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libcurl(3)		       libcurl overview			    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. 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

       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

       To   transfer   files,  you  always  set	 up  an	 "easy	handle"	 using
       curl_easy_init(3), but when you want the file(s) transferred  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  com‐
       pleted,	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  simulta‐
       neously	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 inter‐
       face, as described in the libcurl-share(3) man page.

       There is also a series of other helpful	functions  to  use,  including

		     gets  detailed libcurl (and other used libraries) version

		     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 per‐

       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 distribu‐
       tions 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  CON‐
       STANTS below for details.

       Persistent  connections	means that libcurl can re-use the same connec‐
       tion 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), 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 fol‐
       lowing 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
       curl handle. When you call curl_easy_cleanup(3), all the possibly  open
       connections held by libcurl will be closed and forgotten.

       Note  that  the	options	 set  with curl_easy_setopt(3) will be used on
       every repeated curl_easy_perform(3) call.

       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  capabil‐
       ity  via	 the  GNU  TLS library, there is an elaborate tree inside that
       library that describes the SSL protocol.

       curl_global_init() is the function that you must call.  This may	 allo‐
       cate  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	begin‐
       ning  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	situa‐
       tion  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 mod‐
       ule, you can make  the  constructor  call  curl_global_init()  and  the
       destructor  call	 curl_global_cleanup()	and satisfy libcurl's require‐
       ments 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 situa‐
       tions 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.

libcurl 7.9.6			 19 March 2002			    libcurl(3)

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