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       libcurl-tutorial - libcurl programming tutorial

       This  document  attempts to describe the general principles and some basic approaches to consider when program-
       ming with libcurl. The text will focus mainly on the C interface but might apply fairly well  on  other  inter-
       faces as well as they usually follow the C one pretty closely.

       This  document  will  refer  to  'the user' as the person writing the source code that uses libcurl. That would
       probably be you or someone in your position.  What will be generally referred to as 'the program' will  be  the
       collected  source  code  that you write that is using libcurl for transfers. The program is outside libcurl and
       libcurl is outside of the program.

       To get more details on all options and functions described herein, please refer to their respective man  pages.

       There  are many different ways to build C programs. This chapter will assume a UNIX-style build process. If you
       use a different build system, you can still read this to get general information that may apply to  your  envi-
       ronment as well.

       Compiling the Program
              Your  compiler  needs  to  know  where the libcurl headers are located. Therefore you must set your com-
              piler's include path to point to the directory where you installed them. The 'curl-config'[3]  tool  can
              be used to get this information:

              $ curl-config --cflags

       Linking the Program with libcurl
              When  having compiled the program, you need to link your object files to create a single executable. For
              that to succeed, you need to link with libcurl and possibly  also  with  other  libraries  that  libcurl
              itself  depends on. Like the OpenSSL libraries, but even some standard OS libraries may be needed on the
              command line. To figure out which flags to use, once again the 'curl-config' tool comes to the rescue:

              $ curl-config --libs

       SSL or Not
              libcurl can be built and customized in many ways. One of the things that varies from different libraries
              and  builds  is the support for SSL-based transfers, like HTTPS and FTPS. If a supported SSL library was
              detected properly at build-time, libcurl will be built with SSL support. To figure out if  an  installed
              libcurl has been built with SSL support enabled, use 'curl-config' like this:

              $ curl-config --feature

              And if SSL is supported, the keyword 'SSL' will be written to stdout, possibly together with a few other
              features that could be either on or off on for different libcurls.

              See also the "Features libcurl Provides" further down.

       autoconf macro
              When you write your configure script to detect libcurl and  setup  variables  accordingly,  we  offer  a
              prewritten macro that probably does everything you need in this area. See docs/libcurl/libcurl.m4 file -
              it includes docs on how to use it.

Portable Code in a Portable World
       The people behind libcurl have put a considerable effort to make libcurl work on a large  amount  of  different
       operating systems and environments.

       You  program  libcurl  the  same  way  on  all  platforms  that  libcurl runs on. There are only very few minor
       considerations that differ. If you just make sure to write your code portable enough, you may very well  create
       yourself a very portable program. libcurl shouldn't stop you from that.

Global Preparation
       The  program  must  initialize some of the libcurl functionality globally. That means it should be done exactly
       once, no matter how many times you intend to use the library. Once for your program's entire life time. This is
       done using


       and  it takes one parameter which is a bit pattern that tells libcurl what to initialize. Using CURL_GLOBAL_ALL
       will make it initialize all known internal sub modules, and might be a good default  option.  The  current  two
       bits that are specified are:

                     which  only does anything on Windows machines. When used on a Windows machine, it'll make libcurl
                     initialize the win32 socket stuff. Without having that initialized properly, your program  cannot
                     use  sockets  properly.  You  should  only  do this once for each application, so if your program
                     already does this or of another library in use does it, you should not tell libcurl to do this as

                     which  only does anything on libcurls compiled and built SSL-enabled. On these systems, this will
                     make libcurl initialize the SSL library properly for this application. This only needs to be done
                     once  for  each  application  so  if  your program or another library already does this, this bit
                     should not be needed.

       libcurl has a default protection mechanism that detects if curl_global_init(3) hasn't been called by  the  time
       curl_easy_perform(3)  is  called  and  if that is the case, libcurl runs the function itself with a guessed bit
       pattern. Please note that depending solely on this is not considered nice nor very good.

       When the program no longer uses libcurl, it should call curl_global_cleanup(3), which is the  opposite  of  the
       init  call.  It will then do the reversed operations to cleanup the resources the curl_global_init(3) call ini-

       Repeated calls to curl_global_init(3) and curl_global_cleanup(3) should be avoided. They should only be  called
       once each.

Features libcurl Provides
       It is considered best-practice to determine libcurl features at run-time rather than at build-time (if possible
       of course). By calling curl_version_info(3) and checking out the details of the returned struct,  your  program
       can figure out exactly what the currently running libcurl supports.

Handle the Easy libcurl
       libcurl  first  introduced the so called easy interface. All operations in the easy interface are prefixed with

       Recent libcurl versions also offer the multi interface. More about that interface, what it is targeted for  and
       how  to  use it is detailed in a separate chapter further down. You still need to understand the easy interface
       first, so please continue reading for better understanding.

       To use the easy interface, you must first create yourself an easy handle. You need one  handle  for  each  easy
       session  you  want to perform. Basically, you should use one handle for every thread you plan to use for trans-
       ferring. You must never share the same handle in multiple threads.

       Get an easy handle with

        easyhandle = curl_easy_init();

       It returns an easy handle. Using that you proceed to the next step: setting up your preferred actions. A handle
       is just a logic entity for the upcoming transfer or series of transfers.

       You  set  properties  and  options  for  this handle using curl_easy_setopt(3). They control how the subsequent
       transfer or transfers will be made. Options remain set in the handle until set again  to  something  different.
       Alas, multiple requests using the same handle will use the same options.

       Many  of  the  options you set in libcurl are "strings", pointers to data terminated with a zero byte. When you
       set strings with curl_easy_setopt(3), libcurl makes its own copy so that they don't need to be kept  around  in
       your application after being set[4].

       One  of  the most basic properties to set in the handle is the URL. You set your preferred URL to transfer with
       CURLOPT_URL in a manner similar to:

        curl_easy_setopt(handle, CURLOPT_URL, "");

       Let's assume for a while that you want to receive data as the URL identifies a remote resource you want to  get
       here.  Since  you write a sort of application that needs this transfer, I assume that you would like to get the
       data passed to you directly instead of simply getting it passed to stdout. So, you write your own function that
       matches this prototype:

        size_t write_data(void *buffer, size_t size, size_t nmemb, void *userp);

       You tell libcurl to pass all data to this function by issuing a function similar to this:

        curl_easy_setopt(easyhandle, CURLOPT_WRITEFUNCTION, write_data);

       You can control what data your callback function gets in the fourth argument by setting another property:

        curl_easy_setopt(easyhandle, CURLOPT_WRITEDATA, &internal_struct);

       Using that property, you can easily pass local data between your application and the function that gets invoked
       by libcurl. libcurl itself won't touch the data you pass with CURLOPT_WRITEDATA.

       libcurl offers its own default internal callback that will take care of the data if you don't set the  callback
       with  CURLOPT_WRITEFUNCTION.  It  will then simply output the received data to stdout. You can have the default
       callback write the data to a different file handle by passing a 'FILE *' to a file opened for writing with  the
       CURLOPT_WRITEDATA option.

