Provided by: libcurl4-doc_7.81.0-1ubuntu1.16_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  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 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   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.

       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 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.  It is required that the
       functions be called when no other thread in the program is running.

       These global constant functions are not thread safe, so 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.

       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.