Man Linux: Main Page and Category List


       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 more than thirty custom bindings available that bring libcurl
       access to your favourite language. Look elsewhere for documentation  on

       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  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 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
       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

                     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

       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 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
       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
       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 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
       situation  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 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 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
       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 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.