       Now,  we need to take a step back and have a deep breath. Here's one of those rare platform-dependent nitpicks.
       Did you spot it? On some platforms[2], libcurl won't be able to operate on files opened by the  program.  Thus,
       if  you  use  the  default  callback and pass in an open file with CURLOPT_WRITEDATA, it will crash. You should
       therefore avoid this to make your program run fine virtually everywhere.

       (CURLOPT_WRITEDATA was formerly known as CURLOPT_FILE. Both names still work and do the same thing).

       If you're using libcurl as a win32 DLL, you MUST use the CURLOPT_WRITEFUNCTION if you set  CURLOPT_WRITEDATA  -
       or you will experience crashes.

       There  are  of  course  many more options you can set, and we'll get back to a few of them later. Let's instead
       continue to the actual transfer:

        success = curl_easy_perform(easyhandle);

       curl_easy_perform(3) will connect to the remote site, do the necessary commands and receive the transfer. When-
       ever  it  receives  data,  it calls the callback function we previously set. The function may get one byte at a
       time, or it may get many kilobytes at once. libcurl delivers as much as possible as  often  as  possible.  Your
       callback  function should return the number of bytes it "took care of". If that is not the exact same amount of
       bytes that was passed to it, libcurl will abort the operation and return with an error code.

       When the transfer is complete, the function returns a return code that informs you if it succeeded in its  mis-
       sion  or  not. If a return code isn't enough for you, you can use the CURLOPT_ERRORBUFFER to point libcurl to a
       buffer of yours where it'll store a human readable error message as well.

       If you then want to transfer another file, the handle is ready to be used again. Mind you, it is even preferred
       that  you re-use an existing handle if you intend to make another transfer. libcurl will then attempt to re-use
       the previous connection.

       For some protocols, downloading a file can involve a complicated process of logging in,  setting  the  transfer
       mode,  changing  the  current  directory and finally transferring the file data. libcurl takes care of all that
       complication for you. Given simply the URL to a file, libcurl will take care of all the details needed  to  get
       the file moved from one machine to another.

Multi-threading Issues
       The first basic rule is that you must never simultaneously share a libcurl handle (be it easy or multi or what-
       ever) between multiple threads. Only use one handle in one thread at any time. You can pass the handles  around
       among threads, but you must never use a single handle from more than one thread at any given time.

       libcurl  is  completely  thread safe, except for two issues: signals and SSL/TLS handlers. Signals are used for
       timing out name resolves (during DNS lookup) - when built without c-ares support and not on Windows.

       If you are accessing HTTPS or FTPS URLs in a multi-threaded manner, you are then of course using the underlying
       SSL  library multi-threaded and those libs might have their own requirements on this issue. Basically, you need
       to provide one or two functions to allow it to function properly. For all details, see this:




        is claimed to be thread-safe already without anything required.


        Required actions unknown.


        Required actions unknown.


        Required actions unknown.

       When using multiple threads you should set the CURLOPT_NOSIGNAL option to 1 for all handles. Everything will or
       might  work  fine  except  that  timeouts  are not honored during the DNS lookup - which you can work around by
       building libcurl with c-ares support. c-ares is a library that provides asynchronous  name  resolves.  On  some
       platforms, libcurl simply will not function properly multi-threaded unless this option is set.

       Also, note that CURLOPT_DNS_USE_GLOBAL_CACHE is not thread-safe.

When It Doesn't Work
       There will always be times when the transfer fails for some reason. You might have set the wrong libcurl option
       or misunderstood what the libcurl option actually does, or the remote server might return non-standard  replies
       that confuse the library which then confuses your program.

       There's  one  golden rule when these things occur: set the CURLOPT_VERBOSE option to 1. It'll cause the library
       to spew out the entire protocol details it sends, some internal info and some received protocol  data  as  well
       (especially when using FTP). If you're using HTTP, adding the headers in the received output to study is also a
       clever way to get a better understanding why the server behaves the way it does. Include headers in the  normal
       body output with CURLOPT_HEADER set 1.

       Of  course, there are bugs left. We need to know about them to be able to fix them, so we're quite dependent on
       your bug reports! When you do report suspected bugs in libcurl, please include as many details as you  possibly
       can: a protocol dump that CURLOPT_VERBOSE produces, library version, as much as possible of your code that uses
       libcurl, operating system name and version, compiler name and version etc.

       If CURLOPT_VERBOSE is not enough, you increase the level of debug data your application receive  by  using  the

       Getting  some  in-depth knowledge about the protocols involved is never wrong, and if you're trying to do funny
       things, you might very well understand libcurl and how to use it better if you study the appropriate RFC  docu-
       ments at least briefly.

Upload Data to a Remote Site
       libcurl tries to keep a protocol independent approach to most transfers, thus uploading to a remote FTP site is
       very similar to uploading data to a HTTP server with a PUT request.

       Of course, first you either create an easy handle or you re-use one existing one. Then you set the URL to oper-
       ate on just like before. This is the remote URL, that we now will upload.

       Since  we write an application, we most likely want libcurl to get the upload data by asking us for it. To make
       it do that, we set the read callback and the custom pointer libcurl will pass to our read  callback.  The  read
       callback should have a prototype similar to:

        size_t function(char *bufptr, size_t size, size_t nitems, void *userp);

       Where  bufptr  is  the  pointer  to  a buffer we fill in with data to upload and size*nitems is the size of the
       buffer and therefore also the maximum amount of data we can return to libcurl in this call. The 'userp' pointer
       is  the custom pointer we set to point to a struct of ours to pass private data between the application and the

        curl_easy_setopt(easyhandle, CURLOPT_READFUNCTION, read_function);

        curl_easy_setopt(easyhandle, CURLOPT_READDATA, &filedata);

       Tell libcurl that we want to upload:

        curl_easy_setopt(easyhandle, CURLOPT_UPLOAD, 1L);

       A few protocols won't behave properly when uploads are done without any prior knowledge of  the  expected  file
       size. So, set the upload file size using the CURLOPT_INFILESIZE_LARGE for all known file sizes like this[1]:

        /* in this example, file_size must be an curl_off_t variable */
        curl_easy_setopt(easyhandle, CURLOPT_INFILESIZE_LARGE, file_size);

       When  you  call  curl_easy_perform(3)  this  time,  it'll  perform all the necessary operations and when it has
       invoked the upload it'll call your supplied callback to get the data to upload. The program  should  return  as
       much  data  as  possible in every invoke, as that is likely to make the upload perform as fast as possible. The
       callback should return the number of bytes it wrote in the buffer. Returning 0  will  signal  the  end  of  the

       Many  protocols  use  or even require that user name and password are provided to be able to download or upload
       the data of your choice. libcurl offers several ways to specify them.

