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       libcurl-tutorial - libcurl programming tutorial


       This  document  attempts  to  describe the general principles and some basic approaches to
       consider when programming with libcurl. The text will focus mainly on the C interface  but
       might  apply  fairly  well  on  other  interfaces 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

       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 environment as well.

       Compiling the Program
              Your  compiler  needs  to know where the libcurl headers are located. Therefore you
              must set your compiler'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
                $ 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

              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 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 a
       few minor details that differ. If you just make sure to write your code  portable  enough,
       you can create a portable program. libcurl should not 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 will 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 well.

                     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)  has  not
       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 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 initialized.

       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  runtime  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.

Two Interfaces

       libcurl  first  introduced  the  so  called  easy  interface.  All  operations in the easy
       interface are prefixed with 'curl_easy'. The easy interface lets you do  single  transfers
       with a synchronous and blocking function call.

       libcurl  also  offers  another  interface that allows multiple simultaneous transfers in a
       single thread, the so called multi interface. More about that interface is detailed  in  a
       separate  chapter  further down. You still need to understand the easy interface first, so
       please continue reading for better understanding.

Handle the Easy libcurl

       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 transferring. You must never share  the  same  handle  in
       multiple threads.

       Get an easy handle with
        handle = 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

       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. They are sticky. Multiple requests using the same handle
       will use the same options.

       If you at any point would like to blank all previously  set  options  for  a  single  easy
       handle,  you  can  call curl_easy_reset(3) and you can also make a clone of an easy handle
       (with all its set options) using curl_easy_duphandle(3).

       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 do not 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(3) 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
        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(handle, CURLOPT_WRITEFUNCTION, write_data);
       You  can  control  what data your callback function gets in the fourth argument by setting
       another property:
        curl_easy_setopt(handle, 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 will not touch the data you pass
       with CURLOPT_WRITEDATA(3).

       libcurl offers its own default internal callback that will take care of the data if you do
       not  set  the  callback  with  CURLOPT_WRITEFUNCTION(3).  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(3) 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 will not 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(3), it will crash. You should therefore avoid
       this to make your program run fine virtually everywhere.

       (CURLOPT_WRITEDATA(3) was formerly known as CURLOPT_FILE. Both names still work and do the
       same thing).

       If  you are using libcurl as a win32 DLL, you MUST use the CURLOPT_WRITEFUNCTION(3) if you
       set CURLOPT_WRITEDATA(3) - or you will experience crashes.

       There are of course many more options you can set, and we will get back to a few  of  them
       later. Let's instead continue to the actual transfer:
        success = curl_easy_perform(handle);
       curl_easy_perform(3)  will  connect  to  the  remote  site,  do the necessary commands and
       receive the transfer. Whenever 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 same amount of
       bytes that was passed to it, libcurl will abort the operation and  return  with  an  error

       When  the  transfer is complete, the function returns a return code that informs you if it
       succeeded in its mission or not. If a return code is not enough for you, you can  use  the
       CURLOPT_ERRORBUFFER(3)  to  point libcurl to a buffer of yours where it will 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

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

When It does not 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

       There's one golden rule when these things occur: set the CURLOPT_VERBOSE(3) option  to  1.
       it  will 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 are 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(3) set 1.

       Of  course,  there are bugs left. We need to know about them to be able to fix them, so we
       are 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(3) 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(3) is not enough, you increase the level of debug data your application
       receive by using the CURLOPT_DEBUGFUNCTION(3).

       Getting some in-depth knowledge about the protocols involved is never wrong,  and  if  you
       are  trying  to  do funny things, you might understand libcurl and how to use it better if
       you study the appropriate RFC documents 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 similar to uploading data to an 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 operate on just like before. This is the  remote  URL,  that  we  now  will

       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
        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 callback.
        curl_easy_setopt(handle, CURLOPT_READFUNCTION, read_function);

        curl_easy_setopt(handle, CURLOPT_READDATA, &filedata);
       Tell libcurl that we want to upload:
        curl_easy_setopt(handle, CURLOPT_UPLOAD, 1L);
       A few protocols will not 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(3) for all known file sizes like this[1]:

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

       When you call curl_easy_perform(3) this time, it will perform all the necessary operations
       and when it has invoked the upload it will 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 upload.


