133 lines
4.2 KiB
Markdown
133 lines
4.2 KiB
Markdown
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---
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c: Copyright (C) Daniel Stenberg, <daniel@haxx.se>, et al.
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SPDX-License-Identifier: curl
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Title: CURLOPT_HEADERFUNCTION
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Section: 3
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Source: libcurl
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See-also:
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- CURLOPT_HEADERDATA (3)
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- CURLOPT_WRITEFUNCTION (3)
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- curl_easy_header (3)
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---
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# NAME
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CURLOPT_HEADERFUNCTION - callback that receives header data
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# SYNOPSIS
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~~~c
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#include <curl/curl.h>
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size_t header_callback(char *buffer,
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size_t size,
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size_t nitems,
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void *userdata);
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CURLcode curl_easy_setopt(CURL *handle, CURLOPT_HEADERFUNCTION,
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header_callback);
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~~~
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# DESCRIPTION
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Pass a pointer to your callback function, which should match the prototype
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shown above.
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This callback function gets invoked by libcurl as soon as it has received
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header data. The header callback is called once for each header and only
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complete header lines are passed on to the callback. Parsing headers is easy
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to do using this callback. *buffer* points to the delivered data, and the
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size of that data is *nitems*; *size* is always 1. The provide header
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line is not null-terminated!
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The pointer named *userdata* is the one you set with the
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CURLOPT_HEADERDATA(3) option.
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Your callback should return the number of bytes actually taken care of. If
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that amount differs from the amount passed to your callback function, it
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signals an error condition to the library. This causes the transfer to get
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aborted and the libcurl function used returns *CURLE_WRITE_ERROR*.
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You can also abort the transfer by returning CURL_WRITEFUNC_ERROR. (7.87.0)
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A complete HTTP header that is passed to this function can be up to
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*CURL_MAX_HTTP_HEADER* (100K) bytes and includes the final line terminator.
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If this option is not set, or if it is set to NULL, but
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CURLOPT_HEADERDATA(3) is set to anything but NULL, the function used to
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accept response data is used instead. That is the function specified with
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CURLOPT_WRITEFUNCTION(3), or if it is not specified or NULL - the
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default, stream-writing function.
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It is important to note that the callback is invoked for the headers of all
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responses received after initiating a request and not just the final
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response. This includes all responses which occur during authentication
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negotiation. If you need to operate on only the headers from the final
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response, you need to collect headers in the callback yourself and use HTTP
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status lines, for example, to delimit response boundaries.
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For an HTTP transfer, the status line and the blank line preceding the response
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body are both included as headers and passed to this function.
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When a server sends a chunked encoded transfer, it may contain a trailer. That
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trailer is identical to an HTTP header and if such a trailer is received it is
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passed to the application using this callback as well. There are several ways
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to detect it being a trailer and not an ordinary header: 1) it comes after the
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response-body. 2) it comes after the final header line (CR LF) 3) a Trailer:
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header among the regular response-headers mention what header(s) to expect in
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the trailer.
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For non-HTTP protocols like FTP, POP3, IMAP and SMTP this function gets called
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with the server responses to the commands that libcurl sends.
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A more convenient way to get HTTP headers might be to use
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curl_easy_header(3).
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# LIMITATIONS
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libcurl does not unfold HTTP "folded headers" (deprecated since RFC 7230). A
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folded header is a header that continues on a subsequent line and starts with
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a whitespace. Such folds are passed to the header callback as separate ones,
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although strictly they are just continuations of the previous lines.
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# DEFAULT
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Nothing.
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# PROTOCOLS
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Used for all protocols with headers or meta-data concept: HTTP, FTP, POP3,
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IMAP, SMTP and more.
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# EXAMPLE
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~~~c
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static size_t header_callback(char *buffer, size_t size,
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size_t nitems, void *userdata)
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{
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/* received header is nitems * size long in 'buffer' NOT ZERO TERMINATED */
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/* 'userdata' is set with CURLOPT_HEADERDATA */
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return nitems * size;
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}
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int main(void)
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{
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CURL *curl = curl_easy_init();
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if(curl) {
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curl_easy_setopt(curl, CURLOPT_URL, "https://example.com");
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curl_easy_setopt(curl, CURLOPT_HEADERFUNCTION, header_callback);
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curl_easy_perform(curl);
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}
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}
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~~~
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# AVAILABILITY
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Always
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# RETURN VALUE
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Returns CURLE_OK
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