libcurl(3)libcurl overview libcurl(3)NAMElibcurl - client-side URL transfers
DESCRIPTION
This is a 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 thirty custom bindings available that bring libcurl
access to your favourite language. Look elsewhere for documentation on
those.
libcurl has a global constant environment that you must set up and
maintain while using libcurl. This essentially means you call
curl_global_init(3) at the start of your program and
curl_global_cleanup(3) at the end. See GLOBAL CONSTANTS below for
details.
To transfer files, you always set up an "easy handle" using
curl_easy_init(3) for a single specific transfer (in either direction).
You then set your desired set of options in that handle with
curk_easy_setopt(3). Options you set with curl_easy_setopt(3) will be
used on every repeated use of this handle until you either call the
function again and change the option, or you reset them all with
curl_easy_reset(3).
To actually transfer data 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 returns 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 details 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 inter‐
face, 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 distribu‐
tions 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!
PORTABILITYlibcurl 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.
The global environment functions are not thread-safe. See GLOBAL CON‐
STANTS below for details.
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) etc, 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 possi‐
ble 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
handle.
If you use the easy interface, and you call curl_easy_cleanup(3), all
the possibly open connections held by libcurl will be closed and for‐
gotten.
When you've created a multi handle and are using the multi interface,
the connection pool is instead kept in the multi handle so closing and
creating new easy handles to do transfers will not affect them. Instead
all added easy handles can take advantage of the single shared pool.
GLOBAL CONSTANTS
There are a variety of constants that libcurl uses, mainly through its
internal use of other libraries, which are too complicated for the
library loader to set up. Therefore, a program must call a library
function after the program is loaded and running to finish setting up
the library code. For example, when libcurl is built for SSL capabil‐
ity via the GNU TLS library, there is an elaborate tree inside that
library that describes the SSL protocol.
curl_global_init() is the function that you must call. This may allo‐
cate resources (e.g. the memory for the GNU TLS tree mentioned above),
so the companion function curl_global_cleanup() releases them.
The basic rule for constructing a program that uses libcurl is this:
Call curl_global_init(), with a CURL_GLOBAL_ALL argument, immediately
after the program starts, while it is still only one thread and before
it uses libcurl at all. Call curl_global_cleanup() immediately before
the program exits, when the program is again only one thread and after
its last use of libcurl.
You can call both of these multiple times, as long as all calls meet
these requirements and the number of calls to each is the same.
It isn't actually required that the functions be called at the begin‐
ning and end of the program -- that's just usually the easiest way to
do it. It is required that the functions be called when no other
thread in the program is running.
These global constant functions are not thread safe, so you must not
call them when any other thread in the program is running. It isn't
good enough that no other thread is using libcurl at the time, because
these functions internally call similar functions of other libraries,
and those functions are similarly thread-unsafe. You can't generally
know what these libraries are, or whether other threads are using them.
The global constant situation merits special consideration when the
code you are writing to use libcurl is not the main program, but rather
a modular piece of a program, e.g. another library. As a module, your
code doesn't know about other parts of the program -- it doesn't know
whether they use libcurl or not. And its code doesn't necessarily run
at the start and end of the whole program.
A module like this must have global constant functions of its own, just
like curl_global_init() and curl_global_cleanup(). The module thus has
control at the beginning and end of the program and has a place to call
the libcurl functions. Note that if multiple modules in the program
use libcurl, they all will separately call the libcurl functions, and
that's OK because only the first curl_global_init() and the last
curl_global_cleanup() in a program change anything. (libcurl uses a
reference count in static memory).
In a C++ module, it is common to deal with the global constant situa‐
tion by defining a special class that represents the global constant
environment of the module. A program always has exactly one object of
the class, in static storage. That way, the program automatically
calls the constructor of the object as the program starts up and the
destructor as it terminates. As the author of this libcurl-using mod‐
ule, you can make the constructor call curl_global_init() and the
destructor call curl_global_cleanup() and satisfy libcurl's require‐
ments without your user having to think about it.
curl_global_init() has an argument that tells what particular parts of
the global constant environment to set up. In order to successfully
use any value except CURL_GLOBAL_ALL (which says to set up the whole
thing), you must have specific knowledge of internal workings of
libcurl and all other parts of the program of which it is part.
A special part of the global constant environment is the identity of
the memory allocator. curl_global_init() selects the system default
memory allocator, but you can use curl_global_init_mem() to supply one
of your own. However, there is no way to use curl_global_init_mem() in
a modular program -- all modules in the program that might use libcurl
would have to agree on one allocator.
There is a failsafe in libcurl that makes it usable in simple situa‐
tions without you having to worry about the global constant environment
at all: curl_easy_init() sets up the environment itself if it hasn't
been done yet. The resources it acquires to do so get released by the
operating system automatically when the program exits.
This failsafe feature exists mainly for backward compatibility because
there was a time when the global functions didn't exist. Because it is
sufficient only in the simplest of programs, it is not recommended for
any program to rely on it.
libcurl 7.9.6 19 March 2002 libcurl(3)