       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(3) 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(handle, 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(3).  It  is  used quite similar to the CURLOPT_USERPWD(3) option
       like this:
        curl_easy_setopt(handle, 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  (on  Windows,  libcurl  also checks the %USERPROFILE% environment
       variable if %HOME% is unset, and tries "_netrc" as name). 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  functionality,  libcurl  also  supports  this  file  for  non-FTP
       protocols such as HTTP. To make curl use this file, use the CURLOPT_NETRC(3) option:
        curl_easy_setopt(handle, CURLOPT_NETRC, 1L);
       And a 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  is  not  optional,  like  when  you are using an SSL private key for secure

       To pass the known private key password to libcurl:
        curl_easy_setopt(handle, 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
       (SPNEGO). You can tell libcurl which one to use with CURLOPT_HTTPAUTH(3) as in:
        curl_easy_setopt(handle, CURLOPT_HTTPAUTH, CURLAUTH_DIGEST);
       And when you send authentication to a proxy, you can also set authentication type the same
       way but instead with CURLOPT_PROXYAUTH(3):
        curl_easy_setopt(handle, 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
       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

       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(handle, CURLOPT_POSTFIELDS, data);
           curl_easy_setopt(handle, CURLOPT_URL, "");

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

       Simple enough, huh? Since you set the POST options with  the  CURLOPT_POSTFIELDS(3),  this
       automatically switches the handle to use POST in the upcoming request.

       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 therefore 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(handle, CURLOPT_POSTFIELDS, binaryptr);

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

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

        curl_easy_perform(handle); /* 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 do not 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 are called multi-part because they are  built  by  a  chain  of
       parts, each part being a single unit of data. Each part has its own name and contents. 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 a MIME API consisting in several functions:
       using  those,  you  can  create  and  fill  a multi-part form.  Function curl_mime_init(3)
       creates a multi-part body; you can then append  new  parts  to  a  multi-part  body  using
       curl_mime_addpart(3).   There  are  three  possible  data sources for a part: memory using
       curl_mime_data(3), file using curl_mime_filedata(3) and user-defined  data  read  callback
       using  curl_mime_data_cb(3).   curl_mime_name(3)  sets  a  part's (i.e.: form field) name,
       while curl_mime_filename(3) fills in the remote file name. With curl_mime_type(3), you can
       tell  the  MIME  type  of a part, curl_mime_headers(3) allows defining the part's headers.
       When a multi-part body is no longer needed, you can destroy it using curl_mime_free(3).

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

        curl_mime *multipart = curl_mime_init(handle);
        curl_mimepart *part = curl_mime_addpart(multipart);
        curl_mime_name(part, "name");
        curl_mime_data(part, "daniel", CURL_ZERO_TERMINATED);
        part = curl_mime_addpart(multipart);
        curl_mime_name(part, "project");
        curl_mime_data(part, "curl", CURL_ZERO_TERMINATED);
        part = curl_mime_addpart(multipart);
        curl_mime_name(part, "logotype-image");
        curl_mime_filedata(part, "curl.png");

        /* Set the form info */
        curl_easy_setopt(handle, CURLOPT_MIMEPOST, multipart);

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

        /* free the post data again */

       To  post  multiple  files for a single form field, you must supply each file in a separate
       part, all with the same field name.  Although  function  curl_mime_subparts(3)  implements
       nested  multi-parts, this way of multiple files posting is deprecated by RFC 7578, chapter

       To set the data source from an already opened FILE pointer, use:

        curl_mime_data_cb(part, filesize, (curl_read_callback) fread,
                          (curl_seek_callback) fseek, NULL, filepointer);

       A deprecated curl_formadd(3) function is still supported in libcurl.   It  should  however
       not  be  used  anymore  for new designs and programs using it ought to be converted to the
       MIME API. It is however described here as an aid to conversion.

       Using curl_formadd, you add parts to the form. When you are done adding  parts,  you  post
       the whole form.

       The MIME API example above is expressed as follows using this function:

        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(handle, CURLOPT_HTTPPOST, post);

        curl_easy_perform(handle); /* 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

        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(handle); /* post away! */

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

       Since  all options on an easy handle 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 easy handle to go back
       to GET by using the CURLOPT_HTTPGET(3) option:
        curl_easy_setopt(handle, CURLOPT_HTTPGET, 1L);
       Just setting CURLOPT_POSTFIELDS(3) to "" or NULL will *not*  stop  libcurl  from  doing  a
       POST. It will just make it POST without any data to send!