       Most protocols support that you specify the name and password in the URL itself. libcurl will detect  this  and
       use them accordingly. This is written like this:


       If  you  need any odd letters in your user name or password, you should enter them URL encoded, as %XX where XX
       is a two-digit hexadecimal number.

       libcurl also provides options to set various passwords. The user name and password as shown embedded in the URL
       can  instead  get  set  with the CURLOPT_USERPWD option. The argument passed to libcurl should be a char * to a
       string in the format "user:password". In a manner like this:

        curl_easy_setopt(easyhandle, CURLOPT_USERPWD, "myname:thesecret");

       Another case where name and password might be needed at times, is for those  users  who  need  to  authenticate
       themselves  to  a  proxy they use. libcurl offers another option for this, the CURLOPT_PROXYUSERPWD. It is used
       quite similar to the CURLOPT_USERPWD option like this:

        curl_easy_setopt(easyhandle, CURLOPT_PROXYUSERPWD, "myname:thesecret");

       There's a long time UNIX "standard" way of storing ftp user names and passwords,  namely  in  the  $HOME/.netrc
       file. The file should be made private so that only the user may read it (see also the "Security Considerations"
       chapter), as it might contain the password in plain text. libcurl has the ability to use this  file  to  figure
       out  what set of user name and password to use for a particular host. As an extension to the normal functional-
       ity, libcurl also supports this file for non-FTP protocols such as HTTP. To make curl use this  file,  use  the
       CURLOPT_NETRC option:

        curl_easy_setopt(easyhandle, CURLOPT_NETRC, 1L);

       And a very basic example of how such a .netrc file may look like:

        login userlogin
        password secretword

       All these examples have been cases where the password has been optional, or at least you could leave it out and
       have libcurl attempt to do its job without it. There are times when the  password  isn't  optional,  like  when
       you're using an SSL private key for secure transfers.

       To pass the known private key password to libcurl:

        curl_easy_setopt(easyhandle, CURLOPT_KEYPASSWD, "keypassword");

HTTP Authentication
       The  previous  chapter  showed  how to set user name and password for getting URLs that require authentication.
       When using the HTTP protocol, there are many different ways a client  can  provide  those  credentials  to  the
       server and you can control which way libcurl will (attempt to) use them. The default HTTP authentication method
       is called 'Basic', which is sending the name and password in clear-text in the  HTTP  request,  base64-encoded.
       This is insecure.

       At  the  time  of  this writing, libcurl can be built to use: Basic, Digest, NTLM, Negotiate, GSS-Negotiate and
       SPNEGO. You can tell libcurl which one to use with CURLOPT_HTTPAUTH as in:

        curl_easy_setopt(easyhandle, CURLOPT_HTTPAUTH, CURLAUTH_DIGEST);

       And when you send authentication to a proxy, you can also set authentication type the same way but instead with

        curl_easy_setopt(easyhandle, CURLOPT_PROXYAUTH, CURLAUTH_NTLM);

       Both  these  options  allow  you  to set multiple types (by ORing them together), to make libcurl pick the most
       secure one out of the types the server/proxy claims to support. This method does however add a round-trip since
       libcurl must first ask the server what it supports:

        curl_easy_setopt(easyhandle, CURLOPT_HTTPAUTH,

       For  convenience,  you  can  use the 'CURLAUTH_ANY' define (instead of a list with specific types) which allows
       libcurl to use whatever method it wants.

       When asking for multiple types, libcurl will pick the available one it considers "best"  in  its  own  internal
       order of preference.

       We  get  many  questions  regarding how to issue HTTP POSTs with libcurl the proper way. This chapter will thus
       include examples using both different versions of HTTP POST that libcurl supports.

       The first version is the simple POST, the most common version, that most HTML pages using the <form> tag  uses.
       We provide a pointer to the data and tell libcurl to post it all to the remote site:

           char *data="name=daniel&project=curl";
           curl_easy_setopt(easyhandle, CURLOPT_POSTFIELDS, data);
           curl_easy_setopt(easyhandle, CURLOPT_URL, "");

           curl_easy_perform(easyhandle); /* post away! */

       Simple enough, huh? Since you set the POST options with the CURLOPT_POSTFIELDS, this automatically switches the
       handle to use POST in the upcoming request.

       Ok, so what if you want to post binary data that also requires you to set the Content-Type: header of the post?
       Well, binary posts prevent libcurl from being able to do strlen() on the data to figure out the size, so there-
       fore we must tell libcurl the size of the post data. Setting headers in libcurl requests are done in a  generic
       way, by building a list of our own headers and then passing that list to libcurl.

        struct curl_slist *headers=NULL;
        headers = curl_slist_append(headers, "Content-Type: text/xml");

        /* post binary data */
        curl_easy_setopt(easyhandle, CURLOPT_POSTFIELDS, binaryptr);

        /* set the size of the postfields data */
        curl_easy_setopt(easyhandle, CURLOPT_POSTFIELDSIZE, 23L);

        /* pass our list of custom made headers */
        curl_easy_setopt(easyhandle, CURLOPT_HTTPHEADER, headers);

        curl_easy_perform(easyhandle); /* post away! */

        curl_slist_free_all(headers); /* free the header list */

       While  the  simple examples above cover the majority of all cases where HTTP POST operations are required, they
       don't do multi-part formposts. Multi-part formposts were introduced as a better way to  post  (possibly  large)
       binary  data  and  were first documented in the RFC1867 (updated in RFC2388). They're called multi-part because
       they're built by a chain of parts, each part being a single unit of data. Each part has its own name  and  con-
       tents.  You  can  in fact create and post a multi-part formpost with the regular libcurl POST support described
       above, but that would require that you build a formpost yourself and provide to libcurl. To make  that  easier,
       libcurl  provides  curl_formadd(3).  Using  this  function,  you add parts to the form. When you're done adding
       parts, you post the whole form.

