Provided by: libcurl4-doc_7.88.1-8ubuntu1_all bug


       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)  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.

       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.

       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

       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.

       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:

                     gets detailed libcurl (and other used libraries) version info

                     converts a date string to time_t

                     get information about a performed transfer

                     helps building an HTTP form POST

                     free a list built with curl_formadd(3)

                     builds a linked list

                     frees a whole curl_slist

                     parses a URL


       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  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


       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 handle.

       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 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  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

       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.

       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.