Converting from deprecated form API to MIME API

       Four rules have to be respected in building the multi-part:
       - The easy handle must be created before building the multi-part.
       - The multi-part is always created by a call to curl_mime_init(handle).
       - Each part is created by a call to curl_mime_addpart(multipart).
       - When complete, the multi-part must be bound to the easy handle using CURLOPT_MIMEPOST(3)
       instead of CURLOPT_HTTPPOST(3).

       Here are some example of curl_formadd calls to MIME API sequences:

        curl_formadd(&post, &last,
                     CURLFORM_COPYNAME, "id",
                     CURLFORM_COPYCONTENTS, "daniel", CURLFORM_END);
                     CURLFORM_CONTENTHEADER, headers,
        part = curl_mime_addpart(multipart);
        curl_mime_name(part, "id");
        curl_mime_data(part, "daniel", CURL_ZERO_TERMINATED);
        curl_mime_headers(part, headers, FALSE);

       Setting the last curl_mime_headers argument to TRUE would have caused the  headers  to  be
       automatically  released  upon  destroyed  the  multi-part,  thus saving a clean-up call to

        curl_formadd(&post, &last,
                     CURLFORM_PTRNAME, "logotype-image",
                     CURLFORM_FILECONTENT, "-",
        part = curl_mime_addpart(multipart);
        curl_mime_name(part, "logotype-image");
        curl_mime_data_cb(part, (curl_off_t) -1, fread, fseek, NULL, stdin);

       curl_mime_name always copies the field name. The special file name "-" is not supported by
       curl_mime_file:  to  read  an open file, use a callback source using fread(). The transfer
       will be chunked since the data size is unknown.

        curl_formadd(&post, &last,
                     CURLFORM_COPYNAME, "datafile[]",
                     CURLFORM_FILE, "file1",
                     CURLFORM_FILE, "file2",
        part = curl_mime_addpart(multipart);
        curl_mime_name(part, "datafile[]");
        curl_mime_filedata(part, "file1");
        part = curl_mime_addpart(multipart);
        curl_mime_name(part, "datafile[]");
        curl_mime_filedata(part, "file2");

       The deprecated multipart/mixed implementation of multiple files field is translated to two
       distinct parts with the same name.

        curl_easy_setopt(handle, CURLOPT_READFUNCTION, myreadfunc);
        curl_formadd(&post, &last,
                     CURLFORM_COPYNAME, "stream",
                     CURLFORM_STREAM, arg,
                     CURLFORM_CONTENTLEN, (curl_off_t) datasize,
                     CURLFORM_FILENAME, "",
                     CURLFORM_CONTENTTYPE, "application/zip",
        part = curl_mime_addpart(multipart);
        curl_mime_name(part, "stream");
        curl_mime_data_cb(part, (curl_off_t) datasize,
                          myreadfunc, NULL, NULL, arg);
        curl_mime_filename(part, "");
        curl_mime_type(part, "application/zip");

       CURLOPT_READFUNCTION  callback  is  not  used:  it is replace by directly setting the part
       source data from the callback read function.

        curl_formadd(&post, &last,
                     CURLFORM_COPYNAME, "memfile",
                     CURLFORM_BUFFER, "memfile.bin",
                     CURLFORM_BUFFERPTR, databuffer,
                     CURLFORM_BUFFERLENGTH, (long) sizeof databuffer,
        part = curl_mime_addpart(multipart);
        curl_mime_name(part, "memfile");
        curl_mime_data(part, databuffer, (curl_off_t) sizeof databuffer);
        curl_mime_filename(part, "memfile.bin");

       curl_mime_data always copies the initial data: data buffer  is  thus  free  for  immediate

        curl_formadd(&post, &last,
                     CURLFORM_COPYNAME, "message",
                     CURLFORM_FILECONTENT, "msg.txt",
        part = curl_mime_addpart(multipart);
        curl_mime_name(part, "message");
        curl_mime_filedata(part, "msg.txt");
        curl_mime_filename(part, NULL);

       Use  of  curl_mime_filedata  sets  the  remote file name as a side effect: it is therefore
       necessary to clear it for CURLFORM_FILECONTENT emulation.

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(3) to zero. This
       option is set to 1 by default.

