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libcurl(3)





NAME

       libcurl - client-side URL transfers


DESCRIPTION

       This  is  an  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), 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  return  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 deails 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), 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 in on
       every repeated curl_easy_perform(3) call.


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 changes 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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