An Internet transfer might be brief but can also take a long time. Maybe even an infinite amount of time.

libcurl normally performs transfers until they are complete or until an error occurs. If none of those events happen, the transfer continues.

At times, you might want to stop a libcurl transfer before it would otherwise stop.

easy API

As explained elsewhere, the curl_easy_perform() function is a synchronous function call. It does the entire transfer before it returns.

There are a few different ways to stop a transfer before it would otherwise end:

  1. return an error from a callback
  2. set an option that makes the transfer stop after a fixed period of time

Every callback can return an error, and when an error is returned from one of those functions the entire transfer is stopped. For example the read, write or progress callbacks.

The second way is to set a timeout or other option that stops the transfer after a time or at a particular condition. For example one or more of the following:

  1. CURLOPT_TIMEOUT - set a maximum time the entire transfer may take
  2. CURLOPT_CONNECTTIMEOUT - set a maximum time "connection phase" may take
  3. CURLOPT_LOW_SPEED_LIMIT - set the lowest acceptable transfer speed. The transfer stops if slower than this speed for CURLOPT_LOW_SPEED_TIME number of second.

There is no provided function that allows your application to stop an ongoing curl_easy_perform() call from another thread. The common suggestion is then that you signal that intent in a private way that you can detect in a callback and have that callback return error when it happens.

multi API

The multi interface is generally a non-blocking API, so in most situations you can stop a transfer by removing its corresponding easy handle from the multi handle using curl_multi_remove_handle().

When you use the multi API, you might call libcurl to wait for activities or traffic on sockets libcurl works with. A call that might sit blocking while waiting for something to happen (or a timeout to expire), like curl_multi_poll().

An application can make a blocked call to curl_multi_poll() wake up and return forcibly and immediately by calling curl_multi_wakeup() from another thread.