libcurl(3) -- Linux man page
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 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 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
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 possible 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.
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