Go to main content

man pages section 3: Library Interfaces and Headers

Exit Print View

Updated: Thursday, June 13, 2019

libcurl (3)


libcurl - side URL transfers


Please see following description for synopsis


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

       If libcurl was compiled with support for  multiple  SSL  backends,  the
       function curl_global_sslset(3) can be called before curl_global_init(3)
       to select the active SSL backend.

       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

                     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 systems 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.

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

       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(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.

       See attributes(7) for descriptions of the following attributes:

       |Availability   | web/curl         |
       |Stability      | Uncommitted      |
       This     software     was    built    from    source    available    at
       https://github.com/oracle/solaris-userland.   The  original   community
       source    was    downloaded    from    https://github.com/curl/curl/ar-

       Further information about this software can be found on the open source
       community website at http://curl.haxx.se/.

libcurl 7.9.6                    19 March 2002                      libcurl(3)