Why am I Getting \"curl error 18 - transfer closed with outstanding read data remaining\" and How Can I Fix It?

Resolving "curl error 18 - transfer closed with outstanding read data remaining"

When retrieving data from a remote server through the curl command-line tool, you may encounter the error message "curl error 18 - transfer closed with outstanding read data remaining." This error indicates that an attempt to retrieve data from a URL was interrupted before the entire response was received, resulting in partial or missing data.

Typically, this error is encountered when CURLOPT_RETURNTRANSFER is set to true, indicating that the curl_exec function should return the data retrieved from the URL. However, disabling CURLOPT_RETURNTRANSFER by setting it to false allows the retrieved data to be displayed directly, avoiding the error.

What Causes the Error?

This error occurs due to the use of chunked encoding in the HTTP response. Chunked encoding allows a server to send data in multiple chunks, with each chunk being terminated by a defined character sequence.

When a connection is closed before all chunks are received, libcurl recognizes that the last received chunk was incomplete and generates the error message.

Resolving the Issue

There are two main ways to address this issue:

1. Disable Chunked Encoding:

You can try issuing a HTTP 1.0 request instead by adding the -0 option to the curl command. HTTP 1.0 does not use chunked encoding, so this workaround may resolve the error.

2. Inspect the Server and Network Configuration:

The error may indicate a problem with the server or the network connection. It is recommended to check the following:

• The server configuration to ensure that it is not misconfigured or experiencing issues.
• The network connection between the client and the server to identify any potential bottlenecks or interruptions.

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