libcurl
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 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 elsewhere for documen-
tation 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 com-
pleted, 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 simulta-
neously 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 per-
formed.
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 distri-
butions 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 connec-
tion 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 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 pos-
sible following call to curl_easy_perform(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)
