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NAME

       libcurl-tutorial - libcurl programming tutorial

Objective

       This  document  attempts  to  describe the general principles and some basic approaches to
       consider when programming with libcurl. The text focuses on the  C  interface  but  should
       apply  fairly  well  on  other  language bindings as well as they usually follow the C API
       pretty closely.

       This document refers to 'the user' as  the  person  writing  the  source  code  that  uses
       libcurl.  That  would  probably  be  you  or  someone  in your position. What is generally
       referred to as 'the program' is 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.

Building

       There are many different ways to build C programs. This chapter assumes 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
              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  is  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

              If SSL is supported, the keyword SSL is written to stdout, possibly together with a
              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 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
        curl_global_init()
       and it takes one parameter which is a bit pattern that tells libcurl what  to  initialize.
       Using  CURL_GLOBAL_ALL  makes it initialize all known internal sub modules, and might be a
       good default option. The current two bits that are specified are:

       CURL_GLOBAL_WIN32
              which only does anything on Windows machines. When used on a  Windows  machine,  it
              makes  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.

       CURL_GLOBAL_SSL
              which only does anything on libcurls  compiled  and  built  SSL-enabled.  On  these
              systems,   this  makes  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 performs 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
       transfers.

       You set properties and options for this handle using curl_easy_setopt(3). They control how
       the subsequent transfer or transfers using this handle are made. Options remain set in the
       handle until set again to something different. They are sticky.  Multiple  requests  using
       the same handle 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, "http://domain.com/");

       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(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 does not touch  the  data  you  pass
       with CURLOPT_WRITEDATA(3).

       libcurl offers its own default internal callback that takes care of the data if you do not
       set the callback with CURLOPT_WRITEFUNCTION(3). It simply outputs  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 take a  deep  breath.  Here  is  one  of  those  rare
       platform-dependent nitpicks. Did you spot it? On some platforms[2], libcurl is not able to
       operate on file handles opened by the program. Therefore, if you use the default  callback
       and  pass  in  an  open file handle with CURLOPT_WRITEDATA(3), libcurl crashes. You should
       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 experience crashes.

       There are of course many more options you can set, and we 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) connects to the remote site, does the necessary commands and performs
       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 aborts the operation and returns with an error code.

       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  stores  a  human
       readable error message as well.

       If  you  then  want  to transfer another file, the handle is ready to be used again. It is
       even preferred and encouraged that you reuse an existing handle  if  you  intend  to  make
       another transfer. libcurl then attempts to reuse a 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 takes 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
       information.

When It does not Work

       There are 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  is one golden rule when these things occur: set the CURLOPT_VERBOSE(3) option to 1.
       it causes 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 reuse one existing one. Then  you
       set the URL to operate on just like before. This is the remote URL, that we now 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 passes 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 sizenitems* 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 do 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 performs all the necessary operations and
       when  it has invoked the upload it calls 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 signals the end of the upload.

Passwords

       Many protocols use or even require that username 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
       detects this and use them accordingly. This is written like this:
        protocol://user:password@example.com/path/
       If you need any odd letters in your username  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 username 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 is a long time Unix "standard" way of storing FTP usernames 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 username 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);

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

        machine myhost.mydomain.com
        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
       transfers.

       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 username 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 uses 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);

       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
       supports:

        curl_easy_setopt(handle, CURLOPT_HTTPAUTH, CURLAUTH_DIGEST|CURLAUTH_BASIC);

       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 picks the available one it considers "best" in its
       own internal order of preference.

HTTP POSTing

       We get many questions regarding how to issue HTTP POSTs with libcurl the proper way.  This
       chapter  thus  includes  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(handle, CURLOPT_POSTFIELDS, data);
           curl_easy_setopt(handle, CURLOPT_URL, "http://posthere.com/");

           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 RFC
       1867 (updated in RFC 2388). 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 filename. 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 */
        curl_mime_free(multipart);

       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
       4.3.

