When should curl_multi_remove_handle be called: before or after curl_close? A Detailed Explanation of the Call Sequence

1. Overview of curl_multi_* Functions

Let's first briefly understand the basic functions of the curl_multi_* series:

1. curl_multi_init(): Initializes a multi-handle for managing multiple cURL handles.

2. curl_multi_add_handle(): Adds a cURL handle to the multi-handle.

3. curl_multi_remove_handle(): Removes a cURL handle from the multi-handle.

4. curl_multi_exec(): Executes all the requests added to the multi-handle.

5. curl_multi_close(): Closes the multi-handle and cleans up resources.

2. Functions of curl_multi_remove_handle() and curl_close()

• curl_close(): Closes a single cURL session handle and releases all resources associated with that handle.

• curl_multi_remove_handle(): Removes a cURL handle from the multi-handle, but does not directly close the cURL session. After removal, the handle remains valid and can still be used or closed elsewhere.

3. Impact of Call Sequence

We need to clarify two points:

1. curl_multi_remove_handle() operates on the multi-handle, not the session. Calling this function does not terminate the request or release resources associated with the cURL session.

2. curl_close() releases resources. When we call curl_close(), the associated cURL handle is destroyed. Therefore, we don't want to destroy the handle before removing it.

Based on these characteristics, let's analyze the call sequence:

1. Call curl_multi_remove_handle() before curl_close()

This is the recommended sequence. Because:

• When you remove the cURL handle, it detaches from the multi-handle but does not close the session. You can perform additional actions on the handle afterward (such as logging or debugging).

• If you call curl_close() first, the session will be closed directly, and you won't be able to operate on the handle afterward.

2. Call curl_close() before curl_multi_remove_handle()

This sequence is incorrect because:

• Once you call curl_close(), the cURL session is destroyed and its resources are released. You won't be able to remove the handle from the multi-handle.

• Even if you attempt to remove the session from the multi-handle, cURL may have already released the resources internally, leading to unexpected errors.

Therefore, this sequence can cause crashes or undefined behavior.

4. Complete Correct Call Flow

A standard flow should be:

1. Use curl_multi_add_handle() to add each cURL session to the multi-handle.

2. Call curl_multi_exec() to execute and wait for responses.

3. Use curl_multi_remove_handle() to remove completed or no longer needed cURL sessions from the multi-handle.

4. Finally, call curl_close() to close each cURL session and release associated resources.

5. Call curl_multi_close() to close the multi-handle and release associated resources.

5. Conclusion

curl_multi_remove_handle() should be called before curl_close(). Calling curl_multi_remove_handle() only removes the cURL session from the multi-handle and does not release resources. Therefore, after removing the handle, you can still close the cURL session and free the resources. Be sure to avoid calling curl_close() first, as it will cause an error when removing the handle.