Provided by: libcurl4-doc_7.81.0-1ubuntu1.19_all 
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) 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 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.
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.
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
will be used on 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 completed, 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 simultaneously
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 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
curl_url_set()
parses a URL
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 have installed. See the curl-config(1) man page for
further details.
Unix-like operating system that ship libcurl as part of their distributions 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 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) etc, libcurl will attempt to use an existing
connection to do the transfer, and if none exists it will 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 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 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 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 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 allocate resources (e.g.
the memory for the GNU TLS tree mentioned above), so the companion function
curl_global_cleanup(3) releases them.
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.
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 is not actually required that the functions be called at the beginning and end of the
program -- that is 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 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.
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 will 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 situation 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 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 situations 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.