Determining Return Code and Testing for Network Errors in CURL

[xiaochongzi630]

When using CURL to communicate with a server, what return code is returned if the transmission completes successfully but a network error occurs during reception?
Also, how to test this scenario?

[jay]

what return code is returned if the transmission completes successfully but a network error occurs during reception?

The curl command line tool and libcurl return CURLE_OK (0) on success. Can you please give an example of the scenario you are thinking of?

[jay]

You would have to query the server in a follow up request. If there's no good way to do that then you could make an educated guess using write-out variables. For example check to see whether you received the expected response code via %{response_code}. However just because a response code has not been received (eg transfer failed after request was sent but before response) does not mean the server has not processed the request.

CURLE_COULDNT_CONNECT (7) error occurs in what kind of situations?

Refer to the descriptions I linked to earlier: Failed to connect() to host or proxy.

[xiaochongzi630]

Thanks for your response.

I understood that if a CURLE_COULDNT_CONNECT (7) error occurs, it means we can't determine whether the server has processed the request or not.

[jay]

If the connection could not be established then the request has not been sent.

(edit: some exceptions where the request is sent but must be sent again according to the server, in this case it technically has been sent. for example if --location is used a connection may be established, the request sent, but the server responds with a redirect to a different location to send the request that curl cannot connect to, it will return CURLE_COULDNT_CONNECT in that case. if --retry is used and the server failed the request according to whatever retry reasons you set then the connection may be reestablished and if that fails curl will return CURLE_COULDNT_CONNECT.)

[xiaochongzi630]

Is it possible to determine whether the connection has been established?

[jay]

You can use --write-out time variables. Internally those variables map to libcurl's TIMES. Consider viewing that documentation as well for more detail. (eg time_connect maps to CURLINFO_CONNECT_TIME_T, time_starttransfer maps to CURLINFO_STARTTRANSFER_TIME_T).

[dfandrich]

"Established" how? At the socket level? TCP level? TLS level? HTTP level? Application level? It's not a straightforward answer.