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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 func-
       tion 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 essen-
       tially 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), 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  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 lit-
       tle  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 details.

       Persistent connections means that libcurl can re-use the same connection for several transfers, if  the  condi-
       tions 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 following call to curl_easy_per-
       form(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  capability 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 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 pro-
       gram 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 rep-
       resents 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

       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  func-
       tions  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)