CURLOPT_HEADER.3: clarify problems with different data sizes

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.\" * Copyright (C) 1998 - 2015, Daniel Stenberg, <daniel@haxx.se>, et al.

.\" * Copyright (C) 1998 - 2018, Daniel Stenberg, <daniel@haxx.se>, et al.

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CURLcode curl_easy_setopt(CURL *handle, CURLOPT_HEADER, long onoff);

.SH DESCRIPTION

Pass in \fIonoff\fP set to 1 to tell the library to include the header in the

body output for requests with this \fIhandle\fP. This option is relevant for

protocols that actually have headers or other meta-data (like HTTP and FTP).

Pass the long value \fIonoff\fP set to 1 to ask libcurl to include the headers

in the write callback (\fICURLOPT_WRITEFUNCTION(3)\fP). This option is

relevant for protocols that actually have headers or other meta-data (like

HTTP and FTP).

When asking to get the header info passed to the same callback as the body, it

is not possible to accurately separate them again without detailed knowledge

When asking to get the headers passed to the same callback as the body, it is

not possible to accurately separate them again without detailed knowledge

about the protocol in use.

Further: the \fCURLOPT_WRITEFUNCTION(3)\fP callback is limited to only ever

get a maximum of \fICURL_MAX_WRITE_SIZE\fP bytes passed to it (16KB), while a

header can be longer and the \fICURLOPT_HEADERFUNCTION(3)\fP supports getting

called with headers up to \fICURL_MAX_HTTP_HEADER\fP bytes big (100KB).

It is often better to use \fICURLOPT_HEADERFUNCTION(3)\fP to get the header

data separately.