       The following example sets two simple text parts with plain textual contents, and then a file with binary  con-
       tents and uploads the whole thing.

        struct curl_httppost *post=NULL;
        struct curl_httppost *last=NULL;
        curl_formadd(&post, &last,
                     CURLFORM_COPYNAME, "name",
                     CURLFORM_COPYCONTENTS, "daniel", CURLFORM_END);
        curl_formadd(&post, &last,
                     CURLFORM_COPYNAME, "project",
                     CURLFORM_COPYCONTENTS, "curl", CURLFORM_END);
        curl_formadd(&post, &last,
                     CURLFORM_COPYNAME, "logotype-image",
                     CURLFORM_FILECONTENT, "curl.png", CURLFORM_END);

        /* Set the form info */
        curl_easy_setopt(easyhandle, CURLOPT_HTTPPOST, post);

        curl_easy_perform(easyhandle); /* post away! */

        /* free the post data again */

       Multipart  formposts  are  chains  of  parts using MIME-style separators and headers. It means that each one of
       these separate parts get a few headers set that describe the individual content-type, size etc. To enable  your
       application  to handicraft this formpost even more, libcurl allows you to supply your own set of custom headers
       to such an individual form part. You can of course supply headers to as many parts as you like, but this little
       example will show how you set headers to one specific part when you add that to the post handle:

        struct curl_slist *headers=NULL;
        headers = curl_slist_append(headers, "Content-Type: text/xml");

        curl_formadd(&post, &last,
                     CURLFORM_COPYNAME, "logotype-image",
                     CURLFORM_FILECONTENT, "curl.xml",
                     CURLFORM_CONTENTHEADER, headers,

        curl_easy_perform(easyhandle); /* post away! */

        curl_formfree(post); /* free post */
        curl_slist_free_all(headers); /* free custom header list */

       Since  all  options  on  an  easyhandle  are  "sticky",  they remain the same until changed even if you do call
       curl_easy_perform(3), you may need to tell curl to go back to a plain GET request if you intend to  do  one  as
       your next request. You force an easyhandle to go back to GET by using the CURLOPT_HTTPGET option:

        curl_easy_setopt(easyhandle, CURLOPT_HTTPGET, 1L);

       Just  setting  CURLOPT_POSTFIELDS to "" or NULL will *not* stop libcurl from doing a POST. It will just make it
       POST without any data to send!

Showing Progress
       For historical and traditional reasons, libcurl has a built-in progress meter that can be switched on and  then
       makes it present a progress meter in your terminal.

       Switch  on  the progress meter by, oddly enough, setting CURLOPT_NOPROGRESS to zero. This option is set to 1 by

       For most applications however, the built-in progress meter is useless and what instead is  interesting  is  the
       ability to specify a progress callback. The function pointer you pass to libcurl will then be called on irregu-
       lar intervals with information about the current transfer.

       Set the progress callback by using CURLOPT_PROGRESSFUNCTION. And pass a pointer to a function that matches this

        int progress_callback(void *clientp,
                              double dltotal,
                              double dlnow,
                              double ultotal,
                              double ulnow);

       If  any of the input arguments is unknown, a 0 will be passed. The first argument, the 'clientp' is the pointer
       you pass to libcurl with CURLOPT_PROGRESSDATA. libcurl won't touch it.

libcurl with C++
       There's basically only one thing to keep in mind when using C++ instead of C when interfacing libcurl:

       The callbacks CANNOT be non-static class member functions

       Example C++ code:

       class AClass {
           static size_t write_data(void *ptr, size_t size, size_t nmemb,
                                    void *ourpointer)
             /* do what you want with the data */

       What "proxy" means according to Merriam-Webster: "a person authorized to act for another" but also "the agency,
       function, or office of a deputy who acts as a substitute for another".

       Proxies  are  exceedingly  common  these  days. Companies often only offer Internet access to employees through
       their proxies. Network clients or user-agents ask the proxy for documents, the proxy does  the  actual  request
       and then it returns them.

       libcurl  supports SOCKS and HTTP proxies. When a given URL is wanted, libcurl will ask the proxy for it instead
       of trying to connect to the actual host identified in the URL.

       If you're using a SOCKS proxy, you may find that libcurl doesn't quite support all operations through it.

       For HTTP proxies: the fact that the proxy is a HTTP proxy puts certain restrictions on what can  actually  hap-
       pen.  A requested URL that might not be a HTTP URL will be still be passed to the HTTP proxy to deliver back to
       libcurl. This happens transparently, and an application may not need to know. I say "may", because at times  it
       is  very  important to understand that all operations over a HTTP proxy use the HTTP protocol. For example, you
       can't invoke your own custom FTP commands or even proper FTP directory listings.

       Proxy Options

              To tell libcurl to use a proxy at a given port number:

               curl_easy_setopt(easyhandle, CURLOPT_PROXY, "");

              Some proxies require user authentication before allowing a request, and you pass that information  simi-
              lar to this:

               curl_easy_setopt(easyhandle, CURLOPT_PROXYUSERPWD, "user:password");

              If  you want to, you can specify the host name only in the CURLOPT_PROXY option, and set the port number
              separately with CURLOPT_PROXYPORT.

              Tell libcurl what kind of proxy it is with CURLOPT_PROXYTYPE (if not, it will default to assume  a  HTTP

               curl_easy_setopt(easyhandle, CURLOPT_PROXYTYPE, CURLPROXY_SOCKS4);

       Environment Variables

              libcurl  automatically  checks  and  uses a set of environment variables to know what proxies to use for
              certain protocols. The names of the variables are following an ancient de facto standard and  are  built
              up  as  "[protocol]_proxy"  (note the lower casing). Which makes the variable 'http_proxy' checked for a
              name of a proxy to use when the input  URL  is  HTTP.  Following  the  same  rule,  the  variable  named
              'ftp_proxy'  is checked for FTP URLs. Again, the proxies are always HTTP proxies, the different names of
              the variables simply allows different HTTP proxies to be used.

              The  proxy  environment  variable  contents  should   be   in   the   format   "[protocol://][user:pass-
              word@]machine[:port]".  Where  the  protocol://  part  is simply ignored if present (so http://proxy and
              bluerk://proxy will do the same) and the optional port number specifies on which port the proxy operates
              on  the  host.  If  not specified, the internal default port number will be used and that is most likely
              *not* the one you would like it to be.

              There are two special environment variables. 'all_proxy' is what sets proxy for any URL in case the pro-
              tocol  specific  variable wasn't set, and 'no_proxy' defines a list of hosts that should not use a proxy
              even though a variable may say so. If 'no_proxy' is a plain asterisk ("*") it matches all hosts.

              To explicitly disable libcurl's checking for and using the proxy environment variables,  set  the  proxy
              name to "" - an empty string - with CURLOPT_PROXY.

       SSL and Proxies

              SSL  is for secure point-to-point connections. This involves strong encryption and similar things, which
              effectively makes it impossible for a proxy to operate as a "man in between" which the proxy's task  is,
              as previously discussed. Instead, the only way to have SSL work over a HTTP proxy is to ask the proxy to
              tunnel trough everything without being able to check or fiddle with the traffic.

              Opening an SSL connection over a HTTP proxy is therefor a matter of asking the proxy for a straight con-
              nection  to the target host on a specified port. This is made with the HTTP request CONNECT. ("please mr
              proxy, connect me to that remote host").

              Because of the nature of this operation, where the proxy has no idea what kind of data that is passed in
              and  out  through this tunnel, this breaks some of the very few advantages that come from using a proxy,
              such as caching.  Many organizations prevent this kind of tunneling to other  destination  port  numbers
              than 443 (which is the default HTTPS port number).

       Tunneling Through Proxy
              As  explained  above,  tunneling  is required for SSL to work and often even restricted to the operation
              intended for SSL; HTTPS.