       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 */
        curl_formfree(post);

       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 shows 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,
                     CURLFORM_END);

        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 does not stop libcurl from doing a  POST.
       It just makes 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,
                     CURLFORM_END);

       becomes:
        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(3) 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_slist_free_all(3).

        curl_formadd(&post, &last,
                     CURLFORM_PTRNAME, "logotype-image",
                     CURLFORM_FILECONTENT, "-",
                     CURLFORM_END);

       becomes:
        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(3) always copies the field name. The special filename "-" is not  supported
       by  curl_mime_filename(3):  to read an open file, use a callback source using fread(). The
       transfer is be chunk-encoded since the data size is unknown.

        curl_formadd(&post, &last,
                     CURLFORM_COPYNAME, "datafile[]",
                     CURLFORM_FILE, "file1",
                     CURLFORM_FILE, "file2",
                     CURLFORM_END);

       becomes:
        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, "archive.zip",
                     CURLFORM_CONTENTTYPE, "application/zip",
                     CURLFORM_END);

       becomes:
        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, "archive.zip");
        curl_mime_type(part, "application/zip");

       CURLOPT_READFUNCTION(3)  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,
                     CURLFORM_END);

       becomes:
        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(3) always copies the initial data: data buffer is thus free  for  immediate
       reuse.

        curl_formadd(&post, &last,
                     CURLFORM_COPYNAME, "message",
                     CURLFORM_FILECONTENT, "msg.txt",
                     CURLFORM_END);

       becomes:
        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(3)  sets the remote filename 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 is then called on  irregular  intervals  with  information  about  the  current
       transfer.

       Set  the  progress  callback  by  using  CURLOPT_PROGRESSFUNCTION(3).  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  is  provided.  The  first  argument,  the
       'clientp'  is  the  pointer you pass to libcurl with CURLOPT_PROGRESSDATA(3). libcurl does
       not touch it.

libcurl with C++

       There is 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 */
           }
        }

Proxies

       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 asks the
       proxy for it instead of trying to connect to the actual remote 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 is  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, "proxy-host.com:8080");
              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 hostname 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
              defaults to assuming 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 is 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
              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 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
              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(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 does not stand in the way for such innovative
              actions either!

       Proxy Auto-Config
              Netscape  first  came up with this. It is basically a webpage (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
       go.

       After  each  single curl_easy_perform(3) operation, libcurl keeps 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,
       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 attempts 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, 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 reused 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  passes  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 does 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-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.

       CURLOPT_CUSTOMREQUEST
              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: munged.host.line");

       Delete Headers
              If you replace an existing header with one with no contents, you 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 switches 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 (RFC 959 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  does  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 commands are 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   are   issued   and   libcurl   bails   out  with  an  error  code
              (CURLE_QUOTE_ERROR). Note that if you use CURLOPT_QUOTE(3) to send commands  before
              a transfer, no transfer actually takes place when a quote command has failed.

              If  you  set  the CURLOPT_HEADER(3) to 1, you tell libcurl to get information about
              the target file and output "headers" about it. The  headers  are  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  is
              performed.

       FTP Custom CURLOPT_CUSTOMREQUEST
              If  you do want to list the contents of an FTP directory using your own defined FTP
              command, CURLOPT_CUSTOMREQUEST(3) does 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 does not parse or understand incoming  cookies
       and  they  are  just  be  ignored.  However,  when  the  parser is enabled the cookies are
       understood and the cookies are 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 automatically finds out  what  kind
       of file it is and acts 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 filename with CURLOPT_COOKIEJAR(3), that filename is created
       and all received cookies get 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 detail comes 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 want 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 uses your system's "default IP address". If you want to use  a  particular
       IP,  you  can  set  the  full IP address, a hostname to resolve to an IP address or even a
       local network interface name that libcurl gets the IP address from.

       When doing the "PORT" approach, libcurl attempts 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
       directories.

       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_ZERO_TERMINATED);
        curl_mime_type(part, "text/html");

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

        /* 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: me@example.com");
        headers = curl_slist_append(headers, "To: you@example.com");

        /* 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
       CURLOPT_HEADERFUNCTION(3).

       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
       curl_multi_add_handle(3).

       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  only  performs 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 returns 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
       invoke.

       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)) decreases. 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 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 reuse 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
       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.

Footnotes

       [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.

SEE ALSO

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