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libcurl(3)		       libcurl overview			    libcurl(3)

NAME
       libcurl - client-side URL transfers

DESCRIPTION
       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 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
       these:

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

	      curl_getdate()
		     converts a date string to time_t

	      curl_easy_getinfo()
		     get information about a performed transfer

	      curl_formadd()
		     helps building an HTTP form POST

	      curl_formfree()
		     free a list built with curl_formadd(3)

	      curl_slist_append()
		     builds a linked list

	      curl_slist_free_all()
		     frees a whole curl_slist

LINKING WITH LIBCURL
       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‐
       formed.

       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.

LIBCURL SYMBOL NAMES
       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!

PORTABILITY
       libcurl works exactly the same, on any of the platforms it compiles and
       builds on.

THREADS
       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
       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)  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 possi‐
       ble following 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
       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  for‐
       gotten.

       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.

GLOBAL CONSTANTS
       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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