xref: /aosp_15_r20/external/curl/docs/libcurl/libcurl.md (revision 6236dae45794135f37c4eb022389c904c8b0090d)

---
c: Copyright (C) Daniel Stenberg, <[email protected]>, et al.
SPDX-License-Identifier: curl
Title: libcurl
Section: 3
Source: libcurl
See-also:
  - libcurl-easy (3)
  - libcurl-multi (3)
  - libcurl-security (3)
  - libcurl-thread (3)
Protocol:
  - All
Added-in: n/a
---

# 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.