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For compatibility with `fwrite`, the `CURLOPT_HEADERFUNCTION` callback is passed two `size_t` parameters which, when multiplied, designate the number of bytes of data passed in. In practice, CURL always sets the first parameter (`size`) to 1. This practice is also enshrined in documentation and cannot be changed in future. The documentation states that the default callback is `fwrite`, which means `fwrite` must be a suitable function for this purpose. However, the documentation also states that the callback must return the number of *bytes* it successfully handled, whereas ISO C `fwrite` returns the number of items (each of size `size`) which it wrote. The only way these numbers can be equal is if `size` is 1. Since `size` is 1 and can never be changed in future anyway, document that fact explicitly and let users rely on it. Reported-by: Frank Gevaerts Commit-message-by: Christopher Head Ref: https://github.com/curl/curl/pull/2787 Fixes https://github.com/curl/curl/issues/4758
118 lines
5.1 KiB
Groff
118 lines
5.1 KiB
Groff
.\" **************************************************************************
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.\" * _ _ ____ _
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.\" * Project ___| | | | _ \| |
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.\" * / __| | | | |_) | |
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.\" * | (__| |_| | _ <| |___
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.\" * \___|\___/|_| \_\_____|
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.\" *
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.\" * Copyright (C) 1998 - 2019, Daniel Stenberg, <daniel@haxx.se>, et al.
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.\" *
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.\" * This software is licensed as described in the file COPYING, which
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.\" * you should have received as part of this distribution. The terms
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.\" * are also available at https://curl.haxx.se/docs/copyright.html.
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.\" *
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.\" * You may opt to use, copy, modify, merge, publish, distribute and/or sell
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.\" * copies of the Software, and permit persons to whom the Software is
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.\" * furnished to do so, under the terms of the COPYING file.
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.\" *
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.\" * This software is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY
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.\" * KIND, either express or implied.
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.\" *
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.\" **************************************************************************
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.\"
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.TH CURLOPT_HEADERFUNCTION 3 "17 Jun 2014" "libcurl 7.37.0" "curl_easy_setopt options"
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.SH NAME
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CURLOPT_HEADERFUNCTION \- callback that receives header data
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.SH SYNOPSIS
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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, header_callback);
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.SH 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 function gets called by libcurl as soon as it has received header
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data. The header callback will be called once for each header and only
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complete header lines are passed on to the callback. Parsing headers is very
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easy using this. \fIbuffer\fP points to the delivered data, and the size of
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that data is \fInitems\fP; \fIsize\fP is always 1. Do not assume that the
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header line is zero terminated!
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The pointer named \fIuserdata\fP is the one you set with the
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\fICURLOPT_HEADERDATA(3)\fP option.
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This callback function must return the number of bytes actually taken care of.
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If that amount differs from the amount passed in to your function, it'll signal
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an error to the library. This will cause the transfer to get aborted and the
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libcurl function in progress will return \fICURLE_WRITE_ERROR\fP.
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A complete HTTP header that is passed to this function can be up to
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\fICURL_MAX_HTTP_HEADER\fP (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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\fICURLOPT_HEADERDATA(3)\fP is set to anything but NULL, the function used to
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accept response data will be used instead. That is, it will be the function
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specified with \fICURLOPT_WRITEFUNCTION(3)\fP, or if it is not specified or
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NULL - the default, stream-writing function.
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It's important to note that the callback will be invoked for the headers of
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all 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 will need to collect headers in the callback yourself and use
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HTTP 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 will get
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called with the server responses to the commands that libcurl sends.
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.SH 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 will be passed to the header callback as a separate
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one, although strictly it is just a continuation of the previous line.
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.SH DEFAULT
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Nothing.
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.SH 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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.SH EXAMPLE
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.nf
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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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CURL *curl = curl_easy_init();
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if(curl) {
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curl_easy_setopt(curl, CURLOPT_URL, "http://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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.fi
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.SH AVAILABILITY
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Always
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.SH RETURN VALUE
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Returns CURLE_OK
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.SH "SEE ALSO"
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.BR CURLOPT_HEADERDATA "(3), " CURLOPT_WRITEFUNCTION "(3), "
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