       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 irregular intervals with information about  the  current

       Set  the  progress  callback by using CURLOPT_PROGRESSFUNCTION(3). And pass a pointer to a
       function that matches this prototype:

        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(3). libcurl will
       not 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

       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 are using a SOCKS proxy, you may find that  libcurl  does  not  quite  support  all
       operations through it.

       For  HTTP  proxies:  the fact that the proxy is an HTTP proxy puts certain restrictions on
       what can actually happen. 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  important  to
       understand  that all operations over an HTTP proxy use the HTTP protocol. For example, you
       cannot 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(handle, CURLOPT_PROXY, "");
              Some proxies require user authentication before allowing a request,  and  you  pass
              that information similar to this:
               curl_easy_setopt(handle, CURLOPT_PROXYUSERPWD, "user:password");
              If  you want to, you can specify the host name only in the CURLOPT_PROXY(3) option,
              and set the port number separately with CURLOPT_PROXYPORT(3).

              Tell libcurl what kind of proxy it is with CURLOPT_PROXYTYPE(3) (if  not,  it  will
              default to assume an HTTP proxy):
               curl_easy_setopt(handle, 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
              old tradition 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:password@]machine[:port]". Where the protocol:// part specifies
              which type of proxy it is, and the optional port number specifies on which port the
              proxy operates. 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 protocol specific variable was not 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(3).

       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 an  HTTP  proxy  is  to  ask  the  proxy  to  tunnel
              everything through without being able to check or fiddle with the traffic.

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

              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 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 an HTTP
              proxy. You can in fact use things such as FTP upload or FTP  custom  commands  this

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

              Tell libcurl to use proxy tunneling like this:
               curl_easy_setopt(handle, 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  does  not
              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 past.

              - 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

       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 do not 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(3) option. Default is 5. There is rarely any point  in  changing  this
       value, and if you think of changing this it is often just a matter of thinking 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 are about to operate
       on),  you  can  do that by setting CURLOPT_FRESH_CONNECT(3) 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(3) to 1.

HTTP Headers Used by libcurl

       When  you  use  libcurl  to  do  HTTP  requests,  it  will  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(3) 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" message before it proceeds with sending the  data  part  of  the
              post.  If  the  posted  data  amount  is  deemed "small", libcurl will not use this

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-support.

       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(3) is there for you.
              It is simple to use:
               curl_easy_setopt(handle, 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 are the boss.

       Modify Headers
              HTTP-like  protocols pass a series of headers to the server when doing the request,
              and you are 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(handle, CURLOPT_HTTPHEADER, headers);

               curl_easy_perform(handle); /* 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: do not 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 disable 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 old servers do not 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(handle, 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 an 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 control-connection alone. All kinds of  commands
              that  require  data  interchange  and  thus  need a data-connection must be left to
              libcurl's own judgment. Also be aware that libcurl  will  do  its  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(handle, CURLOPT_QUOTE, headers);

               curl_easy_perform(handle); /* 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(3) 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(3)  to  send  commands
              before  a  transfer,  no transfer will actually take place when a quote command has

              If you set the CURLOPT_HEADER(3) 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(3). If this option is set, no actual file content transfer will
              be performed.

              If you do want to list the contents of an FTP directory using your own defined  FTP
              command,  CURLOPT_CUSTOMREQUEST(3) will do just that. "NLST" is the default one for
              listing directories but you are 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 has not 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 are sent from
       clients to servers with the Cookie: header.

       To  just send whatever cookie you want to a server, you can use CURLOPT_COOKIE(3) to set a
       cookie string like this:
        curl_easy_setopt(handle, 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(3).

       The CURLOPT_COOKIEFILE(3) 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(3) does not
       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 does not exist.

       If  you  would  rather  use  existing  cookies that you have previously received with your
       Netscape or Mozilla browsers, you can make libcurl use that  cookie  file  as  input.  The
       CURLOPT_COOKIEFILE(3)  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 CURLOPT_COOKIEJAR(3), 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-masquerading 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  does  not  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(3) 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  CURLOPT_FTPPORT(3)  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(3) to zero.

MIME API revisited for SMTP and IMAP

       In  addition  to  support  HTTP  multi-part form fields, the MIME API can be used to build
       structured email messages and  send  them  via  SMTP  or  append  such  messages  to  IMAP

       A  structured  email  message  may contain several parts: some are displayed inline by the
       MUA, some are attachments. Parts can also be structured  as  multi-part,  for  example  to
       include  another  email message or to offer several text formats alternatives. This can be
       nested to any level.

