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. 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), 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 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 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'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!
PORTABILITY¶
libcurl 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 CONSTANTS below
for details.
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), 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.
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() 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.