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How To Use Libcurl In Your C/C++ Program
[ libcurl can be used directly from within your PHP or Perl programs as well,
look elsewhere for documentation on this ]
The interface is meant to be very simple for applictions/programmers, hence
the name "easy". We have therefore minimized the number of entries.
The Easy Interface
When using the easy interface, you init your session and get a handle, which
you use as input to the following interface functions you use. Use
curl_easy_init() to get the handle.
You continue by setting all the options you want in the upcoming transfer,
most important among them is the URL itself (you can't transfer anything
without a specified URL as you may have figured out yourself). You might want
to set some callbacks as well that will be called from the library when data
is available etc. curl_easy_setopt() is there for this.
When all is setup, you tell libcurl to perform the transfer using
curl_easy_perform(). It will then do the entire operation and won't return
until it is done or failed.
After the transfer has been made, you cleanup the session with
curl_easy_cleanup() and libcurl is entirely off the hook! If you want
persistant connections, you don't cleanup immediately, but instead run ahead
and perform other transfers. See the chapter below for Persistant
Connections.
While the above mentioned four functions are the main functions to use in the
easy interface, there is a series of other helpful functions to use. They
are:
curl_version() - displays the libcurl version
curl_getdate() - converts a date string to time_t
curl_getenv() - portable environment variable reader
curl_easy_getinfo() - get information about a performed transfer
curl_formparse() - helps building a HTTP form POST
curl_formfree() - free a list built with curl_formparse()
curl_slist_append() - builds a linked list
curl_slist_free_all() - frees a whole curl_slist
For details on these, read the separate man pages.
Portability
libcurl works *exactly* the same, on any of the platforms it compiles and
builds on.
There's only one caution, and that is the win32 platform that may(*) require
you to init the winsock stuff before you use the libcurl functions. Details
on this are noted on the curl_easy_init() man page.
(*) = it appears as if users of the cygwin environment get this done
automatically.
Threads
Never *ever* call curl-functions simultaneously using the same handle from
several threads. libcurl is thread-safe and can be used in any number of
threads, but you must use separate curl handles if you want to use libcurl in
more than one thread simultaneously.
Persistant Connections
With libcurl 7.7, persistant connections were added. Persistant connections
means that libcurl can re-use the same connection for several transfers, if
the conditions are right.
libcurl will *always* attempt to use persistant connections. Whenever you use
curl_easy_perform(), libcurl will attempt to use an existing connection to do
the transfer, and if none exists it'll open a new one that will be subject
for re-use on a possible following call to curl_easy_perform().
To allow libcurl to take full advantage of persistant connections, you should
do as many of your file transfers as possible using the same curl
handle. When you call curl_easy_cleanup(), all the possibly open connections
held by libcurl will be closed and forgotten.
Note that the options set with curl_easy_setopt() will be used in on every
repeat curl_easy_perform() call
Compatibility with older libcurls
Repeated curl_easy_perform() calls on the same handle were not supported in
pre-7.7 versions, and caused confusion and defined behaviour.
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