libcurl(3)libcurl overview libcurl(3)NAMElibcurl - client-side URL transfers
DESCRIPTION
This is an short overview on how to use libcurl in your C
programs. There are specific 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 else-
where 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_perform(3) and let it perform the transfer.
When it is completed, the function return and you can con-
tinue. 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 lib-
curl 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
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libcurl(3)libcurl overview libcurl(3)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 lib-
curl 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 com-
mon 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 func-
tions in the library source code, but other prefixes indi-
cate that the functions are private and may change without
further notice in the next release.
Only use documented functions and functionality!
PORTABILITYlibcurl works exactly the same, on any of the platforms it
compiles and builds on.
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libcurl(3)libcurl overview libcurl(3)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 several 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 exist-
ing 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(3) or
curl_multi_perform(3).
To allow libcurl to take full advantage of persistent con-
nections, 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 Last change: 19 March 2002 3