              This is however not the only time proxy-tunneling might offer benefits to you or your application.

              As tunneling opens a direct connection from your application to the remote machine, it suddenly also re-
              introduces  the  ability to do non-HTTP operations over a HTTP proxy. You can in fact use things such as
              FTP upload or FTP custom commands this way.

              Again, this is often prevented by the administrators of proxies and is rarely allowed.

              Tell libcurl to use proxy tunneling like this:

               curl_easy_setopt(easyhandle, CURLOPT_HTTPPROXYTUNNEL, 1L);

              In fact, there might even be times when you want to do plain HTTP operations using a tunnel  like  this,
              as  it  then  enables  you to operate on the remote server instead of asking the proxy to do so. libcurl
              will not stand in the way for such innovative actions either!

       Proxy Auto-Config

              Netscape first came up with this. It is basically a web page (usually using a  .pac  extension)  with  a
              Javascript that when executed by the browser with the requested URL as input, returns information to the
              browser on how to connect to the URL. The returned information might be "DIRECT" (which means  no  proxy
              should  be  used), "PROXY host:port" (to tell the browser where the proxy for this particular URL is) or
              "SOCKS host:port" (to direct the browser to a SOCKS proxy).

              libcurl has no means to interpret or evaluate Javascript and thus it doesn't support this.  If  you  get
              yourself in a position where you face this nasty invention, the following advice have been mentioned and
              used in the past:

              - Depending on the Javascript complexity, write up a script that translates it to another  language  and
              execute that.

              - Read the Javascript code and rewrite the same logic in another language.

              - Implement a Javascript interpreter; people have successfully used the Mozilla Javascript engine in the

              - Ask your admins to stop this, for a static proxy setup or similar.

Persistence Is The Way to Happiness
       Re-cycling the same easy handle several times when doing multiple requests is the way to go.

       After each single curl_easy_perform(3) operation, libcurl will keep the connection alive and open. A subsequent
       request using the same easy handle to the same host might just be able to use the already open connection! This
       reduces network impact a lot.

       Even if the connection is dropped, all connections involving SSL to the same  host  again,  will  benefit  from
       libcurl's session ID cache that drastically reduces re-connection time.

       FTP  connections  that are kept alive save a lot of time, as the command- response round-trips are skipped, and
       also you don't risk getting blocked without permission to login again like on many FTP servers only allowing  N
       persons to be logged in at the same time.

       libcurl caches DNS name resolving results, to make lookups of a previously looked up name a lot faster.

       Other interesting details that improve performance for subsequent requests may also be added in the future.

       Each  easy  handle  will attempt to keep the last few connections alive for a while in case they are to be used
       again. You can set the size of this "cache" with the CURLOPT_MAXCONNECTS option. Default is 5.  There  is  very
       seldom  any point in changing this value, and if you think of changing this it is often just a matter of think-
       ing again.

       To force your upcoming request to not use an already existing connection (it will even close one first if there
       happens  to  be  one  alive  to  the  same  host  you're  about to operate on), you can do that by setting CUR-
       LOPT_FRESH_CONNECT to 1. In a similar spirit, you can also forbid the upcoming request to be "lying" around and
       possibly get re-used after the request by setting CURLOPT_FORBID_REUSE to 1.

HTTP Headers Used by libcurl
       When  you use libcurl to do HTTP requests, it'll pass along a series of headers automatically. It might be good
       for you to know and understand these. You can replace or remove them by using the CURLOPT_HTTPHEADER option.

       Host   This header is required by HTTP 1.1 and even many 1.0 servers and should be the name of  the  server  we
              want to talk to. This includes the port number if anything but default.

       Accept "*/*".

       Expect When  doing POST requests, libcurl sets this header to "100-continue" to ask the server for an "OK" mes-
              sage before it proceeds with sending the data part of the post. If the  POSTed  data  amount  is  deemed
              "small", libcurl will not use this header.

Customizing Operations
       There is an ongoing development today where more and more protocols are built upon HTTP for transport. This has
       obvious benefits as HTTP is a tested and reliable protocol that is widely deployed and has excellent proxy-sup-

       When  you use one of these protocols, and even when doing other kinds of programming you may need to change the
       traditional HTTP (or FTP or...)  manners. You may need to change words, headers or various data.

       libcurl is your friend here too.

              If just changing the actual HTTP request keyword is what you want, like when GET, HEAD or  POST  is  not
              good enough for you, CURLOPT_CUSTOMREQUEST is there for you. It is very simple to use:

               curl_easy_setopt(easyhandle, CURLOPT_CUSTOMREQUEST, "MYOWNREQUEST");

              When  using the custom request, you change the request keyword of the actual request you are performing.
              Thus, by default you make a GET request but you can also make a POST operation (as described before) and
              then replace the POST keyword if you want to. You're the boss.

       Modify Headers
              HTTP-like  protocols  pass  a series of headers to the server when doing the request, and you're free to
              pass any amount of extra headers that you think fit. Adding headers is this easy:

               struct curl_slist *headers=NULL; /* init to NULL is important */

               headers = curl_slist_append(headers, "Hey-server-hey: how are you?");
               headers = curl_slist_append(headers, "X-silly-content: yes");

               /* pass our list of custom made headers */
               curl_easy_setopt(easyhandle, CURLOPT_HTTPHEADER, headers);

               curl_easy_perform(easyhandle); /* transfer http */

               curl_slist_free_all(headers); /* free the header list */

              ... and if you think some of the internally generated headers, such as Accept: or  Host:  don't  contain
              the data you want them to contain, you can replace them by simply setting them too:

               headers = curl_slist_append(headers, "Accept: Agent-007");
               headers = curl_slist_append(headers, "Host:");

       Delete Headers
              If  you  replace  an  existing  header with one with no contents, you will prevent the header from being
              sent. For instance, if you want to completely prevent the "Accept:" header from being sent, you can dis-
              able it with code similar to this:

               headers = curl_slist_append(headers, "Accept:");

              Both  replacing  and canceling internal headers should be done with careful consideration and you should
              be aware that you may violate the HTTP protocol when doing so.

       Enforcing chunked transfer-encoding

              By making sure a request uses the custom header "Transfer-Encoding: chunked" when doing a  non-GET  HTTP
              operation,  libcurl  will  switch  over  to "chunked" upload, even though the size of the data to upload
              might be known. By default, libcurl usually switches over to chunked upload automatically if the  upload
              data size is unknown.

       HTTP Version

              All  HTTP  requests  includes  the  version  number to tell the server which version we support. libcurl
              speaks HTTP 1.1 by default. Some very old servers don't like getting 1.1-requests and when dealing  with
              stubborn old things like that, you can tell libcurl to use 1.0 instead by doing something like this:

               curl_easy_setopt(easyhandle, CURLOPT_HTTP_VERSION, CURL_HTTP_VERSION_1_0);

       FTP Custom Commands

              Not all protocols are HTTP-like, and thus the above may not help you when you want to make, for example,
              your FTP transfers to behave differently.

