libcurl man page on Manjaro

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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  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

       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  curk_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 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.

       Many Linux and similar sytems 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.

       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)  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

       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‐

       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 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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