libcurl - client-side URL transfers
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
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
To transfer files, you always set up an "easy handle" using
curl_easy_init(3), but when you want the file(s) transferred 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
gets detailed libcurl (and other used libraries) version
converts a date string to time_t
get information about a performed transfer
helps building an HTTP form POST
free a list built with curl_formadd(3)
builds a linked list
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
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
distributions 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!
libcurl works exactly the same, on any of the platforms it compiles and
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
CONSTANTS below for details.
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), 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(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
curl handle. When you call curl_easy_cleanup(3), all the possibly open
connections held by libcurl will be closed and forgotten.
Note that the options set with curl_easy_setopt(3) will be used on
every repeated curl_easy_perform(3) call.
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() 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() 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
beginning 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
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() and
the destructor call curl_global_cleanup() and satisfy libcurl’s
requirements 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
situations 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.