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libcurl(3) libcurl overview libcurl(3)
NAME
libcurl - client-side URL transfers
DESCRIPTION
This is an short overview on how to use libcurl in your C programs. There are spe-
cific man pages for each function mentioned in here. There are also the libcurl-
easy(3) man page, the libcurl-multi(3) man page, the libcurl-share(3) man page and
the libcurl-tutorial(3) man page for in-depth understanding on how to program with
libcurl.
There are more than a twenty custom bindings available that bring libcurl access to
your favourite language. Look elsewhere for documentation on those.
All applications that use libcurl should call curl_global_init(3) exactly once
before any libcurl function can be used. After all usage of libcurl is complete, it
must call curl_global_cleanup(3). In between those two calls, you can use libcurl
as described below.
To transfer files, you always set up an "easy handle" using curl_easy_init(3), but
when you want the file(s) transferred you have the option of using the "easy"
interface, or the "multi" interface.
The easy interface is a synchronous interface with which you call curl_easy_per-
form(3) and let it perform the transfer. When it is completed, the function return
and you can continue. More details are found in the libcurl-easy(3) man page.
The multi interface on the other hand is an asynchronous interface, that you call
and that performs only a little piece of the transfer on each invoke. It is perfect
if you want to do things while the transfer is in progress, or similar. The multi
interface allows you to select() on libcurl action, and even to easily download
multiple files simultaneously using a single thread. See further deails in the
libcurl-multi(3) man page.
You can have multiple easy handles share certain data, even if they are used in
different threads. This magic is setup using the share interface, as described in
the libcurl-share(3) man page.
There is also a series of other helpful functions to use, including these:
curl_version_info()
gets detailed libcurl (and other used libraries) version info
curl_getdate()
converts a date string to time_t
curl_easy_getinfo()
get information about a performed transfer
curl_formadd()
helps building an HTTP form POST
curl_formfree()
free a list built with curl_formadd(3)
curl_slist_append()
builds a linked list
curl_slist_free_all()
frees a whole curl_slist
LINKING WITH LIBCURL
On unix-like machines, there’s a tool named curl-config that gets installed with
the rest of the curl stuff when ’make install’ is performed.
curl-config is added to make it easier for applications to link with libcurl and
developers to learn about libcurl and how to use it.
Run ’curl-config --libs’ to get the (additional) linker options you need to link
with the particular version of libcurl you’ve installed. See the curl-config(1) man
page for further details.
Unix-like operating system that ship libcurl as part of their distributions often
don’t provide the curl-config tool, but simply install the library and headers in
the common path for this purpose.
LIBCURL SYMBOL NAMES
All public functions in the libcurl interface are prefixed with ’curl_’ (with a
lowercase c). You can find other functions in the library source code, but other
prefixes indicate that the functions are private and may change without further
notice in the next release.
Only use documented functions and functionality!
PORTABILITY
libcurl works exactly the same, on any of the platforms it compiles and builds on.
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.
PERSISTENT CONNECTIONS
Persistent connections means that libcurl can re-use the same connection for sev-
eral transfers, if the conditions are right.
libcurl will always attempt to use persistent connections. Whenever you use
curl_easy_perform(3) or curl_multi_perform(3), 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_per-
form(3) or curl_multi_perform(3).
To allow libcurl to take full advantage of persistent connections, you should do as
many of your file transfers as possible using the same curl handle. When you call
curl_easy_cleanup(3), all the possibly open connections held by libcurl will be
closed and forgotten.
Note that the options set with curl_easy_setopt(3) will be used in on every
repeated curl_easy_perform(3) call.
libcurl 7.9.6 19 March 2002 libcurl(3)
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On Apache/1.3.41 (Unix) PHP/5.2.5 mod_perl/1.30 mod_gzip/1.3.26.1a
Under GNU General Public License
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