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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  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
       details.

       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  curl_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
       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.

       Many Linux and similar systems use pkg-config to provide build and link
       options about libraries and libcurl supports that as well.


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

       libcurl  is  thread  safe  but  there  are  a  few exceptions. Refer to
       libcurl-thread(3) for more information.



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(3) 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(3) releases them.

       The basic rule for constructing a program that uses  libcurl  is  this:
       Call  curl_global_init(3), 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(3) 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(3)  and curl_global_cleanup(3).  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 pro-
       gram use libcurl, they all will separately call the libcurl  functions,
       and  that's  OK because only the first curl_global_init(3) and the last
       curl_global_cleanup(3) 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(3) and the
       destructor call curl_global_cleanup(3) and satisfy  libcurl's  require-
       ments  without your user having to think about it.  (Caveat: If you are
       initializing libcurl from a Windows DLL you should  not  initialize  it
       from  DllMain  or a static initializer because Windows holds the loader
       lock during that time and it could cause a deadlock.)

       curl_global_init(3) 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(3) selects the system default
       memory allocator, but you can use curl_global_init_mem(3) to supply one
       of  your  own.  However, there is no way to use curl_global_init_mem(3)
       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(3) 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)

Mac OS X 10.12.3 - Generated Sun Feb 5 15:09:47 CST 2017
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