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Header API
libcurl offers an API for iterating over all received HTTP headers and for extracting the contents from specific ones.
When returning header content, libcurl trims leading and trailing whitespace but will not modify or change content in any other way.
This API was made official and is provided for real starting in libcurl 7.84.0.

Header origins

HTTP headers are key value pairs that are sent from the server in a few different origins during a transfer. libcurl collects all headers and provides easy access to them for applications.
HTTP headers can arrive as
  1. 1.
    CURLH_HEADER - before regular response content.
  2. 2.
    CURLH_TRAILER - fields arriving after the response content
  3. 3.
    CURLH_CONNECT - response headers in the proxy CONNECT request that might have been done before the actual server request
  4. 4.
    CURLH_1XX - headers in the potential 1xx HTTP responses that might have preceded the following >= 2xx response code.
  5. 5.
    CURLH_PSEUDO - HTTP/2 and HTTP/3 level headers that start with colon (:)

Request number

A single HTTP transfer done with libcurl might consist of a series of HTTP requests and the request argument to the header API functions lets you specify which particular individual request you want the headers from. 0 being the first request and then the number increases for further redirects or when multi-state authentication is used. Passing in -1 is a shortcut to the last request in the series, independently of the actual amount of requests used.

Header folding

HTTP/1 headers supports a deprecated format called "folding", which means that there is a continuation line after a header, making the line "folded".
The headers API supports folded headers and returns such contents "unfolded" - where the different parts are separated by a single whitespace character.

When

The two header API function calls are perfectly possible to call at any time during a transfer, both from inside and outside of callbacks. It is however important to remember that the API will of course only return information about the state of the headers at the exact moment it is called, which might not be the final status if you call it while the transfer is still in progress.
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Header origins
Request number
Header folding
When