docs/libcurl/libcurl.3

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.TH libcurl 3 "May 05, 2021" "libcurl 7.78.0" "libcurl overview"

.SH NAME
libcurl \- client-side URL transfers
.SH 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
\fIlibcurl-easy(3)\fP, \fIlibcurl-multi(3)\fP, \fIlibcurl-share(3)\fP,
\fIlibcurl-url(3)\fP and \fIlibcurl-tutorial(3)\fP 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
\fIcurl_global_init(3)\fP at the start of your program and
\fIcurl_global_cleanup(3)\fP at the end.  See \fBGLOBAL CONSTANTS\fP below for
details.

If libcurl was compiled with support for multiple SSL backends, the function
\fIcurl_global_sslset(3)\fP can be called before \fIcurl_global_init(3)\fP
to select the active SSL backend.

To transfer files, you create an "easy handle" using \fIcurl_easy_init(3)\fP
for a single individual transfer (in either direction). You then set your
desired set of options in that handle with \fIcurl_easy_setopt(3)\fP. Options
you set with \fIcurl_easy_setopt(3)\fP stick. They will be used on every
repeated use of this handle until you either change the option, or you reset
them all with \fIcurl_easy_reset(3)\fP.

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
\fIcurl_easy_perform(3)\fP and let it perform the transfer. When it is
completed, the function returns and you can continue. More details are found in
the \fIlibcurl-easy(3)\fP 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 \fIlibcurl-multi(3)\fP 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 \fIlibcurl-share(3)\fP man page.

There is also a series of other helpful functions to use, including these:
.RS
.IP curl_version_info()
gets detailed libcurl (and other used libraries) version info
.IP curl_getdate()
converts a date string to time_t
.IP curl_easy_getinfo()
get information about a performed transfer
.IP curl_formadd()
helps building an HTTP form POST
.IP curl_formfree()
free a list built with \fIcurl_formadd(3)\fP
.IP curl_slist_append()
builds a linked list
.IP curl_slist_free_all()
frees a whole curl_slist
.IP curl_url_set()
parses a URL
.RE

.SH "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
\fIcurl-config(1)\fP 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.

Many Linux and similar systems use pkg-config to provide build and link
options about libraries and libcurl supports that as well.
.SH "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!
.SH "PORTABILITY"
libcurl works
.B exactly
the same, on any of the platforms it compiles and builds on.
.SH "THREADS"
libcurl is thread safe but there are a few exceptions. Refer to
\fIlibcurl-thread(3)\fP for more information.

.SH "PERSISTENT CONNECTIONS"
Persistent connections means that libcurl can re-use the same connection for
several transfers, if the conditions are right.

libcurl will \fBalways\fP attempt to use persistent connections. Whenever you
use \fIcurl_easy_perform(3)\fP or \fIcurl_multi_perform(3)\fP etc, 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 \fIcurl_easy_perform(3)\fP or \fIcurl_multi_perform(3)\fP.

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 \fIcurl_easy_cleanup(3)\fP, all
the possibly open connections held by libcurl will be closed and forgotten.

When you've 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.
.SH "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.

\fIcurl_global_init(3)\fP 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 \fIcurl_global_cleanup(3)\fP releases them.

The basic rule for constructing a program that uses libcurl is this: Call
\fIcurl_global_init(3)\fP, with a \fICURL_GLOBAL_ALL\fP argument, immediately
after the program starts, while it is still only one thread and before it uses
libcurl at all.  Call \fIcurl_global_cleanup(3)\fP 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 \fIis\fP required that the functions be called when no other thread
in the program is running.

These global constant functions are \fInot thread safe\fP, 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
\fIcurl_global_init(3)\fP and \fIcurl_global_cleanup(3)\fP.  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 \fIcurl_global_init(3)\fP and the last
\fIcurl_global_cleanup(3)\fP 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
\fIcurl_global_init(3)\fP and the destructor call \fIcurl_global_cleanup(3)\fP
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.)

\fIcurl_global_init(3)\fP has an argument that tells what particular parts of
the global constant environment to set up.  In order to successfully use any
value except \fICURL_GLOBAL_ALL\fP (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.  \fIcurl_global_init(3)\fP selects the system default memory
allocator, but you can use \fIcurl_global_init_mem(3)\fP to supply one of your
own.  However, there is no way to use \fIcurl_global_init_mem(3)\fP 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:
\fIcurl_easy_init(3)\fP 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.