Provided by: libcurl4-doc_7.35.0-1ubuntu2_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. There are also the  libcurl-easy(3)  man  page,
       the  libcurl-multi(3)  man page, the libcurl-share(3) man page and the libcurl-tutorial(3)
       man page for in-depth understanding on how to program with libcurl.

       There are more than thirty custom 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.

       To transfer files, you always set up an "easy handle" using curl_easy_init(3) for a single
       specific transfer (in either direction). You then set your desired set of options in  that
       handle  with curk_easy_setopt(3). Options you set with curl_easy_setopt(3) will be used on
       every repeated use of this handle until you either call the function again and 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


       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 curl-config(1) 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.


       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.


       Never ever call curl-functions simultaneously using the same handle from several  threads.
       libcurl is thread-safe and can be used in any number of threads, but you must use separate
       curl handles if you want to use libcurl in more than one thread simultaneously.

       The global environment functions are not thread-safe.   See  GLOBAL  CONSTANTS  below  for


       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'll  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'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.


       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() 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() releases them.

       The  basic  rule  for  constructing  a  program  that   uses   libcurl   is   this:   Call
       curl_global_init(), 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() 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  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 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
       curl_global_init()  and  curl_global_cleanup().   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  curl_global_init()  and  the  last
       curl_global_cleanup() 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() and  the  destructor  call  curl_global_cleanup()  and
       satisfy libcurl's requirements without your user having to think about it.

       curl_global_init() 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()   selects  the  system  default  memory  allocator,  but  you  can  use
       curl_global_init_mem() to supply one of your  own.   However,  there  is  no  way  to  use
       curl_global_init_mem()  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() 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.