       To build such a message, you prepare the nth-level multi-part and then  include  it  as  a
       source  to  the  parent  multi-part using function curl_mime_subparts(3). Once it has been
       bound to its parent multi-part, a nth-level multi-part belongs to it  and  should  not  be
       freed explicitly.

       Email  messages  data  is  not  supposed  to  be  non-ascii  and  line  length is limited:
       fortunately, some  transfer  encodings  are  defined  by  the  standards  to  support  the
       transmission  of  such  incompatible data. Function curl_mime_encoder(3) tells a part that
       its source data must be encoded before being sent. It  also  generates  the  corresponding
       header  for  that  part.   If  the part data you want to send is already encoded in such a
       scheme, do not use this function (this would  over-encode  it),  but  explicitly  set  the
       corresponding part header.

       Upon  sending  such  a  message,  libcurl  prepends  it  with  the  header  list  set with
       CURLOPT_HTTPHEADER(3), as zero level mime part headers.

       Here is an example building an email message with an inline  plain/html  text  alternative
       and a file attachment encoded in base64:

        curl_mime *message = curl_mime_init(handle);

        /* The inline part is an alternative proposing the html and the text
           versions of the email. */
        curl_mime *alt = curl_mime_init(handle);

        /* HTML message. */
        curl_mimepart *part = curl_mime_addpart(alt);
        curl_mime_data(part, "<html><body><p>This is HTML</p></body></html>",
        curl_mime_type(part, "text/html");

        /* Text message. */
        part = curl_mime_addpart(alt);
        curl_mime_data(part, "This is plain text message",

        /* Create the inline part. */
        part = curl_mime_addpart(message);
        curl_mime_subparts(part, alt);
        curl_mime_type(part, "multipart/alternative");
        struct curl_slist *headers = curl_slist_append(NULL,
                          "Content-Disposition: inline");
        curl_mime_headers(part, headers, TRUE);

        /* Add the attachment. */
        part = curl_mime_addpart(message);
        curl_mime_filedata(part, "manual.pdf");
        curl_mime_encoder(part, "base64");

        /* Build the mail headers. */
        headers = curl_slist_append(NULL, "From:");
        headers = curl_slist_append(headers, "To:");

        /* Set these into the easy handle. */
        curl_easy_setopt(handle, CURLOPT_HTTPHEADER, headers);
        curl_easy_setopt(handle, CURLOPT_MIMEPOST, mime);

       It should be noted that appending a message to an IMAP directory requires the message size
       to be known prior upload. It is therefore not possible to include parts with unknown  data
       size in this context.

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(3) 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 CURLOPT_HEADERDATA(3).

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

       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 are not actually true
       headers, but in this case we pretend they are! ;-)

Post Transfer Information

       See curl_easy_getinfo(3).

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 does not 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 interface is for multi-threaded programs, but the truth
       is almost the reverse. The multi interface allows a single-threaded application to perform
       the  same  kinds  of  multiple,  simultaneous  transfers  that multi-threaded programs can
       perform. It allows many of the benefits of multi-threaded transfers without the complexity
       of managing and synchronizing many threads.

       To  complicate  matters somewhat more, there are even two versions of the multi interface.
       The event based one, also called multi_socket and the "normal one" designed for using with
       select().  See  the  libcurl-multi.3  man page for details on the multi_socket event based
       API, this description here is for the select() oriented one.

       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 handles into a "multi stack".

       You create the easy handles you want, one for each concurrent transfer, and  you  set  all
       the  options  just  like  you  learned  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 have 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 perform what can be done now and  then
       return  control  to  your program. It is designed to never block. You need to keep calling
       the function until all transfers are completed.

       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 application'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 will 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  long
       timeouts on select() before you call curl_multi_perform(3) again. curl_multi_timeout(3) is
       provided to help you get a suitable timeout period.

       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 in any curl function

       If you want to stop the transfer of one of the easy handles in  the  stack,  you  can  use
       curl_multi_remove_handle(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 automatically 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
       resolving faster, and the  connection  pool  that  is  kept  to  better  allow  persistent
       connections and connection re-use is also shared. If you are using the easy interface, you
       can still share these between specific easy handles by  using  the  share  interface,  see

       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-errors(3), libcurl-multi(3), libcurl-easy(3)