NAME
libcurl - client-side URL transfers
DESCRIPTION
This is an overview on how to use libcurl in your C pro
grams. There are specific man pages for each function men
tioned in here. There's also the libcurl-the-guide document
for a complete tutorial to programming with libcurl.
There are a dozen custom bindings that bring libcurl access
to your favourite language. Look elsewhere for documentation
on those.
All applications that use libcurl should call
curl_global_init() exactly once before any libcurl function
can be used. After all usage of libcurl is complete, it must
call curl_global_cleanup(). In between those two calls, you
can use libcurl as described below.
When using libcurl's "easy" interface you init your session
and get a handle, which you use as input to the easy inter
face functions you use. Use curl_easy_init() to get the han
dle. There is also the so called "multi" interface, try the
libcurl-multi(3) man page for an overview of that.
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 oper
ation and won't return until it is done (successfully or
not).
After the transfer has been made, you can set new options
and make another transfer, or if you're done, cleanup the
session by calling curl_easy_cleanup(). If you want persis
tant connections, you don't cleanup immediately, but instead
run ahead and perform other transfers using the same handle.
See the chapter below for Persistant Connections.
There is also 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_formadd()
helps building a HTTP form POST
curl_formfree()
free a list built with curl_form
parse()/curl_formadd()
curl_slist_append()
builds a linked list
curl_slist_free_all()
frees a whole curl_slist
curl_mprintf()
portable printf() functions
curl_strequal()
portable case insensitive string comparisons
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.
For details, see the curl-config.1 man page.
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 the functions are private and may change without fur
ther 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.
PERSISTANT 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(), 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 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(), 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
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