3: start_thread (pthread_create.c:447)
>> ==8== by 0x4A73A63: clone (clone.S:100)
>> ==8== Allocation context: BSS section of
>> /usr/lib/x86_64-linux-gnu/libcurl-gnutls.so.4.8.0
>> ==8== Other segment start (thread 2)
>> ==8== at 0x4A73A56: clone (clone.S:83)
>> ==8== by 0x49E671F: ??? (allocatestack.c:605)
>> ==8== by 0x67036BF: ???
>> ==8== Other segment end (thread 2)
>> ==8== at 0x4A759DB: connect (connect.c:26)
>> ==8== by 0x48A7DEC: ??? (in
>> /usr/lib/x86_64-linux-gnu/libcurl-gnutls.so.4.8.0)
>> ==8== by 0x48B42E7: ??? (in
>> /usr/lib/x86_64-linux-gnu/libcurl-gnutls.so.4.8.0)
>> ==8== by 0x48B517F: ??? (in
>> /usr/lib/x86_64-linux-gnu/libcurl-gnutls.so.4.8.0)
>> ==8== by 0x48A8FC5: ??? (in
>> /usr/lib/x86_64-linux-gnu/libcurl-gnutls.so.4.8.0)
>> ==8== by 0x48EAED7: ??? (in
>> /usr/lib/x86_64-linux-gnu/libcurl-gnutls.so.4.8.0)
>> ==8== by 0x48ED5D9: curl_multi_perform (in
>> /usr/lib/x86_64-linux-gnu/libcurl-gnutls.so.4.8.0)
>> ==8== by 0x48BBE3A: curl_easy_perform (in
>> /usr/lib/x86_64-linux-gnu/libcurl-gnutls.so.4.8.0)
>> ==8== by 0x4001553: thread_func (test.c:38)
>> ==8== by 0x485B948: vgDrd_thread_wrapper (drd_pthread_intercepts.c:512)
>> ==8== by 0x49E6AA3: start_thread (pthread_create.c:447)
>> ==8== by 0x4A73A63: clone (clone.S:100)
>> ==8==
>>
>> When I run the same valgrind command on the sample code in
>> https://curl.se/libcurl/c/threaded-ssl.html, I see similar warnings, so
>> I'm hoping this is a spurious warning. Has anyone seen this before? Is it
>> safe to add it to my suppressions file?
>>
>>
>> I'm running this on Ubuntu 24.04 (LTS), and I'm using the libcurl3t64-gnutls
>> package.
>>
>>
>>
>> Thanks,
>> Aman Grewal
>>
>>
>>
>> --
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>>
>

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Received on 2025-11-20