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libcurl(3)                          libcurl                         libcurl(3)


       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. 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.


       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 will be used on
       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.


       There is also a series of other helpful functions and interface
       families to use, including these:

                     gets detailed libcurl (and other used libraries) version
                     info. See curl_version_info(3)

                     converts a date string to time_t. See curl_getdate(3)

                     get information about a performed transfer. See

                     helps building an HTTP form POST. See

                     builds a linked list. See curl_slist_append(3)

                     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)


       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 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.


       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
       builds on.


       libcurl is thread safe but there are a few exceptions. Refer to
       libcurl-thread(3) for more information.


       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) etc, libcurl will
       attempt to use an existing connection to do the transfer, and if none
       exists it will open a new one that will be subject for re-use on a
       possible following call to curl_easy_perform(3) or

       To allow libcurl to take full advantage of persistent connections, you
       should do as many of your file transfers as possible using the same

       If you use the easy interface, and you call curl_easy_cleanup(3), all
       the possibly open connections held by libcurl will be closed and

       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 will not affect them. Instead
       all added easy handles can take advantage of the single shared pool.


       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. And 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 will 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.

libcurl 8.2.1                    June 14, 2023                      libcurl(3)

curl 8.2.1 - Generated Wed Aug 16 09:12:22 CDT 2023
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