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libcurl(3) libcurl libcurl(3)
NAME
libcurl - 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. See
libcurl-easy(3), libcurl-multi(3), libcurl-share(3), libcurl-url(3),
libcurl-ws(3) and libcurl-tutorial(3) for in-depth understanding on how
to program with libcurl.
There are many bindings available that bring libcurl access to your
favorite language. Look elsewhere for documentation on those.
TRANSFERS
To transfer files, you create an "easy handle" using curl_easy_init(3)
for a single individual transfer (in either direction). You then set
your desired set of options in that handle with curl_easy_setopt(3).
Options you set with curl_easy_setopt(3) stick. They are then used for
every repeated use of this handle until you either 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.
SUPPORT INTERFACES
There is also a series of other helpful functions and interface fami-
lies to use, including these:
curl_version_info()
gets detailed libcurl (and other used libraries) version
info. See curl_version_info(3)
curl_getdate()
converts a date string to time_t. See curl_getdate(3)
curl_easy_getinfo()
get information about a performed transfer. See
curl_easy_getinfo(3)
curl_mime_addpart()
helps building an HTTP form POST. See
curl_mime_addpart(3)
curl_slist_append()
builds a linked list. See curl_slist_append(3)
Sharing
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.
URL Parsing
URL parsing and manipulations. See libcurl-url(3)
WebSocket communication
See libcurl-ws(3)
LINKING WITH LIBCURL
On unix-like machines, there is 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 have 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 do not provide the curl-config tool, but simply install the
library and headers in the common path for this purpose.
Many Linux and similar systems use pkg-config to provide build and link
options about libraries and libcurl supports that as well.
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
libcurl is thread safe but there are a few exceptions. Refer to
libcurl-thread(3) for more information.
PERSISTENT CONNECTIONS
Persistent connections means that libcurl can reuse the same connection
for several transfers, if the conditions are right.
libcurl always attempts to use persistent connections. Whenever you use
curl_easy_perform(3) or curl_multi_perform(3) etc, libcurl attempts to
use an existing connection to do the transfer, and if none exists it
opens a new one that is subject for reuse 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
handle.
If you use the easy interface, and you call curl_easy_cleanup(3), all
the possibly open connections held by libcurl are closed and forgotten.
When you have 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 do 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 capability
via the GNU TLS library, there is an elaborate tree inside that library
that describes the SSL protocol.
curl_global_init(3) 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(3) releases them.
If libcurl was compiled with support for multiple SSL backends, the
function curl_global_sslset(3) can be called before curl_global_init(3)
to select the active SSL backend.
The global constant functions are thread-safe since libcurl 7.84.0 if
curl_version_info(3) has the CURL_VERSION_THREADSAFE feature bit set
(most platforms). Read libcurl-thread(3) for thread safety guidelines.
If the global constant functions are not thread safe, then you must not
call them when any other thread in the program is running. It is not
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 cannot generally
know what these libraries are, or whether other threads are using them.
If the global constant functions are not thread safe, then the basic
rule for constructing a program that uses libcurl is this: Call
curl_global_init(3), 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(3) immediately before the
program exits, when the program is again only one thread and after its
last use of libcurl.
It is not actually required that the functions be called at the begin-
ning and end of the program -- that is just usually the easiest way to
do it.
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.
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 does not know about other parts of the program -- it does not know
whether they use libcurl or not. And its code does not 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(3) and curl_global_cleanup(3). The module thus
has control at the beginning and end of the program and has a place to
call the libcurl functions. If multiple modules in the program use
libcurl, they all separately call the libcurl functions, and that is OK
because only the first curl_global_init(3) and the last
curl_global_cleanup(3) 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 destruc-
tor as it terminates. As the author of this libcurl-using module, you
can make the constructor call curl_global_init(3) and the destructor
call curl_global_cleanup(3) and satisfy libcurl's requirements without
your user having to think about it. (Caveat: If you are initializing
libcurl from a Windows DLL you should not initialize it from DllMain or
a static initializer because Windows holds the loader lock during that
time and it could cause a deadlock.)
curl_global_init(3) 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(3) selects the system default
memory allocator, but you can use curl_global_init_mem(3) to supply one
of your own. However, there is no way to use curl_global_init_mem(3) 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(3) sets up the environment itself if it has not
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 did not exist. Because it is
sufficient only in the simplest of programs, it is not recommended for
any program to rely on it.
libcurl 8.4.0 August 22, 2023 libcurl(3)
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