              Sending custom commands to a FTP server means that you need to send the  commands  exactly  as  the  FTP
              server  expects  them (RFC959 is a good guide here), and you can only use commands that work on the con-
              trol-connection alone. All kinds of commands that require data interchange and thus need a  data-connec-
              tion must be left to libcurl's own judgement. Also be aware that libcurl will do its very best to change
              directory to the target directory before doing any transfer, so if you change  directory  (with  CWD  or
              similar)  you  might  confuse  libcurl and then it might not attempt to transfer the file in the correct
              remote directory.

              A little example that deletes a given file before an operation:

               headers = curl_slist_append(headers, "DELE file-to-remove");

               /* pass the list of custom commands to the handle */
               curl_easy_setopt(easyhandle, CURLOPT_QUOTE, headers);

               curl_easy_perform(easyhandle); /* transfer ftp data! */

               curl_slist_free_all(headers); /* free the header list */

              If you would instead want this operation (or chain of operations) to happen _after_  the  data  transfer
              took  place  the  option  to  curl_easy_setopt(3) would instead be called CURLOPT_POSTQUOTE and used the
              exact same way.

              The custom FTP command will be issued to the server in the same order they are added to the list, and if
              a  command gets an error code returned back from the server, no more commands will be issued and libcurl
              will bail out with an error code (CURLE_QUOTE_ERROR). Note that if you use CURLOPT_QUOTE  to  send  com-
              mands before a transfer, no transfer will actually take place when a quote command has failed.

              If  you  set the CURLOPT_HEADER to 1, you will tell libcurl to get information about the target file and
              output "headers" about it. The headers will be in "HTTP-style", looking like they do in HTTP.

              The option to enable headers or to run custom FTP commands may be useful to combine with CURLOPT_NOBODY.
              If this option is set, no actual file content transfer will be performed.

              If  you do want to list the contents of a FTP directory using your own defined FTP command, CURLOPT_CUS-
              TOMREQUEST will do just that. "NLST" is the default one for listing directories but you're free to  pass
              in your idea of a good alternative.

Cookies Without Chocolate Chips
       In  the  HTTP  sense,  a  cookie  is  a name with an associated value. A server sends the name and value to the
       client, and expects it to get sent back on every subsequent request to the server that matches  the  particular
       conditions  set.  The  conditions include that the domain name and path match and that the cookie hasn't become
       too old.

       In real-world cases, servers send new cookies to replace existing ones to update them. Server  use  cookies  to
       "track" users and to keep "sessions".

       Cookies  are  sent  from server to clients with the header Set-Cookie: and they're sent from clients to servers
       with the Cookie: header.

       To just send whatever cookie you want to a server, you can use CURLOPT_COOKIE to set a cookie string like this:

        curl_easy_setopt(easyhandle, CURLOPT_COOKIE, "name1=var1; name2=var2;");

       In many cases, that is not enough. You might want to dynamically save whatever cookies the remote server passes
       to you, and make sure those cookies are then used accordingly on later requests.

       One way to do this, is to save all headers you receive in a plain file and when you make a  request,  you  tell
       libcurl  to  read  the previous headers to figure out which cookies to use. Set the header file to read cookies
       from with CURLOPT_COOKIEFILE.

       The CURLOPT_COOKIEFILE option also automatically enables the cookie parser in libcurl. Until the cookie  parser
       is  enabled, libcurl will not parse or understand incoming cookies and they will just be ignored. However, when
       the parser is enabled the cookies will be understood and the cookies will be kept in memory and  used  properly
       in  subsequent  requests  when the same handle is used. Many times this is enough, and you may not have to save
       the cookies to disk at all. Note that the file you specify to  CURLOPT_COOKIEFILE  doesn't  have  to  exist  to
       enable  the  parser, so a common way to just enable the parser and not read any cookies is to use the name of a
       file you know doesn't exist.

       If you would rather use existing cookies  that  you've  previously  received  with  your  Netscape  or  Mozilla
       browsers,  you  can make libcurl use that cookie file as input. The CURLOPT_COOKIEFILE is used for that too, as
       libcurl will automatically find out what kind of file it is and act accordingly.

       Perhaps the most advanced cookie operation libcurl offers, is saving the entire internal cookie state back into
       a  Netscape/Mozilla  formatted  cookie  file.  We  call that the cookie-jar. When you set a file name with CUR-
       LOPT_COOKIEJAR, that file  name  will  be  created  and  all  received  cookies  will  be  stored  in  it  when
       curl_easy_cleanup(3) is called. This enables cookies to get passed on properly between multiple handles without
       any information getting lost.

FTP Peculiarities We Need
       FTP transfers use a second TCP/IP connection for the data transfer. This is usually a fact you can  forget  and
       ignore  but  at times this fact will come back to haunt you. libcurl offers several different ways to customize
       how the second connection is being made.

       libcurl can either connect to the server a second time or tell the server to connect  back  to  it.  The  first
       option is the default and it is also what works best for all the people behind firewalls, NATs or IP-masquerad-
       ing setups.  libcurl then tells the server to open up a new port and wait for a second connection. This  is  by
       default attempted with EPSV first, and if that doesn't work it tries PASV instead. (EPSV is an extension to the
       original FTP spec and does not exist nor work on all FTP servers.)

       You can prevent libcurl from first trying the EPSV command by setting CURLOPT_FTP_USE_EPSV to zero.

       In some cases, you will prefer to have the server connect back to you for the second connection. This might  be
       when the server is perhaps behind a firewall or something and only allows connections on a single port. libcurl
       then informs the remote server which IP address and port number to connect to.  This  is  made  with  the  CUR-
       LOPT_FTPPORT  option. If you set it to "-", libcurl will use your system's "default IP address". If you want to
       use a particular IP, you can set the full IP address, a host name to resolve to an IP address or even  a  local
       network interface name that libcurl will get the IP address from.

       When  doing  the "PORT" approach, libcurl will attempt to use the EPRT and the LPRT before trying PORT, as they
       work with more protocols. You can disable this behavior by setting CURLOPT_FTP_USE_EPRT to zero.

Headers Equal Fun
       Some protocols provide "headers", meta-data separated from the normal data. These headers are  by  default  not
       included  in  the normal data stream, but you can make them appear in the data stream by setting CURLOPT_HEADER
       to 1.

       What might be even more useful, is libcurl's ability to separate the headers from the data and  thus  make  the
       callbacks differ. You can for example set a different pointer to pass to the ordinary write callback by setting

       Or, you can set an entirely separate function to receive the headers, by using CURLOPT_HEADERFUNCTION.

       The headers are passed to the callback function one by one, and you can depend on that fact. It makes it easier
       for you to add custom header parsers etc.

