Tribblix: manual page: libcurl.3

libcurl(3) Introduction to Library Functions 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
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.

SUPPORT INTERFACES

There is also a series of other helpful functions and interface
families 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 data between transfers
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
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 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
allocate 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
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. 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
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.