libcurl(3) libcurl overview 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) 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.
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 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 beginning
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 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.
libcurl 7.86.0 September 20, 2022 libcurl(3)
curl 7.86.0 - Generated Thu Nov 3 18:29:10 CDT 2022