Re: Turning off async resolution at runtime

On Thu, 29 Jan 2026, Andrew Bell via curl-library wrote:

> I have an application that is making many http requests through curl. Each 
> request is handled by a thread. On my OS (OSX) there is a default file 
> descriptor limit of 256. When I make a curl request with curl_easy_perform, 
> it creates two pipes in addition to creating the socket on which to make the 
> request. It appears that these pipes are made for communication internal to 
> curl about async DNS resolution. This means that I'm using three file 
> descriptors for each request, which means get "out of file descriptor" 
> errors when my application has ~80 threads making curl requests.

The socketpair file descriptors are however only used while name resolving.

> It looks like I can disable async resolution at compile-time, which I would 
> think would eliminate the pipes and associated file descriptors, but I can't 
> control how the library was built for users. I don't see any way I can 
> disable this async resolution at runtime, but perhaps I'm missing something.

Nope. Because using sync name resolving is terrible and is normally only used 
as a worst-case fallback.

For example: timeouts for sync name resolving is bad, uses signals and is 
error-prone. It messes up libcurl's ability to work asynchronously.

> Does anyone have any ideas/recommendations to eliminate the FDs being used 
> for DNS resolution or some other ideas on how I can reduce the likelihood of 
> running out of FDs when using lots of simultaneous requests?

I would recommend using the multi interface and avoid threads. It probably 
also increases performance.

-- 

  / daniel.haxx.se || https://rock-solid.curl.dev
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