       "Headers"  for FTP transfers equal all the FTP server responses. They aren't actually true headers, but in this
       case we pretend they are! ;-)

Post Transfer Information
        [ curl_easy_getinfo ]

Security Considerations
       The libcurl project takes security seriously.  The library is written with caution and precautions are taken to
       mitigate  many  kinds  of risks encountered while operating with potentially malicious servers on the Internet.
       It is a powerful library, however, which allows application writers to make trade offs between ease of  writing
       and exposure to potential risky operations.  If used the right way, you can use libcurl to transfer data pretty

       Many applications are used in closed networks where users and servers can be trusted, but many others are  used
       on  arbitrary servers and are fed input from potentially untrusted users.  Following is a discussion about some
       risks in the ways in which applications commonly use libcurl and potential mitigations of those risks. It is by
       no means comprehensive, but shows classes of attacks that robust applications should consider. The Common Weak-
       ness Enumeration project at is a good reference for many of these and  similar  types  of
       weaknesses of which application writers should be aware.

       Command Lines
              If you use a command line tool (such as curl) that uses libcurl, and you give options to the tool on the
              command line those options can very likely get read by other users of your system when they use 'ps'  or
              other tools to list currently running processes.

              To avoid this problem, never feed sensitive things to programs using command line options. Write them to
              a protected file and use the -K option to avoid this.

       .netrc .netrc is a pretty handy file/feature that allows you to login quickly and automatically  to  frequently
              visited  sites.  The  file  contains passwords in clear text and is a real security risk. In some cases,
              your .netrc is also stored in a home directory that is NFS mounted or used on another network based file
              system, so the clear text password will fly through your network every time anyone reads that file!

              To avoid this problem, don't use .netrc files and never store passwords in plain text anywhere.

       Clear Text Passwords
              Many  of  the  protocols  libcurl  supports send name and password unencrypted as clear text (HTTP Basic
              authentication, FTP, TELNET etc). It is very easy for anyone on your network or a network  nearby  yours
              to  just  fire  up a network analyzer tool and eavesdrop on your passwords. Don't let the fact that HTTP
              Basic uses base64 encoded passwords fool you. They may not look readable at a  first  glance,  but  they
              very easily "deciphered" by anyone within seconds.

              To  avoid  this  problem, use HTTP authentication methods or other protocols that don't let snoopers see
              your password: HTTP with Digest, NTLM or GSS authentication, HTTPS, FTPS, SCP, SFTP and FTP-Kerberos are
              a few examples.

              The  CURLOPT_FOLLOWLOCATION  option automatically follows HTTP redirects sent by a remote server.  These
              redirects can refer to any kind of URL, not just HTTP.  A redirect  to  a  file:  URL  would  cause  the
              libcurl  to  read  (or write) arbitrary files from the local filesystem.  If the application returns the
              data back to the user (as would happen in some kinds of CGI scripts), an attacker could leverage this to
              read otherwise forbidden data (e.g.  file://localhost/etc/passwd).

              If authentication credentials are stored in the ~/.netrc file, or Kerberos is in use, any other URL type
              (not just file:) that requires authentication is also at risk.  A redirect such as  ftp://some-internal-
              server/private-file would then return data even when the server is password protected.

              In  the same way, if an unencrypted SSH private key has been configured for the user running the libcurl
              application, SCP: or  SFTP:  URLs  could  access  password  or  private-key  protected  resources,  e.g.

              The  CURLOPT_REDIR_PROTOCOLS  and  CURLOPT_NETRC  options  can  be used to mitigate against this kind of

              A redirect can also specify a location available only on the machine running libcurl, including  servers
              hidden  behind  a  firewall  from  the  attacker.   e.g.  or  http://intranet/delete-
              stuff.cgi?delete=all or tftp://bootp-server/pc-config-data

              Apps can mitigate against this by disabling CURLOPT_FOLLOWLOCATION and handling redirects itself,  sani-
              tizing  URLs  as  necessary. Alternately, an app could leave CURLOPT_FOLLOWLOCATION enabled but set CUR-
              LOPT_REDIR_PROTOCOLS and install a CURLOPT_OPENSOCKETFUNCTION callback function in which  addresses  are
              sanitized before use.

       Private Resources
              A user who can control the DNS server of a domain being passed in within a URL can change the address of
              the host to a local, private address which the libcurl application will then use. e.g. The innocuous URL
     could actually resolve to the IP address of a server behind a firewall,
              such as or Apps can mitigate against this by setting a CURLOPT_OPENSOCKETFUNCTION and
              checking the address before a connection.

              All  the malicious scenarios regarding redirected URLs apply just as well to non-redirected URLs, if the
              user is allowed to specify an arbitrary URL that could point to a private resource. For example,  a  web
              app  providing a translation service might happily translate file://localhost/etc/passwd and display the
              result.  Apps can mitigate against this with the CURLOPT_PROTOCOLS option as well as by similar  mitiga-
              tion techniques for redirections.

              A  malicious FTP server could in response to the PASV command return an IP address and port number for a
              server local to the app running libcurl but behind a firewall.  Apps can mitigate against this by  using
              the CURLOPT_FTP_SKIP_PASV_IP option or CURLOPT_FTPPORT.

              When  uploading,  a  redirect can cause a local (or remote) file to be overwritten.  Apps must not allow
              any unsanitized URL to be passed in for uploads.  Also, CURLOPT_FOLLOWLOCATION should  not  be  used  on
              uploads.  Instead, the app should handle redirects itself, sanitizing each URL first.

              Use  of CURLOPT_UNRESTRICTED_AUTH could cause authentication information to be sent to an unknown second
              server.  Apps can mitigate against this  by  disabling  CURLOPT_FOLLOWLOCATION  and  handling  redirects
              itself, sanitizing where necessary.

              Use  of the CURLAUTH_ANY option to CURLOPT_HTTPAUTH could result in user name and password being sent in
              clear text to an HTTP server.   Instead,  use  CURLAUTH_ANYSAFE  which  ensures  that  the  password  is
              encrypted over the network, or else fail the request.

              Use of the CURLUSESSL_TRY option to CURLOPT_USE_SSL could result in user name and password being sent in
              clear text to an FTP server.  Instead, use CURLUSESSL_CONTROL to ensure that an encrypted connection  is
              used or else fail the request.

              If cookies are enabled and cached, then a user could craft a URL which performs some malicious action to
              a site whose  authentication  is  already  stored  in  a  cookie.  e.g.
              stuff.cgi?delete=all  Apps  can  mitigate  against  this  by  disabling cookies or clearing them between

       Dangerous URLs
              SCP URLs can contain raw commands within the scp: URL, which is a side effect of how the SCP protocol is
              designed.  e.g.  scp://user:pass@host/a;date >/tmp/test; Apps must not allow unsanitized SCP: URLs to be
              passed in for downloads.

       Denial of Service
              A malicious server could cause libcurl to effectively hang by sending a trickle of data through, or even
              no  data  at  all  but  just  keeping the TCP connection open.  This could result in a denial-of-service
              attack. The CURLOPT_TIMEOUT and/or CURLOPT_LOW_SPEED_LIMIT options can be used to mitigate against this.

              A  malicious  server could cause libcurl to effectively hang by starting to send data, then severing the
              connection without cleanly closing the TCP connection.  The app could install a  CURLOPT_SOCKOPTFUNCTION
              callback  function  and  set  the  TCP SO_KEEPALIVE option to mitigate against this.  Setting one of the
              timeout options would also work against this attack.

              A malicious server could cause libcurl to download an infinite amount of data, potentially  causing  all
              of  memory or disk to be filled. Setting the CURLOPT_MAXFILESIZE_LARGE option is not sufficient to guard
              against this.  Instead, the app should monitor the amount of data received within the write or  progress
              callback and abort once the limit is reached.

              A  malicious HTTP server could cause an infinite redirection loop, causing a denial-of-service. This can
              be mitigated by using the CURLOPT_MAXREDIRS option.

       Arbitrary Headers
              User-supplied data must be sanitized when used in options  like  CURLOPT_USERAGENT,  CURLOPT_HTTPHEADER,
              CURLOPT_POSTFIELDS  and  others that are used to generate structured data. Characters like embedded car-
              riage returns or ampersands could allow the user to create additional headers or fields that could cause
              malicious transactions.

       Server-supplied Names
              A server can supply data which the application may, in some cases, use as a file name. The curl command-
              line tool does this with --remote-header-name, using the Content-disposition: header to generate a  file
              name.   An  application could also use CURLINFO_EFFECTIVE_URL to generate a file name from a server-sup-
              plied redirect URL. Special care must be taken to sanitize such names to  avoid  the  possibility  of  a
              malicious server supplying one like "/etc/passwd", "utoexec.bat" or even ".bashrc".

       Server Certificates
              A  secure  application should never use the CURLOPT_SSL_VERIFYPEER option to disable certificate valida-
              tion. There are numerous attacks that are enabled by apps that fail to properly validate server  TLS/SSL
              certificates,  thus  enabling a malicious server to spoof a legitimate one. HTTPS without validated cer-
              tificates is potentially as insecure as a plain HTTP connection.

       Showing What You Do
              On a related issue, be aware that even in situations like when you have problems with  libcurl  and  ask
              someone  for  help,  everything  you reveal in order to get best possible help might also impose certain
              security related risks. Host names, user names, paths, operating system specifics, etc (not  to  mention
              passwords of course) may in fact be used by intruders to gain additional information of a potential tar-

              To avoid this problem, you must of course use your common sense. Often, you can just edit out the sensi-
              tive data or just search/replace your true information with faked data.

Multiple Transfers Using the multi Interface
       The  easy  interface as described in detail in this document is a synchronous interface that transfers one file
       at a time and doesn't return until it is done.

       The multi interface, on the other hand, allows your program to transfer multiple files in  both  directions  at
       the  same time, without forcing you to use multiple threads.  The name might make it seem that the multi inter-
       face is for multi-threaded programs, but the truth is almost the reverse.  The multi interface can allow a sin-
       gle-threaded application to perform the same kinds of multiple, simultaneous transfers that multi-threaded pro-
       grams can perform.  It allows many of the benefits of multi-threaded transfers without the complexity of manag-
       ing and synchronizing many threads.

       To  use this interface, you are better off if you first understand the basics of how to use the easy interface.
       The multi interface is simply a way to make multiple transfers at the same time by adding up multiple easy han-
       dles into a "multi stack".

       You  create  the easy handles you want and you set all the options just like you have been told above, and then
       you create a multi handle with curl_multi_init(3) and add all those easy handles  to  that  multi  handle  with

       When  you've  added the handles you have for the moment (you can still add new ones at any time), you start the
       transfers by calling curl_multi_perform(3).

       curl_multi_perform(3) is asynchronous. It will only execute as little as possible and then return back  control
       to your program. It is designed to never block.

       The  best usage of this interface is when you do a select() on all possible file descriptors or sockets to know
       when to call libcurl again. This also makes it easy for you to wait and respond to actions on your own applica-
       tion's  sockets/handles.  You figure out what to select() for by using curl_multi_fdset(3), that fills in a set
       of fd_set variables for you with the particular file descriptors libcurl uses for the moment.

       When you then call select(), it'll return when one of  the  file  handles  signal  action  and  you  then  call
       curl_multi_perform(3) to allow libcurl to do what it wants to do. Take note that libcurl does also feature some
       time-out code so we advise you to never use very long timeouts on  select()  before  you  call  curl_multi_per-
       form(3),  which  thus  should be called unconditionally every now and then even if none of its file descriptors
       have signaled ready. Another precaution you should use: always call curl_multi_fdset(3) immediately before  the
       select() call since the current set of file descriptors may change when calling a curl function.

       If  you  want  to stop the transfer of one of the easy handles in the stack, you can use curl_multi_remove_han-
       dle(3) to remove individual easy handles. Remember that easy handles should be curl_easy_cleanup(3)ed.

       When a transfer within the multi stack has finished,  the  counter  of  running  transfers  (as  filled  in  by
       curl_multi_perform(3)) will decrease. When the number reaches zero, all transfers are done.

       curl_multi_info_read(3)  can be used to get information about completed transfers. It then returns the CURLcode
       for each easy transfer, to allow you to figure out success on each individual transfer.

SSL, Certificates and Other Tricks
        [ seeding, passwords, keys, certificates, ENGINE, ca certs ]

Sharing Data Between Easy Handles
       You can share some data between easy handles when the easy interface is used, and some data is share  automati-
       cally when you use the multi interface.

       When you add easy handles to a multi handle, these easy handles will automatically share a lot of the data that
       otherwise would be kept on a per-easy handle basis when the easy interface is used.

       The DNS cache is shared between handles within a multi handle, making subsequent name resolvings faster and the
       connection  pool that is kept to better allow persistent connections and connection re-use is shared. If you're
       using the easy interface, you can still share these between specific easy handles by using the share interface,
       see libcurl-share(3).

       Some  things are never shared automatically, not within multi handles, like for example cookies so the only way
       to share that is with the share interface.

       [1]    libcurl 7.10.3 and later have the ability to switch over to chunked  Transfer-Encoding  in  cases  where
              HTTP uploads are done with data of an unknown size.

       [2]    This happens on Windows machines when libcurl is built and used as a DLL. However, you can still do this
              on Windows if you link with a static library.

       [3]    The curl-config tool is generated at build-time (on UNIX-like systems) and should be installed with  the
              'make install' or similar instruction that installs the library, header files, man pages etc.

       [4]    This behavior was different in versions before 7.17.0, where strings had to remain valid past the end of
              the curl_easy_setopt(3) call.

libcurl                           4 Mar 2009               libcurl-